[advances in food research] volume 29 || chemical aspects of the smoking of meat and meat products

ADVANCES IN FOOD RESEARCH, VOL. 29

CHEMICAL ASPECTS OF THE SMOKING OF MEAT AND MEAT PRODUCTS

L. TOTHI AND K. POTTHAST

Institute of Chemistry and Physics, Federal Center for Meat Research Kulmbach, Federal Republic of Germany

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Physical Properties of Smoke.. . . . . . . . . . . . . . , . . . , . . , . . , . . , , . . . . . . . .

111. Smoking Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . A. B. Smoke Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Application of Curing Smoke.. . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . D. Application of Smoke Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production of Smoke Condensates and Smoke Preparations . . . . . . . . . . . . . A. Condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Production of Smoke Preparations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Composition of Smoke and Smoked Products . . . . . . . . . . . . . . . . A. Analytical Methods for the Investigation of Smoke

and Smoked Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI. Effectsof Smoking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Desirable Effects.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Undesirable Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Wood Materials for the Generation of Curing Smoke . . . . . . .

IV.

V.

B. Compounds Identified from Smoke , , . . . . . . . . . . .

87 89 90 90 92 98

100 102 102 103 104

109 112 135 137 146 150 150

I . INTRODUCTION

Along with salting and drying, smoking is considered one of the oldest procedures for preserving meats and other foodstuffs. Archeological findings have revealed the use of smoke in food preparation 90,000 years ago (Mohler, 1978). It seems reasonable to assume that humans have used drying and smoking of

'Present address: Institute of Food Chemistry, J . W. Goethe University, Frankfurt am Main, Federal Republic of Germany.

87 Copyright 0 1984 by Acadcmic Pres,. Inc

All nghl, af reproducrion in any Iorm rcxrved ISBN 0 - 1 2 - 0 I M 2 Y - Y

88 L. TOTH AND K . POTTHAST

foodstuffs since they began using fire. The first publications concerning the use of curing with smoke are quite old. In 1573 Heresbachio gave a short description of curing and smoking as one of the most important treatments for the preservation of meats. According to this publication, meat has to be salted in casks, dried on woodshelves, and smoked in a curing smoke containing oxygen in “not too small amounts” to prevent off-flavors in meat products. Heresbachio described a procedure using a very simple smoke chamber which was used for home-made hams as well as other meats for hundreds of years.

This kind of treatment of meats was applied in industrial countries in general until the last few decades. Now only a very few meat products are prepared in this manner. This does not imply a shortage of production or a loss in desirability of cured, smoked meats-about 60% of the meat products manufactured in the Federal Republic of Germany are cured and smoked-but reflects changes in the methods of production. A modern technology with more complicated smokehouse types has developed from the very simple technique of home-style smoking.

Treatment of meat products, fish, and some other foods with smoke produces a certain amount of preservation in addition to flavoring and coloring. Smoke contains bactericidal and fungicidal constituents that counteract microbial and chemical spoilage. During the smoking procedure, changes such as cooking, ripening, and drying may occur within or at the surface of the product. These processes additionally enhance stability. The tanning effect of smoke on natural casings and the formation of a secondary skin are also of technological interest.

The smoking of meat products was performed empirically for a long time. Flavor and color were determined by experience and local habits. This has led to a wide variability among smoked meat products. Flavor ranges from a very light to a heavy smoke taste accompanied by colors from golden yellow to dark brown (Potthast, 1975, 1981).

In the last few decades smoking has been the topic of numerous scientific publications. In addition to other researchers, Tilgner and co-workers have done much fundamental research, and in 1977 Tilgner published a review of the development of the smoking process. Gilbert and Knowles (1975) reported on the chemistry of smoking procedures, referring to 70 original .publications. A monograph describing the general knowledge of smoking and presenting a survey of known constituents of smoke was written by Mohler (1978). Present analytical publications report about 390 different compounds detectable in curing smoke, but there is no doubt that many more exist. The most important groups are phenols, carbonyls, acids, furanes, alcohols, esters, lactones, and polycyclic hydrocarbons (PAH) (Hamm, 1977a). From these different groups, 70 compounds have been isolated from smoked foods according to Mohler (1978). The spectrum of compounds, in particular, phenols (Tbth, 1982) and PAH (Potthast, 1979), has been completed.

CHEMICAL ASPECTS OF MEAT SMOKING 89

The PAH fraction is the most undesirable fraction in smoke and smoke preparations because some PAH, e.g., benzo[a]pyrene, have been shown to be mutagenic or carcinogenic in animal experiments. For this reason systematic investigations have been carried out to eliminate or reduce these compounds to the lowest levels (Potthast, 1975, 1981). Polycyclic hydrocarbons do not influence flavor, but in this regard phenols are of special importance (Bratzler et al., 1969; Daun, 1969, 1972; Husaini and Cooper, 1957). Thus, efforts have been made to produce smoke preparations free from PAH-containing phenols but enriched with other, desirable constituents of smoke. Some phenols are supposedly carcinogenic or cocarcinogenic (Gibe1 and Gummel, 1967; Kaiser and Bartome, 1966; Kaiser, 1967; Karu et al., 1973; Van Duuren et al., 1966, 1974), but it is not known if this holds true for the mixture of all phenols present in smoke. From animal experiments, it is known that certain substances may have a hazardous effect (Pfeiffer, 1973) in a pure state, yet in mixtures with compounds of similar chemical structure and properties there is no deleterious effect. Thus, one cannot disregard possible toxicological influences from different smoke preparations of differing origin even though they prove harmless alone. This was an impetus for the authors to investigate the phenolic constituents of smoke. Results elaborated during the past several years have been published as a monograph (in German, T6th, 1982), as well as in other publications (T6th, 1980a,b). There are also some original publications concerning PAH by T6th (1971) and Potthast (1975, 1976, 1979, 1980, 1981). The topic of carcinogenicity is discussed again in Section VI,B,2 of this article.

II. PHYSICAL PROPERTIES OF SMOKE

Curing smoke is a complex mixture. The composition is influenced by several generation parameters such as type of wood, humidity of the wood, generation temperature, and amount of oxygen during the generation process as will be discussed later. According to Foster and Simpson (1961), curing smoke consists of a dispersed phase of solid and liquid particles as well as a gaseous phase. Particle size is between 50 and 800 p m (Tilgner, 1958b; Foster and Simpson, 1961). The composition of the particle and gaseous phase is dependent on tern- perature, smoke density, and smoke treatment, e.g., washing or filtering through mechanical or electrical equipment. Tilgner (1967) estimated the gas phase to account for about 10% of the total volume.

Smoke contains high and low molecular weight compounds. From the chemical composition one can see that they are more or less water soluble. This is of great importance in the production of liquid smokes because water-soluble fractions are enriched with constituents desirable for the smoke treatment of foods, while the water-insoluble fraction contains tar, solid particles (e.g., soot), and


PAH, some of which are very potent carcinogenic compounds (e.g., benzo[a]pyrene). The composition of smoke can be influenced by temperature or electrostatic treatment. By cooling smoke before entering the smokehouse, high boiling constituents, e.g., tar and PAH, may be eliminated to a certain extent. By passing smoke through an electrostatic field, the particle phase of the smoke is changed, and then quite a fast separation may occur.

Ill. SMOKING TECHNOLOGY

Smoking is the treatment of meats or meat products, fish, and sometimes other foods with curing smoke. This kind of technology represents two important precepts (Tilgner, 1958a).

1. Thermal treatment of foods is done with the intentions of reducing humidity uniformly, of producing some ripening by autolytic enzymatic action if desired, and of cooking the product to a certain degree.

2 . Smoke treatment influences the overall acceptability by enhancing color or flavor and in addition provides some preservative effect. However, in modem meat processing, preservation seems to be less important than overall acceptability because product stability may be achieved by heating and/or drying.

The aim of this article is to discuss the chemistry of smoke and to show relationships between smoking procedure and smoke-treated foods as well as to discuss other effects accompanying smoke curing.

A. WOOD MATERIALS FOR THE GENERATION OF CURING SMOKE

Curing smoke normally is produced from wood. The composition of wood is approximately 50% cellulose, 25% hemicellulose, and 25% lignin (Fig. 1). The structure of the lignin of softwood (gymnosperms, e.g., pine, fir, spruce) is different from hardwood (dicotyledons, e.g., beech, oak, hickory): lignin from softwood contains fewer methoxy groups than does hardwood. Block (1979) gave the formula for two typical types of lignin:

Lignin from beech: C,H,. 1303(OCH3), ,4,

Lignin from pine: C,H, ,802(OCH3)o~99

where 1.41 and 0.99 denote the ratio of methoxy groups to the rest of the molecule. The lignin of beech mainly consists of sinapinalcohol; that of pine, on the other hand, mainly coniferylalcohol. Both types show differences only in the number of OCH, groups. In addition to the main constituents mentioned, different kinds of wood contain differing amounts of resins of varying composition.


a

b

-0

OH OH

OH CH,OH OH

C

HO

CH,COCH,

CH=CH-CH,OH CH=CH-CH,OH I I

@OCH,

OH

e H,CO @OCH,

OH

FIG. 1. Main constituents of wood: 25% hemicellulose (a); 50% cellulose (b); and 25% lignin (c), from softwood and, with ... OCH,, from hardwood. Arrows in (c) shows splitting points during pyrolysis. Also shown are (d) coniferylalcohol and (e) sinapinalcohol. From Coos (1952).

Examples are etheric oils and other substances which have some influence on smoke flavor.

Addition of spices, seasonings, branches of softwood containing needles, or heather to sawdust makes development of special flavors possible. As was pointed out by T6th and Blaas (1972a), materials with high amounts of resin may be the reason for an intensive development of soot followed by an increase in the


contamination of smoked products with PAH. Potthast (1979) supposed that the increase in soot is caused only by high smoke generation temperatures. During smoking with materials containing up to 20% resin (fresh branches of pines with needles, rootstocks, and seasonings) mixed with sawdust from softwood, no soot and no abnormally high contamination of meat products with benzo[a]pyrene (B[a]p) or other PAH have been detected when smoke generation temperatures were lower than 700°C during a 2-day process. Furthermore, these investigations made clear that smokehouse temperatures did not greatly affect the B[a]p content of smoked meats. After 2 days of smoking ham, the highest B[a]p value detectable was 0.2 pg/kg. The flavor of the products smoked with the above-mentioned materials was of finer quality than that which could be achieved with pure sawdust. Temperature control was maintained by high water levels (50% H,O to 50% dry matter) within the sawdust and by lowering air flow to the smoke generator.

As was observed, the type of wood used for smoke generation exerts great influence on the flavor and color of smoked products (Reuter, 1966). Hardwood from beech, oak, and hickory among others imparts a yellow-brown color. The different phenolic compounds produced by hard- and softwoods influence the taste of the products. Local habits apparently dictate whether people prefer one product or another. Thus Spanyir and co-workers (1960b), Tilgner (1958b), and Rusz (1962) stated that smoke from hardwood or softwood leads to different but comparable smoke flavor and taste. In Germany one can observe more brightly colored smoked products in the north and more dark brown to black smoked products in the south of the country. This arises from customs developed over centuries and reflects every variation in the degree of fineness of the wood particles, water content, as well as ingredients in the wood used for smoking.

B. SMOKE GENERATION

Curing smoke in modem smokehouses is generated by different kinds of generators.

I . Smoke from Smoldering Wood

For centuries, foods, e.g., meat or fish, have been treated with smoke emitted from open fires in living quarters. In Germany special meat products were produced by smoking in one-room houses, or “Katen.” This is sometimes still done.

Experience and local habits have been the origin of different customs of smoking. Smoke developed by burning materials of plant origin, predominantly wood, characterized all smoking procedures common in older times. A preferred quality was achieved when the product was treated with smoke from slowly


burning or smoldering wood. The type of wood chosen for smoking was a matter of experience; sometimes one of a mixture of several types including seasonings was used for smoke generation. Smoke generation and smoking were done in one room.

In modem meat plants most smoke generators are separate from the smokehouses. Often, both the smoke generator and the smokehouse are under elec- tronic control. Scientific findings supported the development of modern smoke generators.

Curing smoke is produced by pyrolytic changes of wood. This process can be divided into two steps (Miler, 1962b): (1) pyrolysis of wood as a primary reaction, and (2) changes in products of pyrolysis by secondary reactions (redox reactions, formation of cyclic and polycyclic compounds, polymerization, condensation, as well as pyrolysis of the formed reaction products).

Dry distillation, a technique of great technological importance, has been studied for a long time (Goos, 1952; Nikitin, 1955; Kuriyama, 1962). Thermal changes observed during the heating of wood in an atmosphere without oxygen are of interest. External heating regulates a periodic increase or decrease of wood temperature (Kuriyama, 1962). From this one can conclude that exothermic reactions take place along with endothermic reactions. The different constituents of the wood decompose at different temperatures. The course of thermal decomposition of the wood is as follows:

1. Drying-up to about 170°C. 2. Pyrolysis of hemicellulose-between 200 and 260°C. 3. Pyrolysis of cellulose-between 260 and 310°C. 4. Pyrolysis of lignin-between 310 and 500°C.

During thermal decomposition in the absence of oxygen there is a development of different gases and fluids, and the remainder is charcoal. Table I shows the percentage of products formed during the pyrolysis of wood by dry distillation. About 35% of the wood remains as charcoal; only 12-17% of the products are water-soluble compounds of interest to the smoking procedure. Tar products account for - 10% of dry distillation products and contain PAH and other undesirable constitutents.

Volatile compounds from dry distillation are less desirable than those of curing smoke in the smoke flavoring of meat products, and result from the lack of oxygen during smoke production. Presence of oxygen, which causes secondary reactions with thermal decomposition products of wood, is of great importance for the development of a good smoke flavor. These reactions take place within neighboring zones. The temperature of burning wood particles (microzone) may rise to 700°C or higher depending on the amount of air during combustion (Miler, 1962a). At these high temperatures, charcoal is degraded to CO, CO,, and H,O. The developing heat accelerates pyrolysis. The higher the amount of


TABLE I PERCENTAGE OF PRODUCTS FORMED BY DRY DISTILLATION

OF DIFFERENT WOODSa

Product Spruce ~~

Pine Birch Beech

Charcoal 37.8 Gases

CO*, CO 13.9 CH4, C2H4 0.8

Water 22.3 Water-soluble organic compounds 12.6 Tar 11.8 Fraction to be condensed down to -80°C 46.7

37.8 31.8

14.1 13.3 0.8 0.7

25.7 27.8 12.1 17.0 8.1 7.9

45.9 52.7

35.0

15.1 0.7

26.6 14.2 8.1

48.9

u From Nikitin (1955) with permission.

oxygen, the more the volatile compounds from thermal decomposition are burned, and with that the curing smoke becomes less flavoring. With excess oxygen, temperatures exceed 1200°C.

Tilgner and co-workers (1962b) pointed out that curing smoke of good quality may be generated if pyrolysis of wood has been performed at temperatures between 350 and 450°C and if the oxidation of volatile decomposition products takes place at about 200°C. From this one can conclude that the most important factor in smoke production is the maintenance of the lowest possible generation temperatures. This requirement had been deduced from practical observation over a long time. Now, however, information from the literature concerning the ideal smoke generation temperatures suggests that they range between 200 and 700°C. These contradictory findings may be explained by difficulties in temperature measurement, which has normally been carried out with thermocouples. This kind of measurement is inaccurate because it is nearly impossible to record the temperature directly within glowing wood particles, where the highest temperatures are achieved for a very short time only. Moreover, the inertia effect of small and large diameter thermocouples leads to quite different results if measurement has to be done in a very short time. Thus, the temperature measured does not represent the real temperature of burning wood but rather the temperature of the environment where the thermocouple is located in the so-called macrozone. Taking temperatures with infrared test equipment, Potthast (1979) and Toth (1982) found that the highest concentration of smoke flavoring constituents (from an equivalent amount of wood) will occur at about 700°C. At this same temperature the relationship between these constituents and PAH is the most favorable, as will be discussed later.


Generation of curing smoke by smoldering wood may cause problems that are reflected in the varying composition of smoke. Modem generators for the production of smoke from smoldering sawdust or other types of fine wood are designed to produce a constant smoke, but in amount and composition of smoke produced, they are unsatisfactory. It is agreed that, when compared to other generation methods, smoke produced by smoldering minced wood is the preferred flavoring agent for meats despite sometimes changing sensory evaluations.

The reason for the differing composition of the smoke is temperature variations within the glowing zone, which depend on the amount of air and the water content of the sawdust. By measuring the temperature with infrared test equipment, Muller (1982) showed that during smoldering of air-dried sawdust containing 10-15% water, temperatures varied from about 700 to 1000°C with the lowest air supply possible with this equipment. After equilibrating the sawdust with 50% water it became possible to maintain stable generation temperatures at about 600°C. Differences in amount and composition of smoke should be vir- tually eliminated by a new temperature- and humidity-controlled smoke generator for smoldering sawdust (Toth and Potthast, 1982). Equilibrium of humidity may be achieved within the sawdust hopper and temperature control by regulation of air flow, which is dependent on the temperature measured within the glowing zone.

2. Smoke Generators Working With Heat Supply

In the last two decades new types of smoke generators have been developed. These generators operate to a certain extent with an external heat supply instead of using heat of combustion for pyrolysis. External heating of wood materials allows better temperature control of pyrolysis and development of smoke of more uniform composition and quality. Smoke generation proceeds as follows. Wood is heated to a temperature between about 300 and 400°C to begin pyrolysis. In general, pyrolysis occurs in the presence of low amounts of oxygen or no oxygen at all, If oxygen is present, pyrolysis starts with endothermic reactions and becomes exothermic to some extent. Pyrolysis without oxygen is mainly endothermic. Combustion temperature is controlled by heat and air supply. If there is an increase in temperature exceeding the set limits, external temperature and air supply are reduced or stopped.

Smoke flavor can be improved after smoke generation by reacting pyrolytic compounds with oxygen in reaction chambers connected to the smoke generator (secondary reactions). These secondary reactions take place at lower temperatures of pyrolysis and have a major influence on the flavoring properties of smoke. The composition of this smoke is quite different from smoke generated


by the heat of combustion. Under combustion conditions, products of pyrolysis are oxidized at higher temperatures than those used for initiating pyrolysis, and thus different reaction products are formed.

3 . Friction Smoke Generator

A very simple and efficient device for smoke generation is the friction smoke generator (Rassmussen, 1956; Tilgner, 1958b; Klettner, 1975). Pyrolysis temperatures are produced by pressing a stick of wood against a rapidly rotating cylinder that has a rough surface. The heat of friction is controlled by peri- odically stopping the cylinder, thus preventing burning of the wooden stick. According to Klettner (1975), pyrolysis takes place at about 400°C. Because of the rapid air current, the smoke is quickly separated from the heated zone and therefore secondary reactions occur at lower temperatures. Special advantages of this technique to meat industries are the production of curing smoke in a short time and the low level of maintenance needed for friction smoke generators.

Miiller (1982), taking temperature measurements with IR test equipment, confirmed the findings of Klettner, which were performed with thermocouples. During the usual time of friction (5-25 sec), the wooden sticks are heated to 380 * 20°C at the contact areas. Glowing wood shavings develop at temperatures higher than 400°C. These glowing particles should be extinguished in a water trap; otherwise they collect in the ash tray, and a second area of pyrolysis develops that produces smoke from normal combustion, exceeding friction generation temperatures. Friction smoke (temperature of pyrolysis about 400°C) shows a distinctly lower generation temperature than that of smoke from smoldering sawdust. This has a particular influence on the chemical composition of smoke as will be discussed later (p. 125 and Fig. 6) . Therefore, the statement of Tilgner (1958b) that meat products treated with friction smoke are of different but good sensory properties may be explained by analytical findings.

4 . Steam Smoke Generator

Inducing pyrolysis by passing superheated steam through chopped wood is another possibility for smoke production (Fessmann, 1965; Reuter and Heinz, 1969). The steam must have a temperature of approximately 300-400°C and contain some oxygen to maintain temperatures for thermal decomposition of the wood and to improve smoke flavor. Measurement of the temperature within the steam smoke generator is difficult and for a long time was not carried out with sufficient accuracy. Moreover, generation temperature was supposed to be iden- tical to steam temperature, that is, 300-400°C. Since smoke generation temperature is the most influential factor in smoke composition, products of steam pyrolysis should be similar to those of friction smoke when both types of smoke


are produced at the same temperature. But according to Toth (1982), smoke from a steam generator is nearer in composition to that of smoldering wood materials, generated at about 650°C. Consequently the pyrolysis of the wood by overheated steam must take place at distinctly higher temperatures than the steam itself. This was shown by Muller (1982), who took temperature measurements with thermocouples fixed within the pyrolysis pipe of the steam smoke generator. He found a relationship between the amount of oxygen in the superheated steam and the smoke generation temperature, which normally varied between 450 and 650°C and sometimes far exceeded these values. To achieve low temperatures of pyrolysis only small amounts of oxygen can be mixed with the steam. The measurements of Muller hint that temperatures within the steam smoke generator are less constant than believed, which would affect the uniformity of smoke composition. However, variations in temperature measured during these experiments might reasonably be due to inaccurate thermocouple measurements which recorded temperatures in the macrozone but not in the microzone.

5 . Fluidization Smoke Generator

The fluidization smoke generator (Nicol, 1960; Klettner, 1979) allows pyrolysis of wood shavings suspended in air that has been heated to 300-400°C. Pyrolysis is carried out within a reaction chamber, and smoke and solid particles are separated by passing a cyclone refiner. Because of its more complicated nature, this type of smoke generation has not been of industrial interest. Al- though actual temperatures have not yet been measured, one can conclude from observations made with the steam smoke generator that smoke generation takes place at temperatures higher than that of the hot air.

6. Laboratory Generators

a. Two-Stage Smoke Generator. In the two-stage smoke generator (Miler, 1962b; Daun, 1966), pyrolysis of wood (primary reaction) and oxidation of thermal decomposition products (secondary reaction) occur at different temperatures. As a first step, pyrolytic changes are induced with hot nitrogen from about 250 to 550°C. The second step is to cool and oxidize products of pyrolysis at temperatures of 125-300°C. This two-stage smoke generator was used only as laboratory equipment to study optimum temperature of pyrolysis and oxidation.

6. Generator for Isothermal Smoke Development. For isothermal smoke development (Ugstad et al . , 1979), smoke is produced from wood chips (con- veyed by a feed screw from a hopper through a smoke generation zone), which are heated by electric elements placed in a steel mantel. Pyrolysis occurs in the


presence of some oxygen. The speed of the feed screw is continuously regulated, which, together with the heating elements, functions to control temperature. Working temperature has to be less than 500°C because of exothermal reactions that occur at higher temperatures. Temperature variation is -+ 10°C.

c. Laboratory Generator According to T6th. The laboratory generator according to T6th (1982) consists of a perforated cylinder in which smoke is produced from smoldering sawdust. The smoke is forced through the holes of the cyclinder by a vacuum, and after cooling it is condensed in several water traps. The smoke generation temperature depends on the water content of the sawdust and on the air velocity regulated by the vacuum. This laboratory generator allows the study of pyrolysis from -400 to greater than 1000°C. Temperature variation is about 250°C.

Additional different types of smoke generators have been described in the literature. Most of them are prototypes, insignificant for industrial purposes. It is remarkable that smoke from all types of generators is compared exclusively with smoke from smoldering wood. This would suggest that the latter method of freshly developed smoke is the most popular one.

C. APPLICATION OF CURING SMOKE

With traditional equipment smoke development and smoking of food are carried out in the same chamber. While this traditional arrangement is becoming rarer in Germany, it can still be found in some meat plants, especially those processing certain smoked meat specialities. In modem technology, the smoke generator and the smokehouse are separated for several reasons.

1. In smokehouses used for the production of meat products with high or low water contents, e.g., dry or frankfurter-type sausages, the high or low relative humidities produced during processing would influence smoke production within the same chamber.

2 . During combustion ash, tar, and soot develop, creating cleanliness and hygienic problems. To prevent sausage contamination, smoke generation is better done outside the smokehouse.

3 . Traditional equipment was operated with little air movement and low air intake. Modem smokehouses use fast air circulation, making temperature control with low temperature pyrolysis nearly impossible.

Smoke from smoke generators cools very fast. During cooling soot and tar are separated and condense within pipes between the smokehouse and generator. The composition of tar varies mainly with the water content of the wood or water vapor used in smoke generation; for example, tar from a steam smoke generator has a greater water content. The separation of tar products leads to a remarkable


loss in carcinogenic PAH in addition to a more desirable flavoring of meat products or other foods. The removal of PAH, soot, and tar may be facilitated by washing or filtering the smoke (Tbth and Blaas, 1972~). Some authors recom- mend complete separation of smoke particles by means of electrostatic filtering equipment (Foster and Simpson, 1961; Rusz et al., 1969; Rusz, 1976). Elec- trostatic treatment of smoke leads to fast soiling of the filter and thus to inefficient installation. Moreover, it causes a significant loss in smoke flavoring constituents, which is also true for washing of smoke. Electrostatic treatment has never become significant for industrial purposes.

The smoke entering the smokehouse equilibrates with the smokehouse temperature, which is dependent on the kind of product to be produced. Processing may be differentiated by temperature as follows: (1) cold smoke processing (15- 25"C), (2) warm smoke processing (25-5OoC), and (3) hot smoke processing (50-85°C). Depending on its humidity, smoke is characterized as dry or wet. Smokehouse temperature and humidity are adjusted to product requirements. Fermented sausages (of the salami type) or raw, fermented ham are in general cold smoked, but may also be warm smoked (up to 40°C). "Briihwurst"-type sausages are hot smoked. Warm smoking of meat products is considered a special treatment (Reuter, 1966) for home-style products. As Potthast (1981) pointed out, warm smoking of ham at temperatures between 35 and 38°C for 48 hr produces a special flavoring that cannot be explained by the smoke treatment alone.

Proteolytic reactions are believed to be responsible to a certain extent for flavor development. In addition, treatment of ham at these temperatures is of great advantage for tenderness and color stability. In this context, it is of interest that the optimal temperature for hydrolytic action of muscle cathepsins is 37°C (Deng and Lillard, 1973). Treatment of meats at higher temperatures (up to 47°C) causes a short-term increase in proteolytic activity, but also a simultaneous advanced denaturation of cathepsins.

During the smoking procedure, smoke constituents are adsorbed or condensed at the surface of the meat products. This condensation is greatest with a cold meat product during a hot smoke treatment. Foster and Simpson (1961) sug- gested that the gaseous constituents of the smoke are the main factors involved in coloring and flavoring of meat. Contrary to this hypothesis is the observation that meat products are smoked faster when the humidity of the smoke is higher. Liquid smoke or a smoke from a steam smoke generator colors and flavors a meat product in a very short time. Alternatively, when passed through an electrostatic field, smoke particles are positively charged (Mohler, 1978; Tilgner, 1958a), thus enhancing their adsorption properties. Treatment of the surface of meat products is completed in a time too short for adequate color development. This is remedied by subsequently heating the smoke-treated products at a higher temperature and at a lower relative humidity, because carbonyl amine reactions

100 L. TOTH AND K . POITHAST

involved in coloring are favored under dry and hot conditions. Increasing temperature and lowering relative humidity are common methods for producing color development and are not used exclusively in electrostatic smoke treatment. Because of the higher temperature treatment this procedure is applied mainly during hot smoke processing.

In recent years one can observe increasing efforts to reduce emissions from smokehouses as environmental requirements evolve. Emissions can be reduced by the condensation of smoke in water traps, electrical filtering, total cornbus- tion, or using less smoke in food processing without less color and flavor development. This idea is realized in certain types of smokehouses in which curing smoke is forced to circulate from the generator through the smokehouse and again through the generator, mixing with freshly developed smoke. This procedure uses only about one-tenth of the amount of smoke normally generated for the treatment of meat products. Smoke development takes place in an oxygen- poor atmosphere, which causes the sawdust to smolder under strong reducing conditions. Consequently, the smoke is of different composition and so is the smoke flavor. An enrichment of low molecular weight hydrocarbons may occur, producing some danger of explosion.

D. APPLICATION OF SMOKE PREPARATIONS

The treatment of meat and meat batters with liquid smoke or flavoring is becoming more and more common. Smoke preparations are also used in fish and cheese production. Their application is not limited by some of the technological hindrances associated with smoking, i.e., they can be mixed with the meat batter or injected into meat pieces as a solution if surface treatment is not desired or impossible, as, for instance, during fabrication of sausages in sterile casings or cans.

These types of smoke preparations are represented by a great number of products. They may be classified as (1) liquid smoke preparations of smoke dissolved in water or oil or of smoke extracts in organic solvents and (2) solid smoke preparations in which smoke is adsorbed to spices, salt, dextrose, proteins, etc.

Sensory evaluations of products treated with these preparations range from unpleasant, nonspecific to intensely aromatic (Toth, 1982). This not only points to different methods of production but also indicates wider ranging differences in composition. Of phenols specifically, most preparations investigated by Toth (1982) show a similar qualitative but a quite different quantitative composition. The amount of phenols in several liquid smoke preparations were less than 30% of the amount found in other preparations. Potthast (1976), Sochtig (1979), and Berger and co-workers (1975) have published similar results. Liquid smoke


preparations are influenced by solvents. If smoke is condensed in pure water, tarry products and PAH are separated after storage of about 10 days. The watery solution desired for smoke flavoring and coloring is decanted. In general, it has a pleasant smoky flavor. Smoke condensed in water that contains solvents, e.g., ethyl alcohol or glycol, has an unpleasant tarry, bitter taste which imparts an off- flavor to foods. During storage of such liquid smokes no separation of tar or PAH is observed.

As mentioned above smoke preparations may be mixed with meat batter or other foods during processing. In addition, treatment of the surfaces of foods is possible by three different procedures: dipping them into a solution of smoke; processing them in an atmosphere of atomized liquid smoke; or stuffing them in casings impregnated with liquid smoke.

Meat products must be dried to a certain extent before liquid smoke application, because liquid smoke easily forms droplets on wet surfaces. This causes a nonuniform deposition of liquid smoke at the surface, which during drying leads to discolorations. Mixing of smoke preparations in meat batters or injection into pieces of meat produces a homogeneous distribution. It mainly influences smoke flavoring. Application of smoke to meat products by dipping or treatment in an atmosphere of atomized solutions causes high surface concentrations. This is advantageous for color development as well as for the inhibition of peroxidation and microorganisms. To produce good color, liquid smoke-treated meat products must be heated in a dry atmosphere. Under dry conditions chemical condensation reactions take place between carbonyls and amines, followed by the formation of the typical smoke color. The color intensity may be determined by the amount of smoke components at the surface and by the duration of the heat treatment. Heating smoked meat products in a dry atmosphere, however, affects surface tenderness. This is of special importance to meat products in natural casings, e.g., certain types of frankfurters. Because of more severe tanning, casings become tougher the longer they are heated in a dry atmosphere. Color development depends mainly on carbonyls in smoke and smoke preparations. Flavoring, on the other hand, is due to a great extent to the types and amounts of phenolic compounds as well as to the amount of acids. The more similar liquid smoke composition is to freshly developed smoke, the more comparable are the taste and flavor of the differently treated meat products (Toth, 1980b; Potthast, 1983). Along with phenols, the acid fraction is of great importance. Liquid smoke preparations show pH values of approximately 1.5-5.5. Treatment with the most acid liquid smokes imparts an unpleasant acid taste, even if the smoke preparation contains only phenols that are desired for a good flavor development.

Surface deposition of liquid smoke may be achieved by producing meat products within casings that have been coated inside with liquid smoke. During ripening at elevated temperature (between 50" and 75°C) a brown-colored surface


develops. Smoke constituents responsible for flavoring penetrate into the outer layers of the meat products. This procedure takes some time; maximum overall acceptance was achieved within 1-2 days.

IV. PRODUCTION OF SMOKE CONDENSATES AND SMOKE PREPARATIONS

A. CONDENSATION

Smoke condensation is required for several reasons, e.g., for the production of the smoke preparation used for flavoring foods. Also, total smoke condensation from smokehouses is desirable since it reduces the amount of smoke discharged into the environment. Finally smoke must be condensed for analytical investigation, etc. Separation of the smoke from transporting air may be done by electrical filtering equipment, by water traps, or by means of cooling systems. With most procedures, condensation is incomplete.

I . Laboratory Equipment for Smoke Condensation

An elaborate condensation system was described by Pettet and Lane (1940), who collected smoke by fractional cooling. The first step was to condense constituents with high boiling points at about 0°C. Traps were flushed with ice- cooled water. Further condensation was carried out between -20 to -80°C by adsorption of the remaining smoke on deep-frozen activated charcoal. Spanyar and co-workers (1960a) used simpler equipment consisting of a water cooler and washing flasks. Since then a variety of condensation devices have been built (Doerr et al., 1966; Fretheim et al . , 1980; Hamid and Saffle, 1965; Jahnsen, 1961; Johnson et al . , 1964; Lustre and Issenberg, 1969; Tilgner, 1970), but their application is inefficient for total smoke condensation since most volatile components pass the traps. For analytical measurements special methods have been used to separate fractions to be investigated. Phenols, for example, were trapped in alkaline solutions. Chemical reaction with 2.4-dinitrophenylhydrazine was recommended to collect aldehydes.

Issenberg et al. (1971), Potthast (1977a), and Tdth (1980a,b,c), among others, have pointed out that treatment of smoke during condensation or analytical investigation leads to a remarkable loss in phenols. T6th (1980a) therefore recommended equipment that is described in detail (which does not use alkaline solutions). His procedure of smoke condensation is based on the observation that smoke passing through glass fibers changes its physical properties to a certain extent and can easily be trapped in water after cooling by a water cooler. About 50% of the phenols are condensed within cooling traps. Almost total condensa-


tion of the rest is achieved by washing flasks that are connected by tubes filled with glass fibers. It seems obvious that adsorption and physical changes of the smoke particle phase caused by glass fibers make smoke condensation more efficient. Only a few volatile components, insoluble in water, pass through the apparatus; their odor is unpleasant and not at all as aromatic as smoke.

2 . Technical Equipment for Smoke Condensation

Most industrial equipment used in smoke condensation work is very similar to laboratory installations. In a Hollenbeck patent (1964) a method of smoke condensation in water was described for liquid smoke production. According to this patent, smoke from smoldering sawdust is cooled, and a separation of ash, tar, and soot takes place. Subsequently, volatile components of smoke are condensed in water. Miler (1970) favored smoke condensation within an electrostatic field where mainly the particle phase is separated. The preparation of liquid smoke from steam smoke generators (Unilever, 1978) may be done most effectively by cooling systems.

T6th (1980a) published a detailed description of a device that allows nearly total smoke condensation by passing smoke through a cyclone separator, a tube filled with glass fibers, and an open column containing small glass tubes and water. Ash, soot, and tar (representing the coarse particle phase) are deposited within the cyclone separator. Smaller particles are adsorbed by several layers of glass fiber or are split and remain with the gas phase from which the smoke constituents are washed with water. The system is connected to a vacuum pump that provides the system with a vacuum sufficient to force the water to remain in the open column and the smoke to pass through the column. Only a few smoke constituents are not condensed, and these are eliminated by suction. A further condensation would be possible by using a second column filled with a suitable organic solvent.

After switching off the vacuum pump the water is collected in a reservoir. It is a yellow to yellow-brown color depending on the amount of smoke constituents dissolved. The particle phase (tar products) from the cyclone separator as well as from the glass fiber filled tube may be cleaned with organic solvents or alkaline solutions, if chemical changes are not critical, e.g., if there is no analysis of the constituents planned. A cleaning device to reduce emissions from smokehouses operating in a similar manner was constructed by T6th and Potthast, and a patent is pending.

€3. PRODUCTION OF SMOKE PREPARATIONS

Smoke consists of water-soluble and -insoluble compounds and is divided into a tarry and an aqueous phase. If freshly developed smoke is condensed in water,


the solution is a bright yellow color. During storage it becomes increasingly dark with the formation of brown-colored condensation or polymerization products. These products, accompanied by PAH, precipitate from the aqueous solution. Precipitation may be prevented by the addition of acids (acetic or citric acid), organic solvents, or detergents (Tween or Span) (Rao and Shoup, 1978).

The production of smoke preparations is described in numerous patent specifi- cations; only a few have some importance for industrial use. The usual method of producing liquid smoke for curing meat is to pyrolyze hardwood by smoldering sawdust and to capture the wood smoke in water (e.g., Hollenbeck, 1964). The smoke condensation is carried out until a given concentration has been reached. The solution is then aged for a certain period of time to allow for polymerization and tar precipitation; afterward the aqueous solution is decanted for use in food treatment. Another method is to generate smoke, e.g., by overheated steam (Unilever, 1978), and to condense tarry fractions from water-soluble fractions in traps cooled to different temperatures. Only the water-soluble fractions are used for smoke flavoring. Miler (1970) recommends smoke condensation by electrical filter equipment in which the whole smoke fraction is deposited. This is separated into three fractions after it is dissolved in water (Fig. 2). A phenolic and an acid fraction are obtained by extraction with organic solvents at different pH values. Neutral compounds are collected by fraction distillation. From these three fractions smoke preparations are formed by mixing certain portions for special flavoring purposes.

A totally different procedure was patented by Moller (1970), who produced smoke preparations from smoked pig skin by extraction and/or steam distillation. These products differ in taste and flavor since they are formed to a certain extent by reactions of smoke and skin constituents. Also different in flavoring properties are some other preparations of smoked foods or food additives, e.g., spices, dextrose, starch, and salts. This is mainly due to the fact that easily volatilized substances are less strongly bound to the substrates and disappear during condensation and subsequent storage. Because it is difficult to isolate smoke-flavored salts by direct adsorption of freshly developed smoke, Henning (1973) recommended condensing smoke to carbohydrates in a first step, then extracting with water and mixing the aqueous solution with the salt followed by drying. Mixtures of pure chemicals with flavoring properties have been described (Spanyar et al . , 1965, 1966; Niinivaara and Luukkonen, 1970) for use as liquid smokes.

V. CHEMICAL COMPOSITION OF SMOKE AND SMOKED PRODUCTS

Curing smoke is generated by pyrolysis of plant materials, and consists of a mixture of constituents that are distributed in air in a gaseous, liquid, or solid state. Physical and chemical changes occur in the smoke between smoke genera-

CHEMICAL ASPECTS OF MEAT SMOKING

Smoke

105

Cooling, condensation, and filtration of smoke

condensate (a) aqueous, (b) tar phase

Adjust to pH 6.5-7.0 with about 5% NaHCO, solution liquid-liquid partition with ethyl acetate (3-4 times)

(tar phase)

/ Steam distillation at 170°C liquid-liquid partition between distillate and ethyl acetate

Phenol

tion and condensation on meat products or other foods. The most favorable conditions for chemical reactions are found in hot, freshly generated smoke where condensation, polymerization, esterification, etc. take place. This demon- strates that smoke may be affected qualitatively as well as quantitatively by treatment under different conditions.

The investigation of possible reactions is very difficult, as smoke is a very complex mixture of reactive constituents. Changes so far described are due mainly to oxidation processes (Tilgner et al., 1962b; Daun, 1966; Miler, 1962a). When analyzed experimentally, the composition of smoke reflects only the constituents of condensed smoke and not those of the smoke as it originates from the wood. There is some reason to believe that smoke constituents on meat surfaces undergo many more chemical changes than they do in the liquid state, for smoke

Filtration through Na$O, (sicc.) and concentration to dryness

extract

Column chromatography through TMS silica gel with 40% ethanol fraction: 0-200 ml

Puritied phenol fraction (PAH free)

106 L. TOTH AND K. POTTHAST

flavor development does not stop after smoke treatment but continues, particularly in the presence of oxygen. Oxygen and smoke do react with one another as well as with meat components, thus initiating typical color and flavor development (Draudt, 1963; Daun, 1979; T&h, 1982). A remarkable loss of some amino acids, e.g., lysine, was observed by Dvorak and Vognarova (1965) and Ziemba (1969a,b). Tanning of casings and the formation of a secondary skin of sausages point to reactions of proteins with smoke. Antioxidative, fungicidal, and bactericidal influences are also direct effects of smoke constituents.

In general, smoke components have been analyzed from condensates in liquids (water, alcohol, etc.); the minor constituents, e.g., some PAH and phenols, have been isolated from smoked meat products or other foods. From analyses of both liquid smoke or smoked products, conclusions concerning smoke generation may be drawn. Analytical results reveal relationships between smoke composition and the different types of wood and generation temperatures used. Phenol profiles change with different types of wood. Guaiacol and other phenols with one methoxy group are predominant in softwood smoke; syringol and phenols with two methoxy groups predominant in hardwood smoke (Toth, 1980b,c). In- creasing amounts of phenols are detected at generation temperatures of about 700°C. The PAH fraction shows an increasing linear temperature dependence between 400 and more than 1000°C. At these high generation temperatures, phenols become less and less abundant in curing smoke (Potthast, 1979, 1982a).

The first general survey on the composition of smoke was worked out by Pettet and Lane (1940) (Table 11). As the results indicate, the amounts of several

TABLE I1 COMPOSITION OF A SMOKE CONDENSATE"

Percentage of Fractions the whole condensate

Formaldehyde 0.12 Aldehydes of higher molecular weight 0.57 Ketones 0.67 Formic acid 0.38 Acetic acid and acids of higher molecular weight 1.71 Methanol 0.96 Tar 4.81 Phenols 0.07 Residue 4.21 Water 82.42

Total 95.92 Extracts from activated charcoal 4.08

" From Pettet and Lane (1940) cited in Draudt (1963). Reprinted from Food Technology 17, 85. Copyright 0 by Institute of Food Technologists.


TABLE 111

CHEMICAL COMPOSITION OF CURING SMOKE

FOR HOT SMOKE PROCESSINGa

Weight of fraction (mgim’)

Smoke from beech Smoke from oak

Isolated fractions Lowh Highb Low High

Ash and soot Ether-insoluble constituents Free bases Aldehydes and ketones Acids Phenols Neutral substances Unknown constituents

7.95 216.00 45.30 47.90 37.28 15.91

298.96 12.16

8.24 99.58

155.76 108.65 203.60 93.31

1757.02 95.56

21.77 154.16 41.96 24.75 64.32 8.97

477.60 21.90

23.50 757.40 48.01 70.95

327.50 73.27

803.64 45.24

Total 681.90 2522.74 815.43 2149.40

107

From Draudt (1963). Reprinted from Food Technology 17, 85. Copyright

Density of smoke analyzed. 0 by Institute of Food Technologists.

fractions of the smoke were determined as the percentage of the whole condensate. The weight of fractions in 1 m3 of smoke (Table 111) was used as a basis for calculation by Draudt (1963) in analyzing smoke of lower and higher density.

More precise information, e.g., smoke generation parameters or the amount of sawdust used for development of a certain volume of smoke, are missing. Thus, the results of Table I11 are not easily compared with other findings. Spanyar and co-workers (1960b) performed their calculations on the basis of the weight of smoke fractions gained from a certain amount of sawdust (Table IV).

The determination of every constituent within single fractions is quite difficult. This is one reason why investigators often prefer group-specific methods for analysis, e.g., total acids, total carbonyls, or total phenols. As most of these methods are quite nonspecific, several authors come to different conclusions even though using similar materials for their investigations. It is therefore not worthwhile to summarize all reported findings; rather the most important results of a few investigators are discussed.

Husaini and Cooper (1957) found that steam-volatile constituents, especially phenols, carbonyls, and acids, are the factors most responsible for the smoke flavoring of meat products. As these fractions are of different compositions, different flavors are to be expected. Table V shows the composition of steam- volatile and -nonvolatile fractions from smoldering sawdust and friction smoke that was condensed at -80°C. From these results an explanation of the development of a different taste is possible.


TABLE IV SMOKE COMPOUNDS FROM DIFFERENT SPECIES OF WOOD"

Amount of compound (gi100 g sawdust)

White Compound Acacia Beech Oak beech Spruce Pine

Monocarbonyls 1.70 1.77 2.40 Oxy- and dicarbonyls 1.86 3.60 1.98 Formaldehyde 0.11 0.15 0.14 Acetaldehyde 0.81 0.66 0.63 Acetone 0.64 0.74 0.37 Furfural 0.24 0.18 0.35 NaOH reducing substances 4.56 6.94 4.68 Reductants 0.13 0. I6 0.17 Deh ydroreductants 1.56 1.32 0.66 Total acids 2.94 4.06 3.56 PH 4.3 4.8 4.0 Esters 8.48 6.69 10.47 Phenols 0.36 0.14 0.16

2.50 3.06 3.10 4.95 6.30 6.12 0.16 0.21 0.20 0.65 1.15 I .oo 0.69 0.69 0.54 0.26 0.21 0.18 9.75 7.83 7.40 0.32 0.21 0.29 2.16 2.10 1.80 6.79 3.07 3.17 4.1 3.8 4.7

10.35 8.08 11.16 0.32 0.29 0.29

~

a From Spanyar et al. (1960b) with permission.

The isolation of smoke constituents from smoked meat products is much more difficult. A procedure applicable to the extraction of all smoke components has yet to be developed because of differences in solubility and chemical properties. Some of the components are lipophilic, some are hydrophilic, and some are bound to proteins. To analyze carbonyls, acids, and phenols from smoked sausage materials, distillation or liquid extraction methods are recommended. When

TABLE V COMPOSITION OF STEAM-VOLATILE AND -NONVOLATILE

CONSTITUENTS OF SMOLDERING SAWDUST AND FRICTION SMOKEa

Smoldering sawdust Friction smoke

Friction Volatile Nonvolatile Volatile Nonvolatile

Total acid (as acetic acid) 1.286 0.183 4.98 0.209 Carbonyls (as acetaldehyde) 0.627 0.164 4.78 2.79 Total phenols (as phenol) 0.068 0.007 0.217 0.067 Water content 86.7 70.6

a From Husaini and Cooper (1957). Reprinted from Food Technology 11, 499. Copyright 0 by Institute of Food Technologists.

Figures are percentage of total smoke.


TABLE VI PHENOL, CARBONYL, AND ACID

CONTENT OF SMOKED BOLOGNA

IN DIFFERENT LAYERS"sb

Layer Phenols Carbonyls

A (outside) 3.76 1.38 B 2.04 1.17 C 1.41 1.23 D 1.02 1.06 E 0.78 1.09 F 0.43 1.22 G 0.26 1.24 H (inside) 0.12 1.05

Acids

4.41 5.48 5.51 6.58 7.01 6.86 6.37 8.72

a From Bratzler et al. (1969). Reprinted from Journal of Food Science 34, 146. Copyright 0 by Institute of Food Technologists.

Content of each layer given in milligrams per 100 g of smoked meat. Distance from layer A to layer G was 1.6 mm.

analyzing bologna, Bratzler and co-workers (1969) used water and alcohol as an eluent. Carbonyls were determined as their 2,4-dinitrophenylhydrazones, phenols with 2,6-trichloro-p-benzoquinonimine by photometry, and acids by titra- tion. Discussion of results encompasses sensory evaluation as well as penetration properties of smoke fractions. As is pointed out in Table VI, phenols are concentrated mainly in the outer layers, carbonyls are distributed homogeneously throughout the product, while somewhat higher concentrations of acids are found in the center of the bologna.

Smoke flavor was correlated to the phenol fraction. Carbonyls and acids had less influence on flavor.

A. ANALYTICAL METHODS FOR THE INVESTIGATION OF SMOKE AND SMOKED PRODUCTS

A comprehensive review on analytical research was published by Hamm (1977a,b). We will therefore refer only to some publications that have special bearing on the following topics.

Analytical investigations of smoke may be conducted with condensates in water, organic solvents, or water-organic solvent mixtures. Depending on the solvent employed, liquid smoke composition is very different. Smoke in pure water separates into aqueous and tarry phases as mentioned above. Organic solvents or water-containing organic solvents do not produce separation at all.


even if the products are stored in solution for a long period of time, which is one reason that published findings are difficult to compare. Another reason is that smoke generation conditions show great variation from experiment to experiment. It is well known that smoke composition is dependent on smoke generation temperature, water content, and kind of wood (Potthast, 1979, 1982a; T6th, 1980b). Furthermore, advances in analytical chemistry in the last few years have given a much more detailed insight into smoke as a whole. Capillary gas chromatography (GC), sometimes in connection with mass spectrometry (GC-MS), has outdated earlier findings.

Gas chromatography has been used for analytical and preparative investigations. Doerr and co-workers (1966) ran smoke fractionation by preparative GC with packed Carbowax 20 M columns, applying total smoke condensates without any previous purification. Fractions were collected and analyzed by optical methods, e.g., infrared (IR) spectrometry. For qualitative and quantitative purposes, isolation of smoke constituents from the aqueous solution prior to GC proved to be advantageous. Extracts from liquid-liquid partition were subjected to GC on columns of different polarity. In such an extract Fiddler and co-workers (1970) detected about 50 compounds. Analytical methods have further improved as more smoke constituents are identified. Hruza and co-workers ( 1 974) used steam distillation to separate volatile and nonvolatile fractions. Acids have been isolated from these fractions and determined to be methyl esters.

In addition, ether-extractable compounds have been fractionated by preparative GC. Thirteen fractions were analyzed by GC-MS. Eighty compounds including acids, PAH, esters, furanes, aldehydes, ketones, and phenols have been identified, about half of them for the first time. Fujimaki et al. (1974) and Kim et al. (1974) were successful in isolating basic, acid, phenolic, and carbonyl compounds by extracting liquid smoke of differing pH. Gas chromatography was carried out with packed Carbowax 20 M columns. Separated compounds were identified by MS, IR, or nuclear magnetic resonance (NMR) spectrometry. One hundred twenty-six compounds have been detected.

T6th (1982) described a similar procedure for the isolation and fractionation of phenolic constituents from smoke condensates, as illustrated in Fig. 3. Smoke condensation is accomplished by cooling and adsorption in water. The liquid smoke is separated into an aqueous (watery) and tarry phase by filtration. Both phases have been used for further investigations, with the tar being dissolved in methanol.

In liquid-liquid partition, ethyl acetate proved to be an excellent solvent for separating phenolic constituents from the aqueous phases. The extraction was carried out at pH 6.5-6.7 since phenols are not degraded at these pH values. After the evaporation of ethyl acetate, an extract with a pleasant smoke flavor is left. This extract is further purified by steam distillation from a solution of 30% LiCl in water (Potthast, 1977a). Of these steam-volatile components, about 90%

Condensate (from the electrostatic f i l ter equipment)

Aqueous phase T a r phase

+ 2% KOH to pH 10 .5 (extraction with ethyl acetate)

Aqueous phase Organic phase

+ 2% KOH to pH 12.8

Organic phase

(neutral compounds)

to be concentrated

distillation

I

to pH 5 (extraction with ethyl acetate)

Aqueous phase Aqueous phase

(extraction with (extraction with ethyl acetate)

Organic phase to be discarded

I

I I Aqueous phase Organic phase Aqueous phase Organic phase (at 10 torr to 2 2 0 0 ~ ) to be discarded

to be concentrated centrated

to be discarded to be con- distillate

I

I I -1 (acids) (phenols) (neutral compounds)

Smoke preparation Dissolved in fraction 1 : fraction 2 : fraction 3

fat 1 0.40 0.20 butter 1 0.25 0.05

salts covered on 1 0 0

FIG. 3 . Scheme for the production of smoke preparations. From Miler (1970).


are phenols. By column chromatography on silanized silica gel, a “purified extract” is achieved that is free of PAH. It contains several hundred phenolic compounds including mono-, di-, and trihydroxyphenols and phenol acids, which are detected by capillary GC-MS (Toth, 1980b; Wittkowski, 1984). “Purified phenol extracts’ ’ have been prepared from smoke preparations as well as from freshly generated smoke in amounts sufficient to alIow toxicological evaluations.

Some PAH have proven to be carcinogenic in animal experiments. Numerous authors have reported that they are found in several foods, especially in smoked meats. In most of the experiments the amount is within a range of several micrograms per kilogram. Numerous methods have been published for the analysis of PAH; best known are the Howard (Howard et al., 1966; Howard, 1979) and the Grimmer (Grimmer and Bohnke, 1975; Grimmer et al., 1981) methods by which PAH are separated from food by liquid-liquid partition using polar and nonpolar organic solvents. Further cleanup is carried out by adsorption or gel chromatography in columns; determination of PAH is accomplished by thin- layer (TLC) or gas chromatography. Gertz (1978, 1981) and Sagredos and Sin- ha-Roy (1979) recommended solutions of caffeine in acetic acid for PAH separation and column chromatography for purification of extracts.

Extraction of PAH from foods with propylene carbonate is favored by Potthast (1975, 1982b). With this method, foods are treated with an organic solvent (e.g., chloroform or methyl ethyl ketone) and mixed with sodium sulfate and Celite to produce a uniform partition of fat. During drying in a vacuum oven (40°C) the formation of thin layers of fat on Celite occurs, from which PAH are eluted by propylene carbonate. Saponification with NaOH and elution with petroleum ether leads to fairly clean extracts that may be analyzed by TLC, GC, or high- pressure liquid chromatography (HPLC). Extracts of insufficient purity are further cleaned by preparative TLC on silanized silica gel. PAH gather in a narrow zone and are sucked off by means of a spot collector. These small amounts of silanized silica gel are eluted with less than 1 ml of acetone. By separating the acetone extract using GC-MS, 78 PAH have been detected in smoke preparations and 42 in smoked meat products.

B. COMPOUNDS IDENTIFIED FROM SMOKE

Estimates of the number of smoke compounds range from about one thousand to several thousands. According to Tilgner (1977) 10,000 different compounds are possible, but only about 500 are supposedly responsible for smoke flavor. Several hundred have actually been detected in smoke. The number analyzed from smoked meat is far less. A survey on detected smoke compounds has been published by Mohler (1978) and Tdth (1982), which includes literature sources. The compounds described in detail belong to several chemical groups (Table


VII). Most of the described compounds in the table were determined by means of GC-MS with separation on capillary columns. A few of them were separated with different chromatographic methods that are less specific. Therefore, there is some question of the preciseness of earlier findings, especially as far as quan- tifications are concerned.

TABLE VII NUMBER OF IDENTIFIED COMPOUNDS

IN CURING SMOKE CLASSIFIED ACCORDING TO CHEMICAL GROUP

In In smoke smoked

Chemical group condensates foods

Hydrocarbons Aliphatic Aromatic PAH

Alcohols Aliphatic Aromatic

Aliphatic Aromatic

Cetoalcohols Aldehydes

Aliphatic Aromatic

Ketones

Aldols Ketols Phenols

Monohydroxy Dihydroxy Pol yhydroxy Aldehydes, ketones

Aliphatic, mono Aliphatic, di and keto Aromatic and phenolic

Aliphatic Aromatic

Aromatic

0-Heteroc yclics N-Heteroc yclics

Acids

Esters

Ethers

Alicyclics

1 10 47

8 2

17 I 5

13 2 1 1

ii 14 } 85

19

18 8 7

5 2

4 23 36

8 Total 295

-

I14 L. TOTH AND K . POTTHAST

I. Aliphatic Compounds

In curing smoke and smoked meat products 77 aliphatic compounds have been detected. The main groups are hydrocarbons, alcohols, aldehydes, ketones, and acids. The maximum number of C atoms in these compounds is five, with the exception of some acids. They originate from pyrolysis of cellulose, hemicellulose, and lignin.

Hydrocarbons in curing smoke are of less importance. Only methane has been detected in hickory smoke (Fiddler et al., 1967), but methane together with CO, and CO were major components in the dry distillation of wood (Nikitin, 1955). The presence of additional aliphatic hydrocarbons in curing smoke is probable. Findings of Drawert and Beck (1974) are surprising since they suggest the presence of hydrocarbons of higher molecular weight, which have never before been found in smoke. They described paraffins and olefins with 9- I8 C atoms detected in smoked ham, which they believed to come from curing smoke. Thus the group of aliphatic hydrocarbons is apparently the only one from which single compounds are determined in a higher number in smoked meat products than in smoke itself.

Some publications indicate the presence of eight alcohols in curing smoke, but only methanol, ethanol, allylalcohol, and n-amylalcohol have been detected by GC-MS analysis. Methanol has been found up to an amount of 700 mgilOO g sawdust. It originates mainly from methoxy groups such as those from lignin. When oxidized it forms formaldehyde or formic acid. While methanol is less desirable because of toxicological considerations, the other alcohols are bene- ficial flavoring constituents.

The carbonyls contribute to smoke color development, by carbonyl-amine reactions, as well as to flavor development. Thirteen aldehydes, 17 ketones, glycolaldehyde, and methylgloxyal have been detected. Major components of curing smoke are acetaldehyde (1 150 mg/ 100 g wood) and formaldehyde (200 mgi 100 g wood). Formaldehyde, a preservation agent, has been detected in amounts up to 50 mg/kg in smoked meat products. It is believed to be mainly involved in the tanning effects of natural casings as well as in the formation of a secondary skin. As formaldehyde has been proven toxic in animal experiments, residual concentrations in smoked foods should be as low as possible. Other than formaldehyde, only acetaldehyde was detected in smoked meats. Aldehydes, especially unsaturated ones, are supposedly bound to food constituents since they tend to form condensation products. The same is true for alcohols (glyoxal) and ketoaldehydes (pyroracemic aldehyde). Ketones are quite reactive as well. Sen- sory evaluations have shown that ketones themselves possess flavoring properties and are involved in the formation of aromatic reaction products with food constituents. The presence of acetone in smoked meat products has not yet been confirmed, though it has been detected in casings. Acetone has been detected in


curing smoke in quantities up to 740 mg/100 g wood. Ketoalcohols such as hydroxybutanone and hydroxypentanone probably act as aliphatic intermediates during formation of hydroxyfuranes (heterocyclics) and cyclopentanones.

Analysis of smoke reveals 18 mono-, 5 di-, and 3 ketocarboxylic acids. While most of these acids have been found by several researchers, only Langner (1966) described the 5 dicarboxylic acids, namely oxalic, malonic, fumaric, maleic, and succinic acid as constituents of smoke preparations.

The pH values measured in liquid smoke preparations range from about 2 to 4. The preparations sometimes contain great amounts of carboxylic acids (6-7 g/lOO g wood, as calculated to acetic acid). The main compound is acetic acid (3.7 g/100 g wood), followed by formic acid (0.8 g/100 g wood). Acetic, butyric, and valeric acids show special aromatizing characteristics. The carboxylic acid fraction has a strong influence on preservation in addition to flavoring properties. In spite of their quantity in smoke, single carboxylic acids have not been definitely determined to be in smoked foods. The previously mentioned determination of carboxylic acid concentration in sausages by Bratzler and co- workers (1969) was just of the total acid fraction.

The presence of some esters in liquid smoke preparations has been confirmed. i.e., the methyl esters of formic, acetic, butyric, and acrylic acids, but no other esters have been detected, and quantitative measurements have not been made. As esters in general are constituents with high flavoring properties, more knowledge would be of interest.

2 . Heterocyclic Compounds

The formation of cyclic compounds is due mainly to pyrolysis of cellulose and hemicellulose. Glucose and pentosans occur as intermediate products (Nikitin, 1955).

Of heterocyclic compounds 0-heterocyclics are the major fraction. They are derived from the following substances.

Butyrolactone 2 - Butenolide Furane

R = CH,; COCH, Position of =O :

R = CH,; CH,CH,; R = CH,; CH,CH3; CH= CH, CH,(CH,),; CH,OH;

CHO; COCH,; COCH,CH,; OCH, 2-one

3-nne

(4 derivatives) (10 derivatives) (16 derivatives)


Butyrolactciles and butenolides originate from corresponding hydroxyacids. y- Butyrolactone is formed from y-hydroxybutyric acid by dehydration:

H,C- CH, I I -H,O H,C-CH,

I Hz7 F=O - H A I o,c+o HO OH

y -Hydroxbutyric y -Butyrolac tone acid

Formation of 2-butenolide is due to a dehydration of y-crotonic acid. Butyrolac- tones and butenolides are seen as intramolecular esters converting aliphatic to aromatic compounds. The flavoring properties of butenolides were characterized by Kim and co-workers (1974) as sweet and caramel-like; smoky aromatic, vanilla-like flavor; burning smell. The presence of 5-hydroxymethyl-Zfuralde- hyde (5-oxymethylfurfurol) in curing smoke points to glucose as an intermediate of cellulose degradation, and is responsible for furane formation. It is formed mainly by dehydration from glucose (Nikitin, 1955):

H H I I

HO-C -C-OH - 3 H,O HC-CH I I ,o - II II

H,C-CH HC-C< H,c,c'o/c' 00 1 1 I H c:H HO OH OH I

HO

Glucose 5-Hydroxymethyl- 2-furaldehyde

Compounds containing an aldehyde and in alcohol group show an easy splitting of the CH,OH group; consequent formation of oxymethylfurfurol, formaldehyde, and furfurol occurs. Furfurol is mainly formed from pentoses, which are degradation products of hemicellulose.

HO-b-k-OH - 3 H,O HC- CH I 1 - II I I

I I H OH OH

H-CH HC-CfO HC,O,C, 50

ch Pentose Furfuraldehyde

(furfurol)

Furfurol tends to condense as do aliphatic aldehydes. Its flavor is sweet, roast meat-, and carmel-like. The homologs of furfurol are described as sweet, good smelling, fruity, and grassy (Kim et af., 1974; Radecki et af., 1976). Conse- quently the furfurols have to be considered important compounds in flavor development. Detection and quantification in smoked foods have yet to be done.

In addition to furfurols, furane and some homologs are found:


R = CH,; CH,CHs; CHz(CHs)z; COCH,; COCH,CH,; OCH,; CH,OH

Furane

(12 derivatives)

Only one six-ring 0-heterocyclic compounds was detected in curing smoke, maltol:

Maltol

Because of the phenolic hydroxyl group, the quantity of maltol could be analyzed in phenol fractions. In smoked meat products it was present in very small amounts (22 kg/kg); therefore, flavoring and antioxidative influence should be of minor importance. Nevertheless maltol is a typical constituent of roasted foods.

The number of N-heterocyclic compounds is low because of the low protein content of wood. Three types of N-containing compounds have been identified:

Pyrrole Pyrazine Carbazole

VR c>R a R

R = CHO; COCH,; R = CH, R = CH, COCH,CH,

(3 derivatives) (3 derivatives) (2 derivatives)

In solutions, pyrrole and pyrazine tend to form dark-colored reaction products by polymerization. Their involvement in smoke color development has not yet been explored. Pyrroles have a sweet, smoky flavor and a smell of burning, and pyrazines have a popcorn-like flavor. All are typical smoke flavor characteristics. Carbazole belongs to heterocyclic PAH compounds, which are found less often in smoke and smoked foods.

3. Alicyclic Compounds

In smoke several alicyclic compounds with five or six C atoms have been detected. Most of them are derivatives of 2-cyclopentanone and cyclopentan-


dione. Cyclopentanones, cyclohexanone, and cyclohex-2-enone may arise from corresponding dicarboxylic acids by decarboxylation and ring formation. Cyclo- pentanones have a grassy, sweetly bitter flavor; cyclopentadiones have a camel- like flavor and a burning odor.

2-Cyclopentanone Cyclopentadione

0

R = CH,; CH,CH,; R = CH,; CH,CH, CH,CH,CH,

(15 derivatives) (4 derivatives)

4. Aromatic Compounds

a. Phenols. Phenols are aromatic hydrocarbons with one or more hydroxy groups directly connected to the benzene ring. They are designated as mono-, di-, tri-, or polyhydroxyphenols, depending on the number of hydroxy groups. When dissolved in water they show an acid pH value. In alkaline solutions phenols become soluble by dissociation as phenolates. They are sensitive to light and to oxygen and form complexes with metals. Pyrocatechol, resorcinol, and pyrogallol, phenols with several hydroxy groups, are most sensitive. The boiling point of phenols is 183°C. A second hydroxy group increases the boiling point by about 100°C. Resorcinol, for example, boils at 270°C.

Phenol ethers are stable in alkaline solutions and are insoluble in water. Some of them are fragrant. At low temperatures they are converted to phenol ketones. Condensation products occur by reaction with aldehydes. Ortho, meta, and para homologs, e.g., o-, m-, and p-methylphenols, exist and are differentiated by the position of the substituent. Chemical and physical properties of phenols may be further influenced by functional groups derived from alcohols, aldehydes, ketones, and acids.

Phenols are classified as monohydroxyphenols, dihydroxyphenols, trihydroxy- and polyhydroxyphenols, and phenolalcohols, -aldehydes, and -ketones.

Monohydroxyphenols. Only four of a great number of possible monohydroxyphenols have been described. These are phenol and o-, m-, and p-cresol.

Phenol o - C resol m-Cresol


The presence of dimethylphenols in curing smoke has been confirmed, but in spite of the determination of their molecular weight, their structure has not yet been elaborated because of the great number of possible isomers with the same molecular weight. The same is true for the trimethylphenols.

Ethyl-, propyl-, and allylphenols, members of the group of homologous compounds with longer side chains, have been detected. The presence of isopropyl-, methyl-, ethyl-, diethyl-, and 2,6-isobutylphenol seems to be unrealistic according to the investigations of Wittkowski and co-workers (1981).

Dihydroxyphenols. The unsubstituted dihydroxyphenols pyrocatechol, resorcinol, and hydrochinone-as well as methyl and ethyl homolog compounds-are present in curing smoke (Toth, 1980b; Wittkowski et al . , 1981). 3-, 4-Methyl- and 3-, 4-ethylpyrocatechol may be analyzed from smoke in relatively high amounts. The most predominant compound is guaiacol, a methyl ether of pyrocatechol. Guaiacol derivatives substituted in the 4 position, the 4- methyl-, 4-ethyl-, 4-vinyl-, 4-propyl-, 4-propenyl-, and 4-allylguaiacols, were detected, too.

Pyrocatechol Guaiac 01

OH

R = CH,; CH,CH,; CH,CH,CH,

Trihydroxyphenols. The symmetric trihydroxyphenol pyrogallol was first recognized by Jahnsen (1961) to be a constituent of curing smoke. Since then some derivatives have been detected. The major compound is syringol, a 1,3- dimethylether of pyrogallol. Toth (1980b) and Wittkowski and co-workers (1981) found 4-methyl, 4-ethyl, 4-viny1, 4-propyl, 4-propenyl, and 4-ally1 derivatives of syringol to be present in curing smoke. Derivatives of 3-methoxypyrocatechol were mentioned as well.

Syringol

Phenolic Compounds with Additional Functional Groups. Phenol alcohols, aldehydes, ketones, acids, and esters are compounds with additional functional groups. From these compounds, numerous derivatives were identified by GC-


MS analysis by Wittkowski et al. (1981), after separation of phenol extracts from smoke using the procedure described by Ishiguro et al. (1976). During liquid-liquid partition between borate-containing aqueous solutions and eth- ylacetate, dihydroxyphenols, as well as the above-mentioned phenols with additional functional groups, remain in the aqueous phase while phenols without these functional groups are separated with the organic solvent. Nineteen of these compounds have been described (R. Wittkowski, personal communication).

Salicylaldehyde, 4-hydroxybenzaldehyde, 4-hydroxyphenone, and 4-hydroxybenzoic acid methyl ester belong to the group of monohydroxyphenolic compounds:

OH I

R = CHO; COCH,; COOCH,

Nine compounds are known of the dihydroxyphenols with additional functional groups

OH I

R2

R' = H; CH,; CH,CH,

RZ = CH=CHCH20H; CH,CH=CH,OH ; CH,CH,CH,OH ; CHO; CH=CHCHO; COCH,; COCH,CH,; COOCH,; CH,COOCH,

Of these substances only vanillin and acetovanillone have been described. The following derivatives of syringol have been detected:

R

R = CHO; CH=CHCHO; CH,CH,CHO; COCH,; COCH,CH,; COOCH,


To date, only syringaldehyde and acetosyringone have been mentioned in the literature.

Formation of Phenols. Phenols originate mainly from the pyrolysis of lignin, which accounts for about 25% of the composition of wood. Lignin is a macromolecule consisting of phenol units having three-carbon side chains. Lig- nin from softwoods includes fewer methoxy groups than lignin from hardwoods. As is shown in Fig. 4, destruction of lignin during pyrolysis leads to the formation of guaiacol derivatives, e.g., ferulic acid, or to syringol derivatives, e.g., sinapinic acid. Thermal decomposition of ferulic acid in an oxygen-free as well as an oxygen-containing atmosphere was carried out by Fiddler et al. (1967). Results indicated that vinyl guaiacol formation is highest at 280"C, independent of the atmosphere. At 380°C only half that amount was produced. Furthermore, formation of guaiacol and the 4-ethyl and 4-methyl derivatives was observed in both atmospheres. Thermal decomposition in air caused an additional formation of cis-isoeugenol, vanillin, acetovanillone, and an unidentified compound that was determined to be C,oH,,02 (mle = 166) by GC-MS analysis. Kossa (1976) considered it to be 4-isopropylguaiacol. Vanillinic acid has been determined as well by use of TLC.

That the presence of oxygen leads to the formation of a greater number of phenolic compounds has also been confirmed by Kossa (1976), who investigated thermal decomposition with and without oxygen at 200°C using cinnamic, p- cumaric, ferulic, and sinapinic acids. Pyrolysis in air provided numerous oxidized aromatics that are known for their special flavoring properties. All of these compounds from thermal decomposition of phenolic acids have also been detected in pyrolysates of lignin and wood (curing smoke) (Block, 1979; Goryaev et al., 1976; Kratzl et al., 1965). Kratzl et al., as well as Block, observed only syringol and a formation of syringol derivatives from lignin of beech wood. This indicates that the composition of the phenol fraction obviously depends on the kind of wood used for pyrolysis. This has also been confirmed by Stahl et al. (1973) and Fujimaki et al. (1974), who carried out experiments with different kinds of wood. According to T6th (1982), syringol and syringol derivatives, as well as guaiacol and guaiacol derivatives, may be found in both hardwood and softwood, but in differing amounts. In smoke from hardwood syringol is predominant; in smoke from softwood guaiacol prevails.

During pyrolysis of ferulic or sinapinic acid in the presence of oxygen, vinylguaiacol is formed as the main compound, accounting for about 80% of the compounds produced. This differs from the pyrolysis of wood, where vinylgua-

Lignin -

HO

H,C-0

Hardwood

p 3

Softwood

H,C-0

Ferulic acid Sinapic acid

1.. H,C-0

Vinylguaiacol

I

Vanillin

Vanillinic acid

H,C-0

i'" 7 3 3

0

H,C- 0

Vinylsy ringol

Sy ringaldehyde

1 J

Syringic acid

CH' 0

H,C-0 HO&

Guaiacol Syringol

FIG. 4. Pyrolysis of lignin


iacol is of minor importance. Obviously, other constituents of wood exert some influence on its thermal decomposition.

The presence of phenols in curing smoke is not only related to lignin. Nikitin (1955) analyzed phenols and cresols from dry distillation products of cellulose. Phenols from cellulose pyrolysis are believed to originate from the conversion of aliphatic compounds to aromatic compounds in the presence of high temperatures. During high frequency pyrolysis at 700°C, Baltes and Schmahl (1978) measured guaiacol and phenol in high amounts and 0-, m-, p-cresol, and salicylaldehyde in only trace amounts. Amylopectin, a constituent of hemicellulose, produced phenol and the three cresols, too. These results show that all of the constituents of the wood participate in the formation of phenolic compounds during generation of curing smoke.

Quantitative Aspects. The phenol content of smoke preparations or smoked foods is probably the most important factor in the development of smoke flavor. Since the determination of total phenol content from single compounds is difficult, it has been analyzed as one fraction by simple colorimetric reactions. Tilgner and co-workers (1962a) pointed out that such methods lead to erroneous results and that extraction followed by gravimetric measurement proved to be more precise. Svobodova et al. (1977a,b) found that color development is dependent on substitution of phenols in different positions. Common reagents such as 4-aminoantipyrene (Foster and Simpson, 1961; Husaini and Cooper, 1957; Tilgner et al., 1962a) or 2,6-dichlorchinon-4-chlorimide (Bratzler et a l . , 1969; Gorbatov et al., 1971) react only with phenols unsubstituted in the ortho or para position. Other reagents such as N,N-dimethyl-p-phenylenediamine (Kramer and Tolentino, 1971), 3-methylbenzthiazolinon-2-hydrazone (Gasparic et al., 1977), and diazotized sulfanilic acid (Sochtig, 1979) show the same properties. It should be mentioned that Sochtig examined mainly phenols with one hydroxy group, so that variations on these findings are possible. Phenol analysis as well as phenol extraction from smoke has been carried out differently by different authors. Furthermore, the influence of smoke generation parameters has often been neglected, making a comparison of results difficult.

Phenol Content of Curing Smoke. During condensation of smoke, chemical reactions and changes in the composition of smoke occur. Therefore, the determination of the original composition of smoke is impossible. To what degree the phenol fraction is involved in these reactions is not yet known. The phenol content of smoke is related to a certain volume of smoke or to the amount of wood used for smoke generation. Depending on the density, phenol contents range from 10 to 200 mg/m3 of smoke. Findings related to the amount of wood show values of 50-5000 mg/ 100 g of wood. These considerable differences can be explained mainly by smoke generation parameters, e.g., smoke generation


temperature. That smoke density varies with the generation temperature is well known, but the optimum temperature for high density smoke generation is debat- able. Tilgner et al. (1962b) found an exponential increase in phenol formation between 300 and 400°C. Fenner and Lephardt (1981) reported 380°C to be the generation temperature resulting in the highest phenol formation. The findings of Fretheim et al. (1980), however, indicate a slight decrease between 350 and 500°C. The investigations of Potthast (1976) demonstrate that highest phenol concentrations develop at generation temperatures from about 600 to 700°C. These temperatures have been confirmed by Toth (1980b,c) for phenol content. For single phenols, T6th’s findings are more precise, as they are the result of investigations carried out with optimal analytical equipment. From Fig. 5 one can see that the highest concentration of phenols are found at generation temperatures of -650°C. With increasing temperature most phenols decrease dras- tically, with the exception of some phenols with two unsubstituted hydroxy groups. Pyrocatechol, for example, seems to be stable to about 900°C.

T6th (1982) supposed that maximal phenol development occurs at about 380”C, which he believes to be the temperature of pyrolysis, and that pyrolysis is followed by an increase in temperature forced by the ignition of intermediately processed charcoal from smoldering wood. If this holds true, there is no explana-

LOO 500 GOO 700 800 O C

FIG. 5 . smoke. Prom Tdth (1980a).

Influence of smoldering temperature on the amount of phenolic constituents in beech


tion for the occurrence of the highest phenol concentration in curing smoke generated at about 65OoC, because smoke generation at 400"C, where the temperature of pyrolysis is already achieved, leads 'to significant lower phenol yeilds. Furthermore, the formation of charcoal seems to be questionable, since the smoldering of sawdust occurs in presence of oxygen, and the reduction of cellulose, hemicellulose, and lignin cannot take place. The amount of phenol produced from the generation of different woods varies considerably. From curing smoke generated at 600°C by smoldering sawdust from beech, T6th (1980~) measured 2.5 g phenol/ 100 g wood; softwoods yielded between 3 and 4 g of phenols/100 g wood.

Simpson and Campell (1962) investigated the contribution of phenols in the gas and particulate phase. They found that guaiacol derivatives are present mainly in the gas phase and pyrogallol and syringol derivatives in the particle phase. On the other hand, Kornreich and Issenberg (1972) detected only 10% of the total phenol and total guaiacol, 5% of 4-methylguaiacol, and 2% of syringol in the gas phase.

Potthast (1976) measured the phenol content in smoke in relation to the amount of wood used for smoke development at different temperatures. From these investigations a marked temperature dependence of phenol formation has been shown. T6th (1980~) confirmed this temperature dependence, but disagreed with findings concerning single phenolic compounds such as syringaldehyde, acetosyringone, and propiosyringone. As main constituents, T6th determined syringol, guaiacol, phenol, and pyrocatechol as well as their alkylated derivatives, as shown in Table VIII. The amounts shown for single phenols are derived from smoke that was generated from beech wood at 600°C. As one can see from Fig. 5, smoldering temperature not only influences the total amount of phenolic compounds, but also leads to different proportions of individual phenols. Syr- ingol from smoke generated between 450 and 850°C is the most predominant phenol, but the relative amount of pyrocatechol increases with temperatures exceeding 650°C. According to T6th (19804, these findings probably allow conclusions to be drawn about generation temperature used in the smoking procedure.

As pointed out before, the type of wood has some influence on smoke composition since it is responsible for a major part of the formation of guaiacol or syringol derivatives. But obviously, different smoke generation conditions may also cause changes in phenol profiles. Thermal decomposition of wood normally gives syringol or guaiacol and related derivatives as main constituents. In smoke from a friction generator these compounds are of minor concentration, but new, as yet unidentified phenols occur. Since friction smoke is generated at temperatures lower than 400°C, pyrolysis takes a different course. Consequently a differentiation of products smoked with a friction generator from those smoked with smoldering wood or steam smoke seems possible (Fig. 6).

126 L. TOTH AND K . POTTHAS?

TABLE VIII IMPORTANT PHENOLS OF A PHENOL FRACTION

FROM SMOLDERING BEECH WOODU

Substance

Weight of the fraction

(%I

Phenol o-Cresol m-Cresol p-Cresol (2,4-Xylenol) 3-Ethylphenol (3,5-xylenol) 2,6-Dimethylphenol 4-Ethylphenol ] 2,3-Xylenol (m - 184) Guaiacol Pyrocatechol (M = 208 + ?) Trimethylphenol + (maltol) (Allylphenol + 208 + 196) (Methylguaiacol) (4-Methylguaiacol) 4-Methylpyrocatechol (3-Methylpyrocatechol) (4-Ethylguaiacol) Syringol 4-Ethylresorcinol + (4-vinylguaiac 3-Methoxypyrocatechol Eugenol (4-Methylsyringol) cis-Isoeugenol (4-Ethylsyringol) trans-Isoeugenol Vanillin (4-Vinylsyringol) 4-Allylsyringol Acetovanillone (cis-4-Propenylsyringol) (trans-4-Propen yls yringol) S yringaldehyde Acetosyringone

1.95 0.86 0.77 0.53

(0.3) 0.16 0.22

0.21

(0.3) 9.06 1.85

(0.4) trace

(0.4) (0.3) 3.0 0.79 (0.9) (1.2) 9.42

:ol) 0.72 0.96 1.31

(8.0) 1.5

(3.0)

0.42 (1.5)

I .68 0.39 (0.5) (1.5) trace trace

Compounds were identified by retention times and GC-MS after separation on glass capillaries Coated with OV-17.


t

FIG. 6. A comparison of the composition of phenols in beech wood smoke from smoldering sawdust and from a friction smoke generator. Open bars, minimum, and hatched area, maximum, of smoke from smoldering wood (450°C). Line bars show minimum and maximum of friction (steam) smoke (380°C).

Different types of woods not only form different amounts of phenols but also influence the phenol profile. Fujimaki et al. (1974) reported that syringol and its derivatives are absent in smoke from softwood. T6th (1980b), on the other hand, also found these compounds in smoke from hardwood. Analyzing phenol fractions from smoke or smoldering sawdust from beech and oak (hardwoods) and from spruce, fir, and pine (softwoods), he found that syringol and its derivatives are predominant in hardwood (total 15-30%), while guaiacol, methyl-, ethyl-, vinyl-, allyl-, and propenylguaiacol account for 20-35% of the total phenols in softwood (Fig. 7). Unpublished findings by the same author report smoke from hickory wood containing a phenolic fraction similar to the above mentioned hardwoods. "Wood" from bamboo (8-14% phenol and 8% cresol), on the other hand, produce smoke of quite a different composition.

Phenol Content of Liquid Smoke Preparations. Numerous results have been published concerning the amount of phenols in liquid smoke preparations. Phe- nol fractions have been analyzed by colorimetric, gravimetric, or gas chromatographic (GC) methods. From GC investigations, Potthast ( 1976) determined


Relative percentage in fraction 15 10 5 0

I I I 0 5 10

I I 1

4-rnethylguaiacol

hardwood softwood

FIG. 7. columns show variations within kinds of hard- and softwood).

Influence of the kind of wood on the amount of different phenols (hatched parts of the

the phenol fraction to be between 4.0 and 219 glliter combining the amounts from known and unknown phenols. Sochtig (1979) found a total of 0.19-28.6 glkg by adding known phenols. According to Knowles and co-workers (1975a) a liquid smoke preparation, produced as described by Miler (1970), consisted of 90% phenols. Berger et al. (1975), Potthast (1976), and Sochtig (1979) published quantitative measurements of single phenolic compounds that demonstrate that liquid smoke preparations differ in total amount as well as in comparative amount of phenols. The majority of preparations included syringol and guaiacol as primary constituents. In some preparations 4-methylguaiacol was predominant, in others phenol or cresol.

The most comprehensive study so far published was presented by Toth (1982). As Table IX shows, total phenol contents ranged from 0.5 to 23.5%, relative to the weight of liquid smoke. From solid smoke preparations (salt, dextrose, etc.) Toth separated only 0.003-1% phenols (Table X). This may be mainly explained by the fact that (1) the binding capacity of the carrier (solid) is weak and ( 2 ) the phenols disappear quickly since they are sensitive to oxidation.

Compared to freshly generated smoke, no new phenol constituents have been detected, but often a great number of phenols are not represented. The compositions of the phenolic fraction of three (Nos. 3 , 4, and 9) of the liquid and one of the solid (No. 11) preparations are quite similar to that of fresh smoke. Individual phenols show a good relative conformity. Fraction Nos. 14 and 15 of the smoke preparation are rich in pyrocatechol(l5.4 and 28.9%), syringaldehyde (13.5 and


TABLE IX PHENOLIC COMPOUNDS IN LIQUID SMOKE"

Compound (molecular weight)b 1 2 3 4 5 6 7 8 9 10

Phenol 0-Cresol m-Cresol p-Cresol 2.4-Xylenol 3,5-Xylenol + 3-Ethylphenol 2,6-Xylenol 4-Ethylphenol 2,3-Xylenol ( 184) Guaiacol Pyrocatechol Trimethylphenol + maltol Allylphenol + (208) + (196) Methylguaiacol + (208) 4-Methylguaiacol 4-Meth ylpyrocatechol 3-Methylpyrocatechol 4-Ethy lguaiacol Syringol + (282) 4-Ethylresorcinol + (238) 3-MethoxypyrocatechoI Eugenoi 4-Methylsyringol + (298) Isoeugenol (c is and rruns) 4-Ethylsyringol Vanillin 4-Vinylsyringol + (326) 4-Allylsyringol + (324) Acetovanillone 4-Propenylsyringol + (252) (cis?) 4-Propenylsyringol (trans?) + (280) Syringaldehyde Acetosyringone

4.2 5.2 5.2 9.8 8.4 7.2 4.2 6.4 6.2 9.8 0.8 1.4 3.6 2.9 2.8 2.1 1.2 1.4 3.2 7.6 1.7 1.8 4.1 4.6 3.4 0.9 1.6 1.9 3.4 5.5 0.9 1.3 2.2 8.7 2.0 2.2 1.2 1.2 3.3 3.3 0.1 0.4 1.5 1.3 0.8 1.3 0.4 0.2 1.7 1.8 0.2 0.4 1.3 1.4 0.8 tr. 0.3 0.4 1.2 0.8 0.2 tr. 0.6 tr. 0.3 0.3 tr. tr. 0.8 1.0 4.9 0.6 0.3 2.5 0.3 0.3 0.3 0.4 0.4 0.4 0.8 - 0.4 tr. - 0.9 0.6 - 0.3 0.4 6.4 4.4 1.0 - 1.5 8.9 4.6 2.8 1.3 0.3 5.7 7.3 10.7 5.7 10.4 10.8 6.5 8.4 7.9 27.5

11.7 17.0 1.5 3.3 9.1 18.7 20.5 17.9 3.0 0.5 0.4 0.3 0.2 - tr. 0.4 0.3 0.3 0.2 0.6 2.2 1.5 0.9 0.4 0.9 2.7 1.9 1.3 1.3 tr. 0.2 - 0.3 tr. 0.4 tr. tr. 0.2 0.2 0.9 2.8 3.5 11.2 7.6 5.9 4.9 3.3 3.7 8.1 14.2 2.6 4.2 0.8 2.3 3.5 1.2 4.8 5.2 1.6 - 2.6 4.2 0.8 2.3 3.5 1.2 4.8 5.2 1.6 - 1.0 1.1 5.7 2.5 2.4 1.5 1.1 0.9 4.0 4.8

13.5 16.9 11.0 14.7 12.9 20.5 17.0 13.9 11.3 5.6 0.3 0.5 0.7 tr. 0.4 - 0.6 0.3 0.5 tr. 9.0 4.9 1.8 0.7 4.0 0.5 5.4 6.6 2.1 0.4 1.2 0.6 1.8 1.4 0.7 0.6 0.6 0.4 2.1 0.4 7.1 6.4 8.6 12.6 7.7 2.9 5.7 6.6 8.9 2.4 0.1 0.2 1.1 0.4 tr. - tr. tr. 1.1 tr. 2.7 2.5 7.4 6.3 4.5 2.1 2.2 2.8 7.5 1.4 0.3 0.8 1.0 tr. 0.7 1.0 0.7 0.5 0.3 0.4 0.4 0.3 0.2 tr. - - - 0.5 1.1 - 0.4 0.7 1.1 2.5 0.9 1.1 0.6 0.5 2.0 tr. 0.4 0.6 tr. - 0.4 0.4 0.5 0.4 0.2 - 0.7 1.3 0.3 - 0.8 0.5 1.1 1.1 0.8 -

0.2 0.1 0.2 tr. 0.9 - tr. tr. 0.5 tr. tr. 0.3 tr. - - - 0.2 tr. tr. tr. tr. 0.4 - - tr. tr. 0.4 0.3 tr. -

Values are given as percentage of the total phenol fraction; tr., traces, < 0.1; -, not detectable. b Numbers in parentheses represent unnamed compounds of the indicated molecular weight.

16.5%), and acetosyringone (6.5 and 7.9%). Syringaldehyde and acetosyringone do not occur in the other smoke preparations nor in freshly generated smoke (or occur only in traces). Pyrocatechol again was present in unusually high concentrations in other samples (Nos. 2 . 6,7, and 8) where other dihydroxy phenols have also been detected in major amounts.

From these investigations one can conclude that flavoring properties of several

I30 L. TOTH AND K . POTTHAST

TABLE X PHENOLIC COMPOUNDS IN SOLID SMOKE PREPARATIONS<'

Compound I 1 12 13 14 15

Phenol o-Cresol rn-Cresol p-Cresol 2,4-Xylenol 3,5-Xylenol + 3-Ethylphenol 2,6-Xylenol 4-Ethylphenol 2,3-Xylenol (184) Guaiacol Pyrocatechol Trimethylphenol + maltol Allylphenol + (208) + (196) Methylguaiacol + (208) 4-Methylguaiacol 4-Methylpyrocatechol 3-Methylpyrocatechol 4-Ethylguaiacol Syringol + (282) 4-Ethylresorcinol + (238) 3-Methoxypyrocatechol Eugenol 4-Methylsyringol + (298) Isoeugenol (cis and trans) 4-Ethylsyringol Vanillin 4-Vinylsyringol + (326) 4-Allylsyringol + (324) Acetovanillone 4-Propenylsyringol (cis and trans) + (252) 4-Propenylsyringol (cis and trans) + (280) S yringaldehyde Acetosyringone

6.5 3.0 3.1 2.9 1.6 1 .0 0.6 0.3 0.3 1.4 6.6 5.0 tr. 0.9 tr. 7.3 2.2 3.8 3.1

15.3 1.6 2.5 1.7 9.8 tr. 4.5 1 . 1

1.8 0.4 0.4 1.6 tr. tr.

-

6.2 3.5 2.6 2.6 I .7 tr. tr. tr. tr. tr.

12.8 11.8 tr. tr. tr.

18.5 6.4 5 . 2 3.7 5.5 tr. 1.3 3.6 3.8 0.6 4.2 2.4

1 . 1 t r . t r .

tr.

-

-

-

3.2 I .4 1.8 I .2 0.9 0.9 tr. tr. tr. I .9 5.1 9 .6 tr. 0 .9 tr. 5 .6 3.3 5.2 2. I

20.5 tr. 4.5 1.4

14.3 I .2 6 .9 1 . 1

2.5 tr. 0.9 tr.

tr.

-

-

0.5 tr. 0.4 0.2 0.7 0.2 tr. tr. tr. 0.4 1 . 1

15.4 0.4 0.1 tr. 0.3 5.6 6.7 0.2

11.6 tr. 6.0 0.7

14.2 tr. 6.7 2.2 2.1 4.1 1.3 tr. tr.

13.5 6.5

I .3 0.5 I .4 1.3 1 . 1 0.7 tr. 0.9 tr. 3.1 1.4

28.9 1.6 0.3 tr. 3.7 9.6

11.0 1.1

14.2 tr. 7.3 0 .9

17.3 tr. 8.4 2.7 2.6 5. I I .6 tr. tr.

16.5 7.9

0 Values are given as percentage of total phenol fraction; tr., trace; -, not detected

smoke preparations must be quite different, because phenolic compounds, which are predominantly responsible for smoke flavor, show great deviations with regard to qualitative and quantitative composition.

Phenol Content of Smoked Meat Products. Smoked meat products contain phenolic compounds in varying amounts, which are mainly dependent on smoke

CHEMICAL ASPECTS OF MEAT SMOKING 13 1

density, duration of smoke treatment, and conditions in the smokehouse (e.g., relative humidity, temperature). Amounts of phenols reported by different researchers (e.g., Lustre and Issenberg, 1970; Potthast, 1976; Mohler, 1978, So- chtig, 1979) range from very low (0.06 mg/kg) to very high (5000 mg/kg). The highest values, which were detected by Potthast, originate from a model expen- ment with frankfurter-type sausages. These sausages, which have a large surface area compared to the volume, were smoked to an unusually dark color. The experiment was carried out to test the degree of contamination of meat products with carcinogenic polycyclic hydrocarbons (PAH), which is associated with increasing smoke flavor intensity. The product is unusual in trade.

Results published thus far should be discussed with regard to analytical possibilities. There is no doubt that modem analytical equipment leads to a more detailed knowledge of smoke constituents. In particular, capillary chromatography in connection with mass spectrometry opens analytical possibilities that are almost unlimited. Extraction methods have been improved with advanced materials for chromatographic purposes, and with a better knowledge of chemical influences on smoke constituents in solutions of different pH values.

The findings of Potthast (1976) and T6th (1980b) show that alkaline treatment of phenolic fractions leads to a drastic loss in total phenol amounts. As T6th (1980b) pointed out, di- and trihydroxyphenols disappear completely under alkaline experimental conditions.

T6th detected 10-200 mg of phenollkg in normal meat products. Heavily smoked meat products may contain up to 500 mg/kg. The phenol content of meat products increases with longer smoking time, but there seems to be no linear relationship between uptake of phenols and the duration of the smoking procedure. For example after 1 hr of smoke application there was 14 mg of phenol detectable and after 5 hr only 45 mg of phenol compounds in 1 kg of frankfurters. This shows that adsorption properties of meat products are affected by changes in the surface of the meat products, e.g., by chemical reactions of smoke constituents with proteins or by a loss in humidity. Lustre and Issenberg (1970), Potthast (1976), and Sochtig (1979) determined phenols by gas chromatography. Lustre and Issenberg found mainly syringol, guaiacol, and some 4-methyl derivatives as well as vanillin. In addition to these Potthast detected higher amounts of syringaldehyde, acetovanillone, acetosyringone, and 4-propiosyringone. Sochtig (1979) analyzed 4-propenylsyringol and eugenol in smoked frankfurter-type sausages; from salami-type sausages syringol, rn-cresol, and p-cresol were determined to be the main phenolic compounds. Knowles and co-workers (1975a) confirmed the findings of Lustre and Issenberg by calculating the relative amounts of different phenols present in smoked bacon.

Because the published results are sometimes contradictory and the dikrepan- cies are not explained by the use of different analytical procedures, Toth (1980b) investigated the composition of the phenol fractions as influenced by smoking


technology. He showed that phenol profiles in smoked meats are mainly dependent on the smoke generation temperature, the type of wood, and the conditions in the smokehouse. As may be seen from Table XI, most of the phenols from smoke may be found, for example, in smoked ham, but in differing amounts. The reason is probably due to chemical reactions of the phenol fraction, with proteins causing a shift in original uptake. Furthermore, reactions between phenols and aldehydes, as well as with nitrite (Knowles et al . , 1975a), are possible. The relative proportions of guaiacol compounds resulting from smoking meat products with smoke from hard- or softwood is different in smoke and smoked meat products (Tdth, 1982). In ham smoked with pine wood, syringol has been detected in an amount of 8% relative to the total phenol fraction, whereas smoke condensed during the same experiment exhibits only 2-3%. Similar changes have been observed in smoking with beech wood. Some phenols seem to become enriched, others diminished.

The climate of the smokehouse plays, as Tdth pointed out, an important role. Under certain conditions it is possible to get a deposition of the same amount of phenols with a similar qualitative composition on meat products, independent of smoke generation parameters and the kind of wood, that is, with smoke of differing composition.

b. Other Monocyclic Aromatic Compounds. Monocyclic aromatic com-

Alcohols: benzylalcohol and phenylethylalcohol with a flavor like rosewood

Aldehydes: acetophenone, m-, and p-methylacetophenone. Ketones: benzaldehyde and anisaldehyde with the odor of almond oil and

having some preservative effects. Acids: benzoic and phenolic acids as well as their derivatives. Most of these

belong to the phenolic acids with one hydroxyl and one carbonyl group. Sali- cyclic acid, the phenolic acid with the simplest structure, has only been detected in smoke.

pounds other than phenols have been detected in smoke:

oil.

OH

Salicylic acid

Resorcylic acid, vanillinic acid, and syringa acid were present in smoke and in smoked meats. p-Hydroxybenzoic acid and homovanillinic acid have been identified from liquid smoke. Benzoic acid as well as salicylic acid are well-known preservative compounds. The other phenolic acids are believed to have preservative effects, too.


TABLE XI COMPARISON OF THE PHENOL COMPOUNDS IN FRESHLY

GENERATED SMOKE AND SMOKED HAMa

133

Smoldering sawdust (pine) Friction smoke (beech, 380°C)

Ham

Smokehouse Smoke Smoke

Tar Water Tar Water temperature ("C) Compounds phase phase Ham phase phase 20 40 55

Phenol 0-Cresol rn-Cresol p-Cresol 2,4-Xylenol 3,5-Xylenol + 3-Ethylphenol 2,6-Xylenol 4-Ethylphenol 2,3-Xylenol (184) Guaiacol Pyrocatechol Trimethylphenol + maltool Allylphenol + (208) + (196) Methylguaiacol + (208) 4-Methylguaiacol 4-Methylpyrocatechol 3-Methylpyrocatechol 4-Ethylguaiacol Syringol 4-Ethylresorcinol + (238) 3-Methoxypyrocatechol Eugenol 4-Methylsyringol + (298) Isoeugenol (cis) 4-Ethylsyringol + isoeugenol Vanillin 4-Vinylsyringol + (326) 4-Allylsyringol + (324) Acetovanillone 4-Propenylsyringol (c is ) + (252) 4-F'ropenylsyringol (trans) + (280) Syringaldehyde Acetosyringone

1.7 0.8 0.9 1 .o 0.5 0.4 0.2 0.4

0.2 6.3 6.5 0.2 0.4

12.0 3.7 3.8 2.8 3.2 4.9 1.2 3.6 3.1 2.0

13.5 5.6 0.3 0.8 2.3 2.2 0.7 0.7 0.2

-

-

5.4 1.7 1.3 1.6 0.6 0.5 tr. tr . tr . 0.5

18.8 12.4 tr. 0.7

16.6 5.0 3.9 2.5 2.5 1.5 1.1 1.3 1.2 0.7 1.9 3.8 tr. tr. 1.5 1.6 tr. 1 .o

-

-

2.0 0.9 1 . 1 1.3 0.8 0.9 0.4 0.8 0.2 1 .o 4.5 3.4 0.8 0.6 0.3

10.2 0.8 2.0 2.9 8.2 7.3 1.7 4.0 5.5 3.0

18.2 1.5

0.7 0.4

0.2

-

-

-

-

X I

x 2

tr. -

-

-

0.9 tr. tr. - -

0.9 0.8 2.6 -

-

- 0.5 0.5 1 .o tr. 5.7 1.4 0.9 0.7 4.0 0.4 4.0 0.4 6.7 3.5 0.6 2.2

10.4 3.5 1 .1 7.0 9.9

I .3 0.5 0.4 0.3 0.9 0.9 tr.

0.2 4.2 5.9 3.0 0.4 0.3

3.0 0.6 1.6 0.5 4.8 1.9 0.9 1 .o 3.0 1.3 2.5 0.8 2.2 1.5 0.3 0.9 2.4 1.6 0.6 1.4 1.9

-

-

2.8 2.3 1.1 0.9 1.2 1 . 1 1.2 1.0

1.5 1.2

tr. tr. 0.4 0.4 0.3 0.2 2.8 1.4

10.5 8.6 2.2 1.7 tr. -

1.0 0.9 tr. -

10.8 10.3 0.5 0.3 0.8 0.6 2.3 2.5

11.6 12.7 6.1 5.8 1.5 1.8 5.5 6.7 5.5 6.7 2.2 3.1 9.6 13.2 0.7 0.7 0.3 0.4 1.4 1.8

tr. 0.2 0.4 0.7

- -

- ~

- -

- - - -

2. I 0.7 0.9 0.7

1.1

-

0.4 0.3 1.7 7.8 3.2

1.2

7.0 0.6 1.2 2.0

12.6 5.5 2.1 4.4 7.0 2.0

10.8 0.9 1 .1 2.9

1 .o 3.1

-

-

-

__ - -

-

a Values are given as percentage of total phenol content; tr., trace; -, not detected.


Esters: Benzoic acid ethyl ester. The existence of more cyclic esters is

Ethers: Anisole, veratrole, 4-methyl-, and 4-ethylveratrole have special fla- probable.

voring properties and may be present in smoke.

c. Bicyclic Aromatic Compounds. Benzofurane and its methyl derivatives occur.

Q-7p (3 derivatives)

1-Indanone and its 2-, 3-, and 6-methyl derivatives are believed to be present in smoke but have not yet been detected in smoked foods.

m: /

1 -1ndanone

From indanone. indane and indene are derived

Indane Indene

These compounds may react with aldehydes and ketones to form colored benzofulvenes.

Naphthalene and methylnaphthalenes have been isolated from smoke and smoked products.

m R / /

They often are considered the first members of the large group of PAH.

d. Polycyclic Aromatic Hydrocarbons. Anthracene, fluorene, and phenanthrene are tricyclic aromatic compounds that may be interpreted as mother substances of the group of PAH:


Anthracene Fluorene Phenanthrene

Smoke is believed to contain more than 200 different isocyclic and heterocyclic compounds (N- , 0-, and S-heterocyclic compounds). From these PAH Potthast (1979) determined 41 in smoked meat products and 61 in smoke by use of GC- MS. Some of the PAH have proved to be extremely carcinogenic in animal experiments as has, for instance, the well-known benzo[aJpyrene (B[a]p).

/ / /

Benzo [ a] pyrene

As stated before, PAH arise from incomplete combustion of wood. Their formation is temperature dependent (Grimmer et al., 1966; T6th and Blaas, 1972c) and increases with increasing temperatures.

5 . Other Compounds

The following additional constituents have also been isolated from smoke:

Soot: CO2: NO: CO COz: NO*: NO3:

1.85 gll00 g wood (Spanyar et al., 1960b) in a 1: 10 relation (Spanyar et a/. , 1960b) 0.4-1.0 pg/liter (Grau and Mirna, 1957) 4 g/100 g wood (Nikitin, 1955) 10 g/lOO g wood (Nikitin, 1955) 1.4-2.5 mg/liter, calculated as NaN02 and 5.0-28 mg/liter, calculated as KN03

both analyzed from a steam smoke condensate (Mima and Schutz, 1971)

These values may vary with changing smoke generation conditions, especially soot, CO, and CO,, because combustion with low amounts of oxygen or with dry or wet wood materials greatly affects soot as well as CO and CO, development.

VI. EFFECTS OF SMOKING

As pointed out before, smoke constituents come into contact with the meat products by condensation or adsorption. Whether condensation or adsorption is


predominant depends on surface properties such as relative leaness and fatness, humidity, temperature, and, if casings are used, the casing materials. Smoke conditions (humid, dry, cold, or warm) and smoke composition are of great influence, too.

Tilgner (1958b) described the phenol content of products with a wet surface smoked during the same period of time to be 20 times greater than that of dry products. Foster and Simpson (1961) reported that volatile compounds are adsorbed more easily with smoke of a higher humidity. Increased smoke humidity was achieved by passing the smoke across water containers. According to investigations of Spanyar and Kevei (1961), different skin materials may favor a rapid or slow uptake of smoke. The better the wettability, the greater the amount of smoke that was deposited on the skin. The importance of humidity was also confirmed by Simon and co-workers (1966), who demonstrated that smoke uptake of cellulose skin filled with water increased with increasing relative humidity and temperature. This particularly held true for phenols. The deposition of smoke on foods may be enhanced by electrostatic treatment. As Mohler (1978) reported, electrically charged smoked constituents become cohesively or ad- hesively bound at the surface of the meat products. Further heat treatment is required for color development.

The major portion of the smoke constituents remains at the surface of the meat products. Polycyclic hydrocarbons are found in the outer layers, and sometimes they may be removed by separating skin or casings from meat products (Tbth, 1972; Potthast, 1975). It has been observed that certain artificial casings are nearly impermeable to PAH while flavoring and coloring compounds penetrate the skin. Phenolic compounds are enriched in surface layers as well.

According to Spanyar and Kevei (1961) total phenol compounds in smoked backfat are concentrated at the surface. It is questionable whether this observation will hold true in general, as phenols show a certain lipophilic behavior and should consequently move. Gorbatov and Kurko (1960) developed an analytical procedure that allows the determination of phenol deposition in a short time. By pressing smoked meat products to a filter paper impregnated with 2,6-di- chlorchinonechlorimide, phenols react producing color development. Using this method the authors found that phenols peretrate only 5-6 mm through the outer layers of frankfurter- and salami-type sausages.

These results were confirmed by Potthast (1976) and Sochtig (1979), who found that the migration velocity of phenols was dependent on the consistency of the sausages. With greater softness and porosity, the penetration depth increased. By quantitative measurements Bratzler et ul. (1969) found that the majority of phenols in smoked bologna was present in an outer 1-2-mm layer. Knowles et al. (1975a) isolated 75% of the phenols present in ham from the outer layers. The same was found with model mixtures of gelatin (Spanyar and Kevei, 1961). These authors also reported that other smoke constituents, e.g., carbonyls and


acids, may penetrate throughout the whole product. Toth (1982) elucidated different factors that possibly affect smoke uptake by subjecting backfat, solutions of gelatin (5%) , and meat batters to smoke:water in open containers with or without NaCl or curing salt (NaC1 + 0.5% NaNO,). The smoke constituents had been separated as described before and analyzed by gas chromatography. The gas chromatograms demonstrate that the compounds detectable in model mixtures are distinctly different from those of the original smoke and that smoke deposition is affected mainly by surface properties. These investigations show that meat products treated with the same curing smoke may contain different amounts of smoke constituents if they'differ with respect to their composition. Furthermore, it seems possible to influence smoke deposition by changing the smokehouse climate, so that the smoke compounds found in smoked meat products are different from the smoke composition itself.

Smoking provides the following effects: coloring, flavoring, preserving by antioxidative and antimicrobial action, tanning of natural casings, formation of a secondary skin, and contamination with toxic compounds, e.g., benzo[a]pyrene.

A. DESIRABLE EFFECTS

1. Color Formation

The formation of the smoke color is believed to originate from an uptake of colored smoke constituents, oxidation and polymerization of smoke compounds (e.g., phenols, aldehydes), and reaction of smoke compounds with meat proteins.

Of these processes affecting color, reactions between proteins and smoke compounds have been best investigated. Ziemba (1969a,b) studied such reactions by using model mixtures that resulted in a loss of amino groups. A remarkable loss of lysine in smoked meat has been reported by several authors (Dvorak and Vognarova, 1965; Ruiter, 1970; Chen and Issenberg, 1972). In these reactions, mainly carbonyl compounds from smoke are involved. According to Ruiter (1979) the following aldehydes were analyzed as the most important ones from a smoke condensate:

Formaldehyde Glycolaldehyde Glyoxal Acetone Acetol Methylglyoxal Diacetyl Furfural

700 mg/kg I500

60 I70

1390 830 140 90

4890 mg/kg


Compounds carrying a keto or an aldehyde group, e.g., some keto and aldehyde phenols, are of some additional influence. As Chen and Issenberg (1972) stated, formaldehyde does not cause color changes, but the other aldehydes are more reactive (Ruiter, 1979).

From model mixtures containing glyoxal and ethylamine, it was determined that browning was most intense at a molar relation of 2: 1 to 3: 1. This relation between the aldehydes of the smoke and meat proteins may be achieved by heavy smoking of meat products. Dark-colored meat products with a strong but pleasant smoke flavor result.

For color development Ruiter described the course of the reaction shown in Fig. 8. 1 -Hydroxyethyl-3-hydroxymethyl-2-pyrrolaldehyde was isolated from smoked meats, indicating that there is a good probability that the reaction occurs as described. The aldosamine browning shows a great similarity to the better known Maillard reactions. The main differences is that Maillard reactions take place between carbohydrates and amino compounds, whereas browning of meats from smoke is caused by the thermal decomposition products arising from mac- romolecular carbohydrates during smoke generation.

Ruiter suggests that a reaction similar to this is responsible for color formation by aldehydes and proteins in smoked meats, but there is no doubt that additional reactions take place in smoked color development. Curing smoke consists of a great number of compounds that may react by condensation and polymerization as well as by oxidation. Such reactions are responsible for the changes occurring within the very first minutes after smoke generation and condensation begin. The condensate turns from a bright yellow to a brown color that becomes increasingly dark brown, followed by a precipitation of brown to black tarry products. Ac- cording to Block (1979), the precipitates consist of high molecular weight substances from which phenols and furfural have been detected after thermal decomposition. These brown-colored products may be adsorptively bound to proteins, thereby intensifying the smoke color. The increase in color of steam- smoked products observed during storage may be explained by such polymerization and oxidation reactions. The presence of acids from smoke has a stabilizing influence. Furthermore, brown-colored products occur from the reaction of phenols and nitrite (Toth, 1982). The formation of nitrosophenols and their polymerization products is pH dependent: with decreasing pH values it increases. At pH 5-6 (the pH of most meat products) these reactions take place to a limited extent.

In addition to color, the flavor of the smoke plays a major role. As pointed out before, flavor development shows a close relationship to phenol content. Flavor- ing constituents of the smoke are believed to have an appetizing effect, but if the smoking procedure is not done properly, smoked products may cause intestinal disturbances (Bechthold, 1971).


Polymerization

t

Ethylamine Schiff's base aldehyde

0H 0 H C W H - R C"H-R II

OH >( !!' CH

I / ,H I /" ,H c=c-c

CH,OH

c-c-c\ "0 I I /H'OH\'

I ',H

I

+ CH,OHCHO

- H,O

H H I

C-NH-

CH I I I

C-OH

Polymerization

[R = CH,CH,OHJ

FIG. 8. .:thylamine. From Ruiter (1979).

Reaction course as supposed in color formation by the reaction of glycolaldehyde and

2. Flavoring of Foods

Curing smoke with good flavoring properties may be obtained from different kinds of wood (Spanyar et al., 1960a; Tilgner, 1958b). Some types of wood such as beech, oak, maple, birch, spruce, and pine are said to provide smoke of preferred quality, while lime, alder, aspen, and fir are less suitable for smoke production. However, we believe that every kind of wood may be used success- fully in smoking if the proper generation parameters are maintained, in particular generation temperature. Tilgner (1 96 1) recommends smoke generation at tem-


peratures ranging from 200-6OO0C, where wood materials are just in a glowing state. According to Miler (1962a) and Tilgner and Daun (1970), temperatures of pyrolysis should not exceed 350-450°C and subsequent oxidation should be done at about 200°C. Smoke generation at higher temperatures provides an unpleasant bitter and/or tarry smoke (Reuter, 1966). The composition of smoke varies. As Tilgner (1970) states, production of a uniform smoke over a longer period of time is nearly impossible, independent of the kind of smoke generation, if a one- or a two-step procedure is used.

More recent investigation (Potthast, 1979) shows that there is a good possibility of obtaining smoke with a permanently unchanging composition when smoke is generated with the same type of wood of the same humidity at constant temperatures. The best generation temperature is between 650 and 700"C, and the water content of wood should be about 50%. By using wood with a high water content, high density smoke is obtained. The humidity works as a temperature-controlling mechanism since the heat of evaporation required for steam development cools the smoldering materials. Between 650 and 700°C, phenols and PAH are in the most desirable proportions (Fig. 9). At these temperatures it is possible to smoke products to quite a dark color without producing a high degree of contamination with benzo[a]pyrene or other carcinogenic PAH (Pot- thast, 1982a).

The composition of curing smoke and its flavoring properties may be affected by different treatments. Foster and Simpson (1961) achieved well-flavored meat products from an electrostatically treated smoke. The smoke color was also described as good. About 40% of the smoke constituents are lost by electrostatic treatment as Table XI1 shows. The highest loss of flavoring smoke compounds is due to the loss of phenols, which accounts for 64%. Ninety-eight percent of the PAH have been trapped. Obviously, the more volatile compounds are responsible for a good flavor development, and therefore the removal of tar products is desirable. Thus, the simplest method to produce smoke with good flavoring

I ,T ' LOO0 700" 1Mx) OC

FIG. 9. value u for the phenol content corresponds to more than 1000 times the value of benzo[a]pyrene.

Temperature dependency of benzo[a]pyrene and phenol contents in curing smoke. The


TABLE XI1

CHEMICAL COMPOSITION OF ELECTROSTATICALLY TREATED OR UNTREATED CURING SMOKE"

Smoke condensate

Untreated Treated Loss (mg/100 g) (mg/100 g) (%)

Acids 211.0 150.0 29.0 Phenols 1.5 2.7 64.0 Carbonyls 341.0 192.0 44.7 Total 565.5 344.7 X 39.1

Benzo[a]pyrene 38.5 pg/liter 0.8 p,g/liter 98.0

141

a From Rusz (1976) with permission.

properties is to separate the generator and the smokehouse, that is, to cleanse the smoke of high-boiling compounds by condensation in the connecting pipe (T6th and Blaas, 1972a).

To evaluate the flavoring constituents, smoke is generally divided into several fractions. The most common fractionation method is steam distillation. By using this procedure, Husaini and Cooper (1957) and Krylowa et al. (1963) were able to demonstrate that acids, aldehydes, and phenols have special flavoring properties. Miler and co-workers (1965) reported the favorable contribution to flavor of a phenol extract obtained from smoke during the treatment of sausages. Bratzler et al. (1969) observed a close correlation between smoke flavor and phenol content of smoked products. This finding has been confirmed by Tilgner et al. (1962~) and by Daun (1969, 1972), who separated phenol fractions by preparative column chromatography prior to sensory evaluation.

Fiddler and co-workers (1966) separated a good-smelling smoke fraction by extracting smoke condensate in water with ether. This extract consisted mainly of phenols. As main compounds guaiacol and syringol were identified. Similar results have been achieved by T6th (1981), who used ethyl acetate in the extraction procedure. T6th's investigations showed that other compounds were involved in smoke flavoring as well. This is in agreement with the findings of Daun (1972) and Fujimaki et al. (1974), who found a certain phenolic chemical or uncharacteristic smoky taste. To overcome these disadvantages of pure phenol fractions, Miler (1970) recommended mixing several smoke fractions in liquid smoke production on the basis of flavoring considerations. After separation of flavoring constituents from liquid smoke by liquid-liquid partition and determination by gas chromatography, Fujimaki et al. (1974) came to the following conclusions. Apart from phenols, smoke flavor is due mainly to carbonyls and


lactones with higher boiling points. Some alkylated 1,2-~yclopentadiones and 2- butenolide derivatives show a sweet, burnt, or caramel-like smell and should be important for smoke flavor. The smell of burning is mainly caused by 2- butenolides.

Furfurol, 5-methylfurfurol, acetylfurane, and acetophenone provide a sweet, pleasant smelling, flowery flavor. They soften the strong smoky flavor of the phenols. The numerous derivatives of cyclopentanones have a bitter taste and smell like grass. Their influence on the smoke flavor is not significant.

Single phenols have been described as follows:

Phenols, cresols, 2,3-, and 2,4-xylenols

2,6-, 3,4-, 3,5-Xylenols, 2- and 3-ethyl-5-methylphenols, and 2,3,5-trimethylphenoIs

Guaiacol

4-Methyl-, 4-ethyl-, 4-propyl-, and 4-propenylsyringol

Pyrocatechol and its derivatives

4-Methyl-, 4-ethyl-, and 4-vinylguaiacol

4-Allylguaiacol

Syringol

Characteristic, biting

Cresolic

Sweetly smoky. a little bit hot

Mild, strong, smell of burning

Sweet, with a burning taste

Sweet, smoky

Wood-like

Smoky

Olsen (1976), as well as Barylko-Pikielna and co-workers (1976), demonstrated the flavoring properties of several phenol fractions distilled from a liquid smoke (after Miler, 1970) by mixing them with milk. The first fraction (60-90°C) consisting of phenol, cresols, guaiacol, methyl-, and ethylguaiacol produced a hot and bitter taste. The second fraction (91-132°C) containing cis- and trans- isoeugenol, syringol, and methylsyringol had a pure and mild smoke flavor. The third fraction (133-200°C) provided an acid, chemical taste of poor flavor. From these investigations it may be concluded that the phenols of medium volatility are most desirable in curing smoke, as was stated by Daun (1972).

Wasserman (1966) and Sochtig (1979) dealt with the question of whether single phenols or mixtures of single phenols are sufficient for smoke flavor development, Wasserman used a taste panel to identify guaiacol, methylguaiacol, and syringol in meat products. Of the panelists, 67% classified the products as smoke-like, the rest as phenolic. A real smoke flavor was not found. In the Sochtig experiments, 12 panelists tested single phenols in solutions of differing concentrations. The results are summarized in Table XIII. All of the tested phenols showed smoke-like characteristics together with other sensory attributes.


TABLE XI11 SENSORY EVALUATION OF SMOKE COMPOUNDS

AS DERIVED FROM MODEL MIXTURESa ~ ~ ~ ~~ ~

Optimum concentration

Compound (mg/100 ml H20) Smell

~~

Taste

Guaiacol 3.15 Phenolic, smoky, aromatic, hot, sweet

2,6-Dimethoxyphenol 7.50 Smoky, spicy, aromatic, like smoked ham, phenolic, hot, sweet

4-Methylguaiacol 1.90 Sweet , vanilla-like, fruity, like cinnamon smoky, pleasant hot

Isoeugenol

Cyclotene

0-Cresol

Dimethylphenol

9.80 Sweet, fruity, vanilla-

1.90 Like Maggi,b smoked like, phenolic

ham, spicy, aromatic, like clove

aromatic, caramel-like, like smoked ham

matic, sweet

7.50 Phenolic, sweet fruity,

0.90 Phenolic, like ink, aro-

Phenolic, hot spicy, flavor of smoked ham, sweet, dry

Phenolic, smoky, like burning, charcoaled wood, whiskey-like, dry, hot

Sweet, vanilla-like, caramel-like, aromatic, pleasant smoke flavor, burning

Sweet-fruity, mild smoke flavor, dry, hot

Like Maggi,b like smoked ham, sweet, hot

Sweet, hot, unpleasant smoky, burning

Phenolic, hot, a little bit carbonized, sweet, dry

a From Siichtig (1979) with permission. Common flavor-enhancing protein hydrolysate in Germany.

Cyclotene, tested at the same time, ought to enhance meat flavor by its “Mag- gi”-like taste (Maggi is a flavor protein hydrolysate common in Germany). A mixture of the sensorilly most favored compounds syringol, guaiacol, 4-methylguaiacol, and cyclotene in a concentration was judged only weak in smoke flavor but sweeter, fruity, and generally aromatic. Obviously, a mixture of the several components produces a flavor different than single compounds normally do. There is no doubt that phenols are mainly responsible for smoke flavor, but nonetheless a great number of nonphenolic compounds participate in smoke flavor development. Tilgner (1977) estimates the number of smoke flavoring compounds to be about 500. If so, one may not expect to produce an artificial mixture comparable to a natural smoke flavor by merely combining a few compounds that play a predominant flavor role in smoke treatments.

The typical flavor of smoke-treated products is not fully explained by a deposi-


tion of smoke constituents on the surface. A great number of smoke flavor compounds are composed of reactional groups that may react with food constituents. These reaction products provide a taste different from that of the original reaction components. It is most probable that carbonyl compounds from smoke and aliphatic as well as aromatic carbonyls are mainly involved in these reactions. Whether some compounds, e.g., phenols with reactional carbonyl groups, disappear at all, as Lustre and Issenberg (1970) found, or only are reduced (Toth, 1980a, 1982) is of less importance. The extent of the reactions may be remarkable, as has been shown in the investigations of Knowles and co-workers (1975a) (Table XIV).

3. Preservation Effect

Smoke is known to have an antioxidative and an antimicrobial effect (Ker- sken, 1973), both of which are influenced mainly by phenolic compounds.

TABLE XIV COMPOSITION OF PHENOL EXTRACTS FROM LIQUID

SMOKE AND LIQUID SMOKE-TREATED BACON"

Percentage in the phenol extract

Compounds Liquid smokeh Baconh

Furfurylalcohol Cycloten Guaiacol Phenol 0-Cresol 4-methylguaiacol m-, p-Cresol 4-Ethylguaiacol 4-Propylguaiacol 4-Allylguaiacol 4-Vinylguaiacol cis-Isoeugenol trans-Isoeugenol S yringol 4-Methylsyringol 4-Ethylsyringol 4-Propylsyringol 4-Allylsynngol 4-Propenylsyringol

1 .o 2.1 0.1 0.1 2.3 0.3 0.2 0.1 6.0 11.4 5.2 2.9 2.6 3.5 4.0 2.9 1.6 1.3 0.2 0.5

10.5 13.6 10.1 8.6 2.1 2.6 3.8 4. I 5.1 4.1 1 .o 0.9 3.6 2.3 0.6 0.4 4.3 2.9 0.8 1.2 1.3 1.7 2.2 0.5 0.4

11.0 6.8 1.6 I .9 8.5 11.9 15.5 16.7

14.5 12.5 13.9 17.6 3.1 3.9 2.9 3.4 1 .o 5.0 3.8 14.5 16.7 5.0 3.8 0.5 0.1

- - -

- __ -

0 From Knowles et al. (1975a) with permission. b Two separate experiments were conducted for each.


TABLE XV

PHENOLIC COMPOUNDS IN CURING SMOKE WITH PROVED ANTIOXIDATIVE PROPERTIESa

Phenols Phenol aldehydes Phenol acids

145

~~

Pyrocatechol Vanillin 2-Hydroxybenzoic acid Hydrochinone Salicylaldehyde 4-Hydroxybenzoic acid Guaiacol Eugenol Isoeugenol

a From Seher (1967).

During the storage of foods the spoilage of fat by rancidity is the limiting factor if microbial spoilage is prevented by cooling, refrigeration, or drying. Fat oxidation, known as the peroxidation process, increases with decreasing water activity. Hydroperoxides are present, which decompose to aldehydes and ketones, and are responsible for the rancid taste. The peroxidation process may be prevented by antioxidants, all of which have a phenolic structure. Seher (1967) described antioxidative properties of some phenols present in smoke. Besides the tested phenols (Table XV), other phenols supposedly act as antioxidants in as much as they are able to turn into quinoid structures.

Kurko (1959) investigated basic, acid, and neutral fractions of smoke for antioxidative properties in mixtures with pork fat. Only the neutral fraction that contained most of the phenols showed excellent antioxidative effects. The acid fraction had minor antioxidative properties. The basic fraction proved to be prooxidative. The neutral fraction was separated again by vacuum distillation to isolate the most effective compounds. Fractions with lower boiling points, with phenol, cresol, and guaiacol as main constituents, were less antioxidative than fractions of higher boiling points, containing mainly syringol and its derivatives. Pokorny and co-workers (1963) took the phenols with two unsubstituted hydroxy groups in the ortho or para position and phenols with several alkyl groups to be most effective.

Tilgner and Daun (1970) compared the properties of smoke from smoldering sawdust and from a friction smoke generator and found that smoke from smoldering sawdust was more antioxidative. This observation may be explained by the findings of T6th (1980b) that friction smoke is less rich in dihydroxyphenols. Whether the effectiveness of phenols is enhanced by dicarboxylic acids (fumaric, succinic, etc.) has not yet been investigated.

Smoke constituents exert a restraining effect on bacteria in pure cultures as well as in meat products (Kersken, 1973). However, there are factors other than smoke affecting the growing of microorganisms, such as lowering the water

146 L. TOTH AND K . PO'ITHAST

activity and the pH value. Heat treatment may be of additional influence (Lerche et al . , 1957). Bacteria that do not form spores or bacteria in a germinating state are killed. Spores of bacteria and molds remain unaffected. Cocci in particular are most resistant.

The antimicrobial effects of smoke occur mainly at the surface, but to a certain extent smoke can influence the interior of sausages, too. As most smoke constituents are enriched at the surface of the meat products, several authors explain germicidal effects by pH changes. Hess (1928), Kochanowski (1962), and Incze (1965) believe formaldehyde to be the most bactericidal compound. Kurko and Perova (1961) contradict this opinion since in their experiments only organic acid and phenol fractions of the smoke proved to be bactericidal. The fungicidal effect of smoke is produced by phenols and formaldehyde as well as by other compounds of the smoke (Kersken, 1973). This effect is of particular importance in the prevention of mycotoxic mold growth.

4. Other Effects

During surface treatment of fish or meat products with smoke, formation of a secondary skin is observed. The reactions of carbonyls and proteins are mainly responsible for this formation (the tanning effect).

2 >N + RCH - >NCHRN, / + H,O

; This reaction is favored by high temperatures and dry smoking conditions or, if smoke was applied in a wet atmosphere, by subsequent reduction of the humidity. The extent of tanning shows a strong relation to the smoke composition and the climate of the smokehouse. This reaction also takes place with natural casings and artificial casings made of collagen, and is responsible for the resistance of a sausage in a natural casing to bursting when heated in a hot water bath. In general, this reaction imparts greater firmness, influences sausage consistency, and stabilizes microbial growth.

B. UNDESIRABLE EFFECTS

Besides the above desirable effects, smoking or smoke flavoring of meat products may cause some disadvantages in the form of a reduction of essential nutrients and contamination with toxic compounds.

I . Influence on Nutrients

The formation of color and flavor as well as the tanning effect are based on reactions of proteins with carbonyl compounds of the curing smoke. These


reactions lead to a loss of amino groups. With curing smoke, only surface proteins are damaged. Mixing liquid smoke with foods distributes smoke compounds evenly, and a more extensive degradation of proteins is possible. Experi- ments with formaldehyde-treated collagen show that the reaction products are not digested in the stomach but are split by the enzymes of the intestine (Mohler, 1978). Proteins with free amino groups undergo changes very easily.

Losses of lysine, an essential amino acid, may occur (Chen and Issenberg, 1972), and these increase with increasing smoke deposition on the foods. Despite remarkable losses of lysine from the surface layers of some heavily smoked meat products, a nutritional deficiency problem is not apparent. Whether other essential amino acids or vitamins are affected as well is not yet known.

2. Contamination with Carcinogenic Polycyclic Aromatic Compounds

The first observations that smoke may cause cancer were made by Pott (1773, who described skin cancer in a chimney sweep. Since about 1940 some PAH have been perceived as being carcinogenic. One of the best known PAH is benzo[a]pyrene (B[a]p), which was demonstrated to be mutagenic and carcinogenic in numerous animal experiments. Benzo[a]pyrene and other PAH have been detected in foods of vegetable and animal origin. Vegetables are contaminated during growing by air pollution or by an uptake from the soil. Foods of animal origin such as milk products or meats are practically free of PAH, until they become contaminated during processing, in particular with smoke. In meat cookery, barbacuing is a secondary source of PAH contamination. Polycyclic hydrocarbon content may show a wide range of concentration as is shown for B[a]p (Table XVI).

TABLE XVI

BENZO[n]PYRENE CONTENT OF SMOKED AND BARBECUED MEAT PRODUCTS

Remarks

Smoked sausages 0-6.15 Highest concentration in small-diameter sausages Ham, bacon, etc. Ham, bacon, etc. Ham, bacon, etc.

0.1-3.10 0.1-15.0 1%300

Fermented or cooked; smoked to a bright color Fermented or cooked; smoked to a dark color Fermented or cooked; smoked to a dark color; more or less soot

may be found on the surface on country-type ham produced according to home-style procedures

Depending on the fuel and fire condition; increasing with increasing fat content of the sausages

Depending from the fuel and fire condition; increasing with increasing fat content of the meat

Barbecued sausages

Barbecued meat

0.1-86.0

0.657.0


Studies on the influence of smoking technology (Toth, 1972; Potthast, 1975, 1979, 1982a; Spanyar et al., 1960a; Tilgner, 1958a, 1977; Fritz, 1977) have revealed that the contamination of B[a]p shows a close relationship to smoke generation parameters and that very high smoke generation temperatures, in particular when soot-developing materials have been used, lead to B[a]p concentrations higher than 1 ppb.

Although there is no proof that B[a]p causes cancer in humans as a result of eating smoked or barbecued meat products, a potential danger cannot be ex- cluded. As Fritz and Soos (1980) pointed out, stomach and colon cancer are significantly higher in countries or areas where meat products with high B[a]p and other carcinogenic PAH are common. From the estimated 200 PAH present in smoke, some are mutagenic or carcinogenic and some have a cocarcinogenic or antagonistic effect. Carcinogenicity or mutagenicity has been proven for the compounds listed in Table XVII. The total possible carcinogenicity of PAH is estimated to be about 10-fold that of B[a]p alone.

From a technological point of view smoked meat products need not contain more than 1 ppb (1 pg/kg) B[a]p. even if the meat products have been smoked to a very dark color and have a smoky flavor. For this reason the maximum B[a]p content in smoked meat products has been limited by regulation to 1 ppb in the Federal Republic of Germany.

3. Formaldehyde

Constituents of smoke other than PAH have toxic effects. One such compound is formaldehyde, shown to be mutagenic by animal experiments. Since formaldehyde is degraded by enzymatic action in a relatively short time, foods may contain up to 50 mgikg without any health hazard (Mohier, 1978). Recent studies by the Chemical Institute of Industrial Toxicology and the Institute of Occupational Safety and Health revealed that formaldehyde vapor may induce nasal cancer. There is consequently a keen interest in keeping formaldehyde concentrations in the workshop as low as possible. The maximum concentration allowed in Germany is 1.2 mg/m3. This value is never reached under normal conditions.

4 . Phenolic Compounds with Mutagenic Activity

The phenols of curing smoke are of some interest with respect to toxicological properties. Carcinogenic or cocarcinogenic influences have been reported from phenols of tobacco smoke and tea (Gibe1 and Gummel, 1967; Kaiser and Bar- tene, 1966, 1967). Pool and Lin (1981, 1982) found mutagenic effects arising from some phenols detected within purified total phenol fractions of curing smoke (Pool and Lin, 1982). In addition, phenols may react with nitrites in cured


TABLE XVII POLYCYCLIC AROMATIC HYDROCARBONS

FROM CURING SMOKE WITH CARCINOGENIC OR MUTAGENIC PROPERTIES",'

PAH Carcinogenic Mutagenic

7,12-Dimethylbenz[a]anthracene 3-Methylcholanthrene Benzo[ alpyrene Dibenz[ a, hlanthracene Dibenzo[a, ilpyrene Benzo[ clphenanthrene Benzo[y lanthracene Indeno[ 1,2,3-c,d]pyrene Benzo[ blfluoranthene Chrysene Benzo[ klfluoranthene Picene Benzo[g.h, ilperylene 1 -Methylpyrene Dibenz[a,c]anthracene Benzo[ elpyrene Anthanthrene 2-Methylanthracene 9-Methylanthracene 2-Methylphenanthrene Pyrene Benzo[ alfluorene Triphenylene Fluoranthene Perylene

+ + + + + -

+ -

+ + + -

+ + + + + + + + + + + +(+++) +(+++)

" From Fretheim (1976), Kaden et al. (1979), Umweltbun- desamt (1979).

- , Not carcinogenic (only sometimes considered); -t , un- certain; + , carcinogenic, mutagenic; + + , + + + , + + + + : increasing carcinogenicity; -, not determined.

meat products, thereby forming nitro- and nitrosophenols (Knowles et al., 1975b). Nitrosophenols are potential catalysts in nitrosamine formation (Davies and McWeeny, 1977; Walker et al., 1979, 1982). Nitrosophenols themselves show a mutagenic effect (Gilbert et al., 1980). Whether nitro- and nitrosophenols are present in smoked meat products in an amount dangerous to human health is not yet known. If liquid smoke is mixed with nitrite containing brine, nitro- and nitrosophenols are formed. Therefore, an application of smoke-flavored brines to meat products may be undesirable.

150 L. TOTH AND K . POITHAST

VII. CONCLUSION

In the last few decades curing smoke has been analyzed with increasing success. Now several hundred compounds have been detected, and most of them identified by gas chromatography or GC-MS, using pure substances for their identification. The molecular weights of more compounds are known. Several smoke fractions have been investigated for sensory, technological, and toxicological effects. Browning reactions, flavor development, and preservation have been studied. These studies demonstrate that phenols play a predominating role in flavoring foods but that a real smoke flavor is due to other additional cyclic and noncyclic compounds. From chemical analyses we learned that smoke composition varies with different smoke generation conditions. It is dependent on the type of wood (softwood or hardwood), the humidity of the wood, and the generation temperature. The temperature influence is most important. High generation temperatures are responsible for contamination of smoked foods with high amounts of PAH and low temperatures for the development of an unpleasant smoke flavor. Smoke has to be generated in the presence of small but adequate amounts of oxygen, as oxygen causes the temperature to rise. The generation temperatures should not exceed 650-700°C. This optimal generation temperature, at which the smoke composition shows the most favorable ratio of desired to undesired compounds, may be easily achieved by wetting the wood (50-60% H,O). With high water content of the wood, the smoke density increases, and the smoking procedure may be done in a shorter time. Smoke of higher humidity is easier to condense in water; thus a reduction of emissions seems to be possible. Coloring, flavoring, and preserving properties are excellent. Temperature and humidity control in curing smoke generation are the most important factors whether the smoke is used in food or liquid smoke production.

ACKNOWLEDGMENT

We would like to thank R. McCormick for his help in reviewing and correcting this article

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[advances in food research] volume 29 || chemical aspects of the smoking of meat and meat products

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