[cheese: chemistry, physics and microbiology] major cheese groups volume 2 || cheddar cheese and...

Cheddar Cheese and Related Dry-salted Cheese Varieties R.C. Lawrence, J. Gilles* L.K. Creamer, V.L. Crow, H.A. Heap, C.G. HonorS, K.A. Johnston and P.K. Samal, Fonterra Research Centre, Palmerston North, New Zealand

In the warm climates in which cheesemaking was first practised, cheeses would have tended to have a low pH as a result of the acid-producing activity of the lactic acid bacteria and coliforms in the raw milk. In colder climates, it would have been logical either to add warm water to the curds and whey to encourage acid production (the prototype of Gouda-type cheeses) or to drain off the whey and pile the curds into heaps to prevent the temperature falling. In the latter case, the piles became known as 'Cheddars', after the village in Somerset, England, where the technique is said to have been first used about the middle of the nine- teenth century. The concept of cheddaring was quickly adopted elsewhere. The first Cheddar cheese factory, as opposed to farmhouse cheesemaking, was in operation in the United States (NY State) in 1861, followed by Canada (Ontario) in 1864 and by New Zealand and England in 1871.

Development of cheddaring

Cheddar cheese was apparently made originally by a stirred curd process without matting, but poor sanitary conditions led to many gassy cheeses with unclean flavours (Kosikowski and Mistry, 1997). Cheddaring was found to improve the quality of the cheese, presumably as a result of the faster and greater extent of acid production. As the pH fell below about 5.4, the growth of undesirable, gas-forming organisms, such as coliforms, would have been increasingly inhibited. The piling and repiling of blocks of warm curd in the cheese vat for about 2 h also squeezed out any pockets of gas that formed during manufacture. Cheesemakers came to believe that the characteristic texture of Cheddar cheese was a direct result of the cheddaring process. It is now clear that recently developed methods of manufacturing Cheddar cheese do not involve a traditional cheddaring step but the cheese obtained has a texture identical to that of traditionally made Cheddar.

* Deceased 19 January 2003.

The development of the fibrous structure in the curd of traditionally made Cheddar does not com- mence until the curd has reached a pH of 5.8 or less (Czulak, 1959). The changes that occur are a consequence of the development of acid in the curd and the consequent loss of calcium and phosphate from the protein matrix. Therefore, it is important to recognize that 'cheddaring' is not confined only to Cheddar cheese. All cheeses are 'cheddared' in the sense that all go through this same process of chemical change. The only difference is one of degree, i.e., the extent of flow varies due to differences in calcium level, pH and moisture (Lawrence et al., 1983, 1984). In addition, with brine-salted cheeses, flow is normally restricted at an early stage in manufacture by placing the curd in a hoop. However, if Gouda curd is removed from a hoop, it flows in the same way as Cheddar curd. Similarly, the stretching induced in Mozzarella by kneading in hot water is best viewed as a very exaggerated form of 'cheddaring'. All young cheese, regardless of the presence of salt, can be stretched in the same way as Moz- zarella, provided that the calcium content and pH are within the required range (Lawrence et al., 1993).

Development of dry-salting

In the early days of cheesemaking, the surface of the curd mass was presumably covered with dry salt in an attempt to preserve the cheese curd for a longer period. In localities where the salt was obtained by the evaporation of seawater, it would have been a rational step to consider using the concentrated brine rather than wait for all the liquid to evaporate. The technique of dry-salting, i.e., salting relatively small pieces of curd before pressing, appears to have evolved in England, probably in the county of Cheshire, where rock salt is abundant. Cheshire has been manufactured for at least 1000 years and is thus a more ancient cheese than Cheddar. Variants of Cheshire and Cheddar were developed in specific localities of Britain and have come to be known as British Territorial cheeses. Blue- veined cheeses such as Stilton, Wensleydale and Dorset are also dry-salted.

Cheese: Chemistry, Physics and Microbiology, Third edition - Volume 2: Major Cheese Groups ISBN: 0-1226-3653-8 Set ISBN: 0-1226-3651-1

Copyright �9 2004 Elsevier Ltd All rights reserved

72 Cheddar Cheese and Related Dry-salted Cheese Varieties

Dry-salting overcomes the major disadvantage of brine-salting, i.e., the 'blowing' of the cheese due to the growth of such bacteria as coliforms and clostridia, but introduces new difficulties because the starter organisms and lactic acid formation are also inhibited by the salt. This inhibition is not a problem when the pH of the curd granules is allowed to reach a relatively low value prior to the application of salt, as in Cheshire and Stilton manufacture. However, the manufacture of a dry-salted cheese in the medium pH range (5.0-5.4), such as Cheddar, is more difficult than that of the Gouda-type cheeses in which the pH is controlled by limiting the lactose content of the curd by the addition of water to the curds/whey mixture in the vat. At the time of salt addition, a relatively large amount of lactose is still present in Cheddar curd (Turner and Thomas, 1980). However, this is not detrimental to the quality of the cheese provided that the salt-in-moisture (S/M) level is greater than 4.5% and the cheese is allowed to cool after pressing (Fryer, 1982).

Differences obviously exist in the procedures used for the manufacture of dry- and brine-salted cheeses but these have relatively little effect on the finished cheeses; the production of dry-salted cheeses is similar in principle to that of brine-salted cheeses. Clearly, the rate of solubilization of the casein micelles and the activity of the residual rennet and plasmin in the curd �9 , , i l l k ~ , ,~ f fo , - , t c , , - t , -n . . . . . . i , t l , , ~ , , A . . . . . . l t l n c , t h ~ n k , , . . . . . u~- u t t v v t k u - , - , , . , - k - u l J - ~ . . - j v u,v , , a t T - o o t t t t t - ~ t - - u t , u jv

brining but only during the first few weeks of ripening. There is no evidence to suggest that the mechanisms by which the protein is degraded are affected by the changes in salt concentration as the salt diffuses into the curd. Any differences between dry- and brine- salted cheeses of the same overall chemical composition will therefore decrease as the cheeses age.

Traditional Cheddar cheese is visually different from the common brine-salted cheeses such as the Gouda- and Swiss-type cheeses, which are more plastic in texture and have 'eyes'. However, both these characteristics are a result of the relatively high pH and moisture of these cheeses and not of brine-salting itself. The texture of a brine-salted cheese is less open than that of traditionally-made Cheddar cheese because the curd is pressed under the whey to remove pockets of air before brining. As a close texture is a pre-requisite for the formation of 'eyes', it has come to be generally believed that 'eyes' can be obtained only in brine-salted cheese. The technique of vacuum pressing allows the removal of air from between the particles of dry-salted curd. This can result in a closeness of texture similar to that of Gouda-type cheeses. Therefore, it is now possible to manufacture dry-salted cheese with 'eyes' provided that the chemical composition is similar to that of tradi-

tional brine-salted cheeses and if the starter contains gas-producing strains (Lawrence et al., 1993).

Present and future role of Cheddar-like cheeses

Traditionally, Cheddar was a so-called 'table cheese' and was purchased by the consumer shortly before consumption. In line with the global changes in the dairy and food industries (Creamer etal., 2002), cheese, Cheddar in particular, is commonly purchased from the manufacturer, repackaged, often in vacuum packs, and sold on to supermarkets or food wholesalers. It is also used as the base material for a range of processed cheeses ('Pasteurized Processed Cheese and Substitute/ Imitation Cheese Products', Volume 2) and 'cheese- food' products ('Cheese as an Ingredient', Volume 2). Because of our understanding of the factors controlling the development of Cheddar cheese flavour and texture during maturation, it is possible to produce cheeses with a range of pre-determined characteristics using semi-automated mechanized manufacture ('General Aspects of Cheese Technology', Volume 2).

Cheese, as a major ingredient in a food, needs to fulfil certain requirements, such as retention of the flavour and textural characteristics it confers on the food over a substantial storage period. This is coupled with strict composition and price criteria. A good example of meeting this challenge is outlined in detail by Chen and Johnson (2001) in producing a dry-salted cheese using a mesophilic starter suitable for hot-melt products, such as Pizza pies, without using the pasta- filata (Mozzarella) process.

During the latter half of the twentieth century, there were a number of significant changes to the way in which Cheddar cheese is manufactured. The single most important factor supporting those changes has been the availability of reliable starter cultures. The successful development of continuous mechanized systems for Cheddar manufacture has depended upon the ability of the cheesemaker to control precisely both the expulsion of moisture and the increase in acidity required in a given time. This in turn has led to the recognition that the quality of cheese, now being made on a very large scale in modern cheese plants, can be guaranteed only if its chemical composition falls within pre-determined ranges. Nevertheless, Cheddar cheese is still a relatively difficult variety to manufacture because the long ripening period necessary for the development of the required mature flavour can also be conducive to the formation of off-flavours. In addition, its texture can vary considerably. The intermediate position of

Cheddar Cheese and Related Dry-salted Cheese Varieties 73

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Swiss

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Cheddar

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Stilton

I I I I I I I I I

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mmoles calcium/kg solids-not-fat

Classification of traditionally manufactured cheese varieties by their characteristic ranges of the ratio of calcium to solids-not-fat and pH.

Cheddar cheese in the total cheese spectrum (Lawrence et al., 1984) (Fig. 1) is particularly exemplified by its textural properties, which lie between the crumbly nature of Cheshire and the plastic texture of Gouda.

The traditional manufacture of Cheddar cheese consists of: (a) coagulating milk, containing a starter culture, with rennet; (b) cutting the resulting coagulum into small cubes; (c) heating and stirring the cubes with the concomitant production of a required amount of acid; ( d ) w h e y removal; (e) fusing the cubes of curd into slabs by cheddaring; (f) cutting (milling) the cheddared curd; (g) salting; (h) pressing; (i) packaging and ripening (Fig. 2). Although it is impossible to separate the combined effects of some of these operations on the final quality of the cheese, they will, as far as possible, be considered individually.

Effect of milk composit ion and starter culture

Cheesemaking basically involves the removal of moisture from a rennet-induced coagulum (Fig. 3). The four major factors involved are the proportion of fat in the curd, the curd particle size, the cooking (scalding) temperature and the rate and extent of acid production

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The main factors in the expulsion of moisture from a rennet-induced milk coagulum.

(Whitehead and Harkness, 1954; Lawrence et al., 1983; Johnston et al., 1991). In order to achieve uniform cheese quality in large commercial plants, the manufacturing procedures must be as consistent as possible. The first requirement is uniformity of the raw milk. This is achieved by bulking the milk in a silo to even out differences in milk composition from the various districts supplying milk to the cheese plant. Preferably, the milk should be bulked before use so that its fat content can be standardized accurately. For Cheddar cheese varieties, the milk is normally standardized to a casein/fat ratio between 0.67 and 0.72. The more fat present in the cheese milk, and therefore in the rennet coagulum, the more difficult it is to remove moisture under the same manufacturing conditions because the presence of fat interferes mechanically with the syneresis process. Standardization has traditionally involved manipulation of the fat content of the cheese milk to give a specific casein/fat ratio. This is usually achieved either by partially removing the fat from the whole milk stream or by removing all the fat from the whole milk and adding back a portion to the skim milk stream. However, recent developments in membrane

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A typical manufacturing schedule for Cheddar cheese.


processing technologies have meant that the protein component of the whole milk can now be standardized also. There are a number of options by which the protein content of cheese milk can be standardized. An example is concentrating the level of protein in a skim milk stream by uhrafihration and adding the retentate back to the whole milk stream to boost the protein concentration in the whole milk to the target level, which is typically between 3.5 and 4%.

The manufacture of Cheddar cheese is more dependent on uniform starter activity than that of washed curd cheeses, such as Gouda. The proper rate of acid development, particularly before the whey is drained from the curd, is essential if the required chemical composition of the cheese is to be obtained (Whitehead and Harkness, 1954; Lawrence etal . , 1984). However, the curd is 'cooked' to expel moisture at a temperature that normally adversely affects the starter bacteria. The cheesemaker must therefore exert judgement to ensure that the desired acid development in the curd is reached at about the same time as the required moisture content.

The starter system used in New Zealand cheese plants is based on the continuous use of a single triplet starter comprising three defined strains of Lactococcus lactis subsp, cremoris selected primarily on the basis of their acid production, phage resistance and flavour development (Heap, 1998). Defined starter systems are now widely used in the United States (Richardson �9 -,~- ,-,1 1 0 Q I ) t ,.,-,1,-, ~ ,--1 ( T ~ o~ ,11 1 ~ Q Q ' ~ C , - , , - , , 1 , - , , . ,A ~ L L,4,L.,~ I ~ ' U ,,j LLr,,,..,LOLILU ~K I L L L L I L I U I I D I~L (LA.L.,~ JL .~rOO) , j J L U I I O L I L L U

and Australia (Heap and Lawrence, 1988" Limsowtin et al., 1996) and have replaced the undefined commercial mixed-strain cultures of the type still used exclusively for the manufacture of Gouda-type cheeses in The Netherlands (Stadhouders and Leenders, 1984). If the cooking temperature is kept constant (for instance at 38 ~ throughout the cheesemaking year and standardized milk is used, by far the most important factor in producing Cheddar cheese of uniform quality is the extent of acid production in the vats. In New Zealand, this is managed successfully in two ways: (a) the use of reconstituted skim milk or suppliers' milk of good quality for the preparation of bulk cheese starter; (b) the ability of the cheese industry to produce neutral- ized bulk cheese starter and to control the ratios of the starter strains added to the cheese milk (Heap, 1998).

To compensate for seasonal changes in milk composition, it is normally necessary only to vary the percentage inoculum of starter to achieve the required acidity at draining.

Effect of coagulant

The amount of rennet added should be the minimum necessary to give a firm coagulum in the set-to-cut time

(time between rennet addition and cutting) required. In Cheddar cheese manufacture, the set-to-cut time is usually in the range 35-45 min. There is a range of animal, microbial and recombinant rennets to choose from and their advantages and disadvantages are discussed in 'Rennets: General and Molecular Aspects', Volume 1. Calf rennet, high in chymosin, has been used traditionally for Cheddar cheese production. The advantage of using a high chymosin content calf rennet is that the flavour and the texture of aged Cheddar are more predictable, with less bitterness. The same could be said for the recombinant chymosins. How- ever, some customers have strong aversions to the use of genetically engineered ingredients in cheese. Some cheese manufacturers are now investigating the use of microbial rennets, which provide the added advantage of being suitable for Kosher, Halal and some vegetar- ian products. In addition, use of microbial rennets in Cheddar cheese production opens up the options for downstream whey products (whey protein concentrates, milk protein concentrates, etc.).

Changes in the volume of rennet added, an increase or decrease in the setting temperature, addition of calcium chloride and/or pH adjustment may be required to avoid any seasonal changes in milk composition and functionality.

The rennet-induced coagulum consists of a continuous network of protein that entraps both water and fat

UL ~LUUULL~.. .~. I ILK.. [ J l UL~..LLL LLK..L~CUL R i S I . . .ULLLL.JU~EU D l l l ~ l t l t

units of protein held together by various forces. Several reports (Eino et al., 1976; Green et al., 1981, 1983) have concluded that the microstructure of the coagulum produced by different types of milk coagulant is a major factor determining the structure and texture of Cheddar cheese. It has been suggested (Green et al., 1981) that 'the structure of the protein network is laid down during the initial curd-forming process and is not funda- mentally altered during the later stages of cheesemaking and that the fibrous and more open framework of curd formed by bovine and porcine pepsins might be a reason for the softer curd associated with their use' (Eino et al., 1976). This implies that different milk coagulants significantly affect the initial arrangement of the network of protein structural units. However, it is more likely that the proportion of minerals lost from the coagulum, as a result of the change in pH throughout the entire process, largely determines the texture of a cheese. As one would expect, the type of rennet used and the amount retained in the cheese curd affect the degree of proteolysis as the cheese ripens (Stanley and Emmons, 1977; Creamer etal . , 1985) (cf. 'Rennet- induced Coagulation of Milk', 'Biochemistry of Cheese Ripening: Introduction and Overview', 'Metabolism of Residual Lactose and of Lactate and Citrate', 'Lipolysis


and Catabolism of Fatty Acids in Cheese', 'Proteolysis in Cheese during Ripening', and 'Catabolism of Amino Acids in Cheese during Ripening', Volume 1).

The early stages of Cheddar cheese manufacture, specifically gel assembly and curd syneresis, have been reviewed (Fox, 1984; Green, 1984) ('Formation, Struc- tural Properties and Rheology of Acid-coagulated Milk Gels', Volume 1). Electron microscopy studies (Kimber et al., 1974; Kalab, 1977; Stanley and Emmons, 1977) have shown that the casein micelles, which are separate initially, aggregate, coalesce and finally form a multi-branched casein network. The fat globules, also separate at first, are gradually forced together as a result of shrinkage of the casein network. After the coagulum is cut, the surface fat globules are exposed and washed away as the curd is stirred. This leaves a thin layer depleted of fat at the curd granule surface. During matting, the layers of adjacent curd granules fuse, leading to the formation of fat-depleted junctions (Lowrie et al., 1982). Starter bacteria are trapped in the casein network near the fat-casein interface, which has been shown to be the region of highest water content in the mature cheese (Kimber et al., 1974). In all cheese varieties, the outline of the original particles of curd formed when the rennet-induced coagulum is cut can be readily distinguished by scanning electron microscopy (Kalab et al., 1982). In addition, in traditionally-made Cheddar cheese, the boundaries of the milled curd pieces can be seen (Lowrie et al., 1982). These curd granules and milled curd junctions in Cheddar cheese are permanent features, which can still be distinguished in aged cheese.

Effect of cutting

The objective of cutting the coagulum, and indeed the objective of the heating and stirring stages that follow cutting, is to facilitate syneresis ('The Syneresis of Rennet-coagulated Curd', Volume 1). However, the cutting operation, together with the speed of stirring following cutting, also influence how large the particles will be at draining and how much of the original milk components (fat and protein) are lost to the whey. The size distribution of the particles at draining is one of the key factors for controlling the moisture content of cheese. The larger the particles, the more moisture that is retained (Whitehead and Harkness, 1954). Maximizing moisture (or moisture in the non- fat substance (MNFS)) and minimizing losses (fat and cheese fines) to the whey will ensure the highest possible yield and profitability (Lawrence and Johnston, 1993). Therefore, cutting is a key operation in cheesemaking and influences not only the composition but also the yield of the finished cheese.

Johnston et al. (1991) showed that the speed and duration of cutting in 20 000 1 Damrow cheese vats during commercial Cheddar cheese production determines the curd particle size distribution at draining and hence the moisture content of the final cheese. The whey fat losses could be minimized by the choice of the cutting protocol used. They concluded that, as cutting proceeded, the particle size distribution increasingly favoured smaller particles and that there were two different effects (Fig. 4). In region I, where the cutting cycle is too short, large curd particles remaining after cutting will be reduced in size by smashing during the subsequent stirring phase. Smashing results in small curd particles and fines at draining and high whey fat losses. Between regions I and II, the curd particle size following cutting is small enough to avoid smashing during subsequent stirring and therefore the curd particle size is at a maximum and whey fat losses are at a minimum. In region II, continued cutting gives rise to a greater proportion of smaller curd particles and, in the absence of smashing, whey fat losses remain low.

Based on this explanation, Johnston et al. (1991) proposed a model (Fig. 5) for cutting that explains how variations in cutting speed and duration of cutting, followed by a constant stirring speed, determine the curd particle size distribution in a Damrow cheese vat. Each of the five curves (Fig. 5) represents the variations in curd particle size distribution with the duration of cutting, for a constant speed of cutting. Each curve depends on the duration of cutting and is characterized by a specific duration of cutting at which the curd particle size is at a maximum. As the cutting speed is reduced and the duration of cutting is increased to avoid shattering during stirring, the maximum curd particle size increases. Cutting beyond a certain duration, irrespective of the speed of cutting, does not further reduce the curd particle size.

A similar study (Johnston et al., 1998) using Ost vats (30 000 1) showed similar trends. However, the Ost vat study also showed that, although similar, the trends were sufficiently different to warrant the charac- terization of each vat type as to the effect of the speed and duration of cutting, before implementing a specific cutting regime.

Effect of heating (cooking) the curd

During cooking, the curds are heated to facilitate syneresis and aid in the control of acid development. The moisture content of the curds is normally reduced from approximately 87% in the initial gel to below 39% in the finished Cheddar cheese. The expulsion of whey is aided by the continued action of rennet as

76 C h e d d a r C h e e s e a n d R e l a t e d D r y - s a l t e d C h e e s e V a r i e t i e s

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Curves showing the effect of the speed and duration of cutting on the proportion of particles <7.5 mm. Cutting speeds were: (�9 2 rev/min; (A), 4 rev/min; (A), 5 rev/min; (I-1), 6 rev/min; ( i ) , 8 rev/min. The continuous lines show the data of Johnston et al. (1991) and the dotted lines are the anticipated trends.

well as the combined influence of heat and acid. The temperature should be raised to 38-39 ~ over a period of about 35 min. The curds shrink in size and become firmer during cooking.

A c i d p r o d u c t i o n at t h e v a t s t a g e

The single most important factor in the control of Cheddar cheese quality is the extent of acid production in the vat (Fig. 6) because this largely determines its final pH (Lawrence and Gilles, 1982; Creamer et al., 1988) and the basic structure of the cheese (Lawrence et al., 1983). As the pH of the curds decreases, there is a concomitant loss of colloidal calcium phosphate from the casein sub-micelles and, below about pH 5.5, the sub-micelles dissociate into smaller aggregates (Roefs et al., 1985). As the amount of rennet added and the temperature profile are normally constant in the manufacture of Cheddar cheese, the pH change in the curd becomes the important factor in regulating the rate of whey expulsion (Van Slyke and Price, 1952; Lawrence et al., 1984). In mechanized cheesemaking systems, the cheese is usually made to a fixed time schedule. In New Zealand, the time between 'setting' (addition of milk coagulant) and 'running' (draining of whey from the curds; also called 'pump out') is normally 2 h 40 min +_ 10 min. The percentage of starter added determines the increase in titratable acidity or


Acid production in vat

%> Decrease in pH of curd

Loss of calcium and phosphate

pH and mineral content of curd at draining

Basic structure of cheese

%> Breakdown of casein network during ripening

Cheddar texture and flavour

Relationship between the extent of acid production up to the draining stage and the production of Cheddar cheese flavour and texture.

decrease in pH between 'cutting' and 'running'. The extent of acidity increase at this stage is particularly important because it also controls the increase in acidity from 'drying' (when most of the whey has been removed) onwards (Dolby, 1941). The actual increase in acidity may need to be adjusted at intervals throughout the year to achieve the required pH in the cheese at 1 day. This depends upon changes in the chemical composition of the milk, which, in turn, are determined by both the feed of the cow and the lactational cycle. The pH at draining also determines the proportions of residual chymosin (calf rennet) and plasmin in the cheese (Holmes et al., 1977; Lawrence et al., 1984; Creamer et al., 1985). Chymosin plays a major role in the degradation of the caseins during ripening and in the consequent development of characteristic cheese flavour and texture.

While curds remain in the whey, there is a contin- ual transfer of lactose to the curds. The whey thus provides a reserve of lactose which prevents any great decrease in lactose concentration in the curd. After the whey has been removed, this reserve is no longer available and the lactose content of the curd falls rapidly as the fermentation proceeds. Curd that has been left in contact with the whey for a longer period has a

higher lactose content than curd of the same pH value from which the whey has been removed earlier (Dolby, 1941; Czulak et al., 1969). Acid production can be under complete control only if defined starter systems, such as the single triplet starter, where the individual strains have been selected based on their sensitivity to the manufacturing temperature profile, are used (Lawrence and Heap, 1986; Heap, 1998). Use of this culture has allowed New Zealand cheesemakers to reduce the time from 'set' to 'salt' to about 4 h 30 min. Even shorter times are potentially possible but these are limited by the rate at which moisture can be expelled from the curds in the traditional Cheddar process. Experience has shown that it is preferable to produce lactic acid relatively slowly during the early stages of curd formation and cooking, followed by an increasing rate after draining the whey from the curds. This procedure retains more of the calcium and phosphate in the curd.

A recent trend in Europe has been to include a thermophilic strain in starter blends comprising mesophilic strains used for making pressed cheeses (Beresford and Cogan, 1997), as well as soft-ripened cheeses. The rationale for the inclusion of this strain would appear to be in terms of providing a relatively slow rate of acid production at a low temperature of manufacture (high-pH, white-mould cheeses). How- ever, phage attack of the mesophilic strains in these starter blends has led to variable rates of acid production in the cheese vats and problems controlling the final moisture content of the cheese (Heap, personal observation).

Effect of cheddaring

The series of operations consisting of packing, turning, piling and re-piling the slabs of matted curd is known as cheddaring. The curd granules fuse under gravity into solid blocks. Under the combined effect of heat and acid, matting of the curd particles proceeds rapidly. The original rubber-like texture gradually changes into a close-knit texture, with the matted curd particles becoming fibrous. The importance attached to flow in the past varied markedly from country to country. In Britain, it was common for each Cheddar block to be made to spread into a thin, hide-like sheet covering an area of about a square metre, whereas in New Zealand only moderate flow was induced, the final Cheddar block being little different in dimensions from when first cut. Czulak and his colleagues (Czulak and Hammond, 1956; Czulak, 1958, 1959; King and Czulak, 1958) initially concluded that extensive deformation and flow were essential in Cheddar cheesemaking. However, further research in Australia


(Czulak, 1962), New Zealand (Harkness et al., 1968) and Canada (Lowrie et al., 1982) slowly led to the view that 'cheddaring' is not an essential step and serves no purpose other than to provide a holding period during which the necessary acidity develops and further whey can be released from the curds. This loss of whey is controlled by the acidity and temperature of the curd. The temperature is important, both directly and indirectly, because the rate of acid development is also influenced by temperature. In general, a higher temperature during cheddaring increases the expulsion of whey from the curds. In the traditional process, manipulations of the curd, i.e., the cutting of the matted curd into different sized blocks, the height of piling and the frequency of turning the curd blocks, also aid in moisture control.

Mechanical forces- pressure and f l o w - have been shown (Czulak and Hammond, 1956; Czulak, 1959) to be an important factor in the development of the fibrous structure in the curd. This is clearly seen in the arrangement of the fibres, which follow the direction of the flow. However, a fibrous structure cannot be brought about by pressure and deformation unless the curds have reached a pH of 5.8 or less (Czulak, 1959). This suggested that pressure and flow serve to knit, join, stretch and orientate the network of casein fibres already partly formed in response to rising acidity. The readiness to flow, the type of fibres and the density of t h p i r n p t ~ x z n r b a r o , J l c n i n f l l l o n e o r h , r t o m n o r a t , , r o a n d L . L l ~ 4 .L t l .L~,-- L Y V ~_P l I~L. q,.~ Jl_ lh,.~ ( ~ l ~,_J l . J J. I l R l IL,~ ~..- l R 14,.. ~,.~ I~.AL JL_J 7 L I~ . . l l l L .P q~.- t G& L L.& t ~,.- G,41, l 1. ~*,L

moisture. The warmer the curd and the higher its moisture content, the more readily it flows and the finer, longer and denser are the fibres. Czulak (1959) also concluded that it is possible to influence curd structure by manipulating pH, pressure and temperature and that a direct relationship exists between the structure and the water-holding capacity of the curd. This was confirmed by Olson and Price (1970), who showed that extension and rapid flow of curd during cheddaring produced a higher moisture content in the resulting cheese.

Fluorescence microscopy has demonstrated the change of the casein from spherical granular particles to a fibrous network (King and Czulak, 1958). Whereas some granular structure was evident in curd grains, the conversion to the fibrous form was complete in cheddared curd. The fibrous shreds of cheddared curd consist of flattened, elongated curd particles that overlap each other, forming a network- type structure with the protein as a continuous phase. The exact mechanism responsible for these observed changes in cheddared curd is not known with cer- tainty but the loss of minerals from the casein micelles in the curd is likely to be the major factor. The loss of calcium phosphate will destabilize the casein micelles,

resulting in a change in the conformation of the caseins. The concomitant loss of moisture from the casein micelles may also possibly contribute to the conforma- tional change.

Czulak (1962) concluded that the characteristic close texture of Cheddar cheese could be obtained without cheddaring. However, he suggested that in mechanizing the cheesemaking process it was probably most convenient, while holding the curd for acidity to develop, to allow the particles to mat together 'but to apply no labour or equipment for its fusing beyond that necessary for ready handling'. Almost all modern mechanized Cheddar cheesemaking systems are based upon these conclusions and involve little or no flow of the curd mass. This development was supported by the success achieved in the manufacture of cheese of normal Cheddar characteristics, particularly in the United States, by 'the stirred curd' process. This strongly indicates that flow and the cheddaring process itself are of little or no significance in the Cheddar cheesemaking process. Similar conclusions were also reached by research workers (Harkness et al., 1968) in New Zealand.

Effect of milling

The milling operation consists of mechanically cutting the cheddared curd into small pieces in order to: (,~'~ i . . . . . . . t h . . . . . f ~ . . . . . . . f t h . . . . . A a r i a . . . . . ]--~lo ~ 4 [ ~ ] L L t1~. . t q~ . .GAO %.. L L t%..- O I.~. l t (r~l, qJ,,~ I~.. (r..~L L %... f..L U L t l Ilk,. ~. . IL~L L ql~L ( ~ L L R 3 1 k . / V L . L ( . ~ U L q ~

more uniform salt distribution into the curd; (b) encourage whey drainage from the curd; (c) assemble the curd in a convenient form for hooping or block-forming. There is a practical upper limit to the cross-section of milled curd before salting for two reasons: (a) there is inadequate whey drainage after salting with large particles; (b) the larger the curd particles, the smaller is the surface/volume ratio. With larger particles, a higher salting rate is therefore required to achieve a given final level of S/M in the cheese. This increases the chance of seaminess (Conochie and Sutherland, 1965a) and gives higher salt losses in the whey (Gilles, 1976). The longer time required for salt penetration allows a greater development of acid in the centre of large curd particles than in smaller particles and this may result in a 'mottled' appearance of the final cheese.

Gilbert (1979) pointed out that ideally the curd should be cut into spheres to obtain a uniform mass/surface area profile. However, the best that can be achieved (Breene et al., 1965; Gilbert, 1979) is to use a curd mill that produces a shredded curd, flakes of curd or tinge>like pieces of curd. The curd mill speed can be increased or decreased to change the curd particle size and shape, which in turn affect the


cheese S/M ratio (Samal, personal observation). The more uniform the ratio of surface area to curd mass after milling, the more uniform will be the rate of salt diffusion into the milled curd particles and more consistent will be the amount of salt retained. It is worth noting that these conditions are more closely satisfied if the curd is not cheddared but is kept in the granular state prior to salting. Milling has little role in granular curd cheesemaking; see 'Stirred curd or granular cheese'.

Mellowing prior to salting In the traditional procedure for Cheddar cheese manufacture, the milled 'chips' were left until the newly cut surfaces glistened as a mixture of whey and fat exuded from them. The mellowing period provided time to produce sufficient surface moisture to dissolve the salt crystals when they were applied and gave rise to better salt retention. The purpose of the traditional mellowing period ('dwell time') was to allow for further moisture release and acidity increase. In modern mechanized Cheddar cheese plants, salt is added to the curd pieces immediately after milling, and continuous agitation of the milled particles is used to encourage whey flow and salt absorption.

Effect of salting

Salt (and more specifically S/M) plays a number of roles in the quality of Cheddar cheese by controlling: (a) the final pH of the cheese (Thomas and Pearce, 1981; Lawrence and Gilles, 1982), (b) the growth of microorganisms, specifically starter bacteria and undesirable species such as coliforms, staphylococci and clostridia, and (c) the overall flavour and texture of the cheese. The S/M level controls the rate of proteolysis of the caseins by the rennet, plasmin and bacterial proteases. Proteolysis, and thus the incidence of bitterness and other off-flavours, decreases with an increase in salt concentration (Thomas and Pearce, 1981; Pearce, 1982). At S/M levels >5.0%, bitter flavours are rarely encountered (Lawrence and Gilles, 1969); below this level there is more or less an inverse linear relationship between S/M and the incidence of bitterness. General aspects of salt in cheese are considered in 'Salt in Cheese: Physical, Chemical and Biolog- ical Aspects', Volume 1; some specific aspects in relation to Cheddar are considered below.

Salting of milled curd The salt crystals dissolve on the moist surfaces of the milled curd particles and form a brine. This diffuses into the curd matrix through the aqueous phase, causing the curd to shrink in volume, and more whey is

thereby released to dissolve more salt. The proportion of moisture in the curd and the amount of salt added both affect the rate of solution of the salt. The high salt content of the surface of the milled curd particles reduces the tendency of the particles to fuse together. The difference between dry-salting and brine-salting is, in effect, the availability of water at the surface of the curd. With brine-salting, salt absorption begins immediately; release of whey occurs, as in dry-salting, but is not a pre-requisite for salt absorption.

In modern cheese plants, it is essential that the curd particles prior to salting are consistent from day to day with respect to moisture content, particle size and shape, acidity level and temperature, and that the application of salt is uniform. This gives the cheesemaker control over both the mean salt content and, equally important, variations (standard deviation) within a day's manufacture. Cheese specifications normally require both moisture and S/M to be within specified ranges. This means that in practice variations in the moisture content of the curd prior to salting must not be greater than ___ 1%.

It has been suggested (Sutherland, 1974; Gilbert, 1979) that the size of the salt crystals used is important for both salt uptake and moisture control. In practice, however, the major requirement in mechanized cheese plants is that the size range of the salt crystals should be narrow. If the range is variable, the delivery of salt from the equipment is erratic. The presence of large amounts of very fine crystals also results in excessive salt dust within the plant environment.

Although salt promotes syneresis, it should not be used in mechanized Cheddar cheesemaking as a means of making a significant adjustment to the moisture content of the curd. However, in practice, because of variations in milk buffering capacity, starter activity and plant breakdowns, day-to-day variations in the curd pH and moisture are not uncommon. Therefore, slight adjustments are made to the quantity of added salt to attain consistency in the salt and moisture contents in the curd.

The salting techniques commonly used in mechanized cheesemaking are: boom-salting, in one or two stages, and trommel salting. The former uses salt addition to the curd on a mass/volume ratio whereas the latter uses a mass/mass ratio. More details on salting systems are included in 'General Aspects of Cheese Technology', Volume 2.

Mellowing after salting Sufficient time must be allowed after salting (the mellowing time) to ensure the required absorption of salt on the curd surface and continued free drainage of whey. It was suggested earlier that the curd could be


hooped as soon as it had been salted. However, this led to problems in cheese made by these shorter processes (Czulak, 1963), specifically to the entrapment of whey and consequently to excessive moisture and uneven colour in the cheese. As a result, a number of investigations have been carried out to determine the factors that influence the amount of salt absorbed and the speed of its absorption (Breene et al., 1965" Suther- land, 1974; Gilles, 1976; Gilbert, 1979).

The amount of salt absorbed by the curd and the rate of subsequent whey drainage are related to the availability of dissolved salt on the curd particle surfaces, and to the physical characteristics of the curd, e.g., fat-free curd allows faster diffusion (Sutherland, 1977). Even when a mellowing time of more than 30 min is maintained and the level of salt addition is uniform, large variations may still occur in the salt content of cheeses because other conditions that affect salt absorption are not controlled. For instance, the curd temperature, the depth of curd, the extent of stirring after salt addition and the degree of structure development in the curd are also significant factors in the control of salt absorption and subsequent whey drainage (Sutherland, 1974; Gilles, 1976). Therefore, it is not surprising that there have been conflicting reports as to how long the mellowing time after salting should be. It is clear that holding for at least 15 rain is necessary to minimize the loss of salt during pressing (Rreene at a l 1065"1 Other renort.~ ,~lltJ~e.qt that x . . . . . . , . o - - r - - - c ~ o . . . . . .

the pressing of the salted curd should be delayed for at least 30 min (Gilles, 1976) and preferably for 45-60 min (Breene et al., 1965). Some loss of salt occurs even when the mellowing time is extended to 60 min. However, an increase in the mellowing time sub- stantially reduces the proportion of whey expelled during pressing and greatly improves the degree of salt absorption (Sutherland, 1974). Mechanized cheese plants nowadays have a mellowing time of 20-40 min, which is usually adequate for satisfactory salt uptake and whey removal (C.G. Honor~ and P.K. Samal, unpublished results).

The irregular effect of curd temperature on the extent of salt absorption was thought (Breene et al., 1965) to be caused by a protective layer of fat exuding from the surface of curd particles. Less fat was present on curd surfaces at 26 ~ than at 32 ~ Above 38 ~ such fat was melted and dispersed in the brine solution that was present on the surface. In general, however, a decrease in the curd temperature at salting increases the S/M of the final cheese (Sutherland, 1974). Curd salted at a high pH retains more salt (Dolby, 1941) and is more plastic than curd salted at a low pH. Similarly, for a given salting level, the S/M is high when the titratable acidity is low (Gilles, 1976).

Salting the curd under the most favourable conditions for salt absorption reduces the proportion of salt required and reduces salt losses (Gilles, 1976) and also helps to overcome the defect of seaminess (Czulak, 1963; Czulak et al., 1964).

Equilibration of salt within a cheese The rate of penetration of salt into cheese curd is very slow (McDowall and Dolby, 1936" Guerts et al., 1974; Sutherland, 1977; Morris et al., 1985) and a mean diffusion rate of 0.126 cm2/day for salt in the water of Cheddar cheese has been reported (Sutherland, 1977). This corresponds well with salt migration values for Gouda cheese of the same moisture content (Guerts et al., 1974), suggesting that the matrix structures of the two cheese types are similar. Despite the low rate of salt diffusion, it was nevertheless believed that the S/M concentration in Cheddar cheese was essentially uniform within a few days (McDowall and Dolby, 1936). Reports (Morris, 1961" Fox, 1974; Sutherland, 1977; Thomas and Pearce, 1981) suggested that wide variations occurred in salt content between blocks from the same vat and even within a block. Also, there was appreciable variation in the salt and moisture contents of small plug samples taken from different cheeses from the same vat (Sutherland, 1977; Thomas and Pearce, 1981). With increased plant mechanization and automation, better designed cheese vats and improved salting devices, the S/M variation within a block of cheese and between blocks of cheese from the same vat is reasonably well controlled. As the consistency of cheese flavour is directly related to the extent of variability in S/M, the need to produce a curd mass consisting of particles of uniform cross-section at the time of salting cannot be over-emphasized (Fig. 7).

Moisture , , ' content of curd

Acidity development

Curd particle Curd acidity/pH cross-section

at salting at salting

. . . . . 1~> Salt uptake <~

Salt-in-moisture I

Salting rate

The main factors that affect the salt uptake and S/M level in Cheddar cheese.


Seaminess and fusion When curd particles are dry-salted, discrete boundaries are set up between the individual particles, in contrast to brine-salted cheeses where there is only one boundary, i.e., the cheese rind or exterior. The addition of dry salt causes shrinkage of the curd and a rapid rate of release of whey containing calcium and phosphate, particularly in the first few minutes of pressing. It has been suggested that the salted surface of the curd particle acts as a selective permeable membrane, thereby concentrating calcium and phosphate at the surface of the curd particle (McDowall and Dolby, 1936). It is possible that this calcium gradient is also accentuated, under some circumstances, by the variations in pH between the surface and the interior of the salted curd particle owing to inhibition of starter activity by the high salt concentration at each curd boundary. The establishment of a pH gradient leads, in turn, to a shallow calcium gradient (Le Graet et al., 1983), the magnitude of which will depend on the size of the curd particle and the proportion of salt added. In its most extreme form, the deposition of calcium phosphate crystals results in the phenomenon of seaminess in Cheddar cheese (Czulak, 1963; Conochie and Sutherland, 1965a; A1-Dahhan and Crawford, 1982), a condition in which the junctions of the milled curd particles are visible after pressing. Seami- ness is more frequent and more marked with cheese of low moisture and high salt content and in some cases persists after the cheese has matured (Czulak et al., 1964). The binding between curd particles is usually weak, due to incomplete fusion. This often leads to crumbling when the cheese is sliced or cut into small blocks for packing.

Photomicrographs show that, in both seamy and non-seamy Cheddar cheese, crystals of calcium ortho- phosphate dihydrate are dispersed throughout the cheese mass (Conochie and Sutherland, 1965a), but in seamy cheese they are concentrated in the vicinity of the surfaces of the milled curd particles to which salt was applied. To a depth of about 20 b~m below these surfaces, the protein appears to be denser than elsewhere, suggesting that severe dehydration of the surface occurs on contact with dry salt. The observation (Van Slyke and Price, 1952; Czulak, 1963) that seaminess is reduced by washing the curd after milling and before salting can be explained by the removal of calcium and/or phosphate from the surface layer. In addition, the provision of more water will lessen the dehydrating and contracting effect of salt on the surface layer.

Seaminess and poor bonding between the curd particles occur together and treatment with warm water corrects both defects. Poor fusion of the curd as a con-

sequence of heavy salting results from changes in the protein at the surface, from poor contact between the hardened surfaces, from the physical separation brought about by the presence of salt crystals and, when these have disappeared, from the growth of the calcium ortho-phosphate crystals (Conochie and Sutherland, 1965a). Fusion of the particles is improved by an increase in the pH, temperature or moisture content of the curd.

Effect of pressing

Traditionally, Cheddar cheese was pressed overnight using a batch method. The development of the 'block- former' system (Wegner, 1979; Brockwell, 1981; Tamime and Law, 2001; 'General Aspects of Cheese Technol- ogy', Volume 2) offered two major advantages for modern cheesemaking plants: firstly it is a continuous process and secondly the residence time is reduced to about 30-45 min. The curd is fed continuously into an extended hoop (tower) under a partial vacuum, and mechanical pressure is applied at the base of the tower for a very short period, usually for about 1 min.

In traditionally made Cheddar, the two common types of textural defect are mechanical- and slit- openness. Mechanical-openness (occurrence of irregu- larly shaped holes) is evident in very young cheese but decreases markedly during the first or second week after manufacture and changes little thereafter (Czulak et al., 1962; Irvine and Burnett, 1962; Price et al., 1963). How- ever, in Cheddar cheese blocks from block-formers, mechanical-openness is barely visible immediately after manufacture and becomes prominent at about 4 weeks after manufacture (Samal, personal observation). Slit- openness is usually absent in freshly made cheese (Robertson, 1965a) but develops during maturation (Hoglund et al., 1972a). The extreme expression of this defect, known as fractured texture, is found only in mature cheese. A comprehensive survey of commercial Cheddar cheese on the United Kingdom market carried out during 1958-1961 showed that mechanically open cheese was usually almost free of fractures and con- versely that badly fractured cheese usually had few mechanical openings (Robertson, 1965b). The term 'fracture' is normally used to describe long slits, i.e., slits longer than about 3.5 cm. As a result of the growth of the cheese pre-packaging trade, the importance attached to fractures in cheese has greatly increased because fractures can result in the break-up of cheese during pre- packaging. The basic mechanisms for the formation of openness in Cheddar cheese depend firstly on mechanical-openness, i.e., microscopic nuclei or larger air spaces in the cheese structure, and secondly on gas production by microorganisms (Martley and Crow, 1996).


From the observation (Walter etal . , 1953) that cheese hooped under whey had a completely close texture and from their own studies of curd behaviour, Czulak and Hammond (1956) concluded that air entrapped during compression of curd was responsible for mechanically open texture. They considered that during compression of the salted, granular curd, the spaces between the granules diminish until they form a complex of narrow channels filled with air. Under further pressure, some of the air is forced out, the escape of the remainder being blocked by closure of the channels at various points and the high surface tension developed by traces of whey in the remaining narrowed outlets. The isolated pockets of trapped air form numerous small irregular holes in the cheese. Conventionally-made Cheddar cheese has a significantly closer texture than Granular cheese. The effect of cheddaring on texture appears to be due to the presence of milled strips of curd (fingers) compared with the relatively small granules of uncheddared curd present in Granular cheese. The larger the fingers of curd, the fewer are the pockets of trapped air and the closer is the texture of the cheese. As mentioned previously, however, there is a practical limit to the size of milled curd because large curd fingers may result in inadequate whey drainage after salting.

During the last 50 years, there has been a marked reduction in the incidence of both texture defects by: (,-,'~ t h~ , , c , . ,-,f h i , ~h , . , - , - , . - , . c c , , , - , . c A , , , - i , ~ ,~ r f , , c i r ~w

(Whitehead and Jones, 1946)" (b) the change from the manufacture of 36 kg rinded cheese to smaller 20 kg rindless cheese" (c) the introduction of vacuum pressing; (d) the use of defined single-strain cultures from which gas-producing strains have been omitted. The beneficial effects of these modifications are undoubtedly associated with a reduction in the gas content of the cheese. The production of carbon dioxide during ripening by non-starter bacteria has been associated with the development of slit-openness (Hoglund et al., 1972a) but gas production is considered to be of sec- ondary importance compared with manufacturing conditions (Hoglund et al., 1972b). It is the relatively insoluble and biologically inactive nitrogen in the entrapped air that contributes to the ultimate openness of the cheese because the oxygen is rapidly metabolized during ripening.

Vacuum pressing It was a logical step to prevent the entrapment of air between the curd particles by pressing the curd under vacuum, a procedure first patented in Canada by Smith etal . (1959). A moderately high vacuum, approximately 33 kPa pressure, is required. Vacuum- treated cheese is free, or almost so, of mechanical-

openness when 2 weeks old and remains free throughout maturation (Robertson, 1965a). There was some disagreement among the various research groups as to the optimum conditions for vacuum pressing (Robertson, 1965b). Initially, the cheddared and salted curd was pre-pressed under vacuum for 30 min before d res s ing - or trimming, followed by normal pressing (Czulak et al., 1962). Later work suggested that pres- sures greater than 180 kPa appeared to be required during and after vacuum pressing to achieve a close texture (Robertson, 1965b).

An important development in Australia was the hooping of granular, salted curd and pressing under vacuum (Czulak, 1962). It was found that the use of vacuum pressing ensured the characteristic close texture of Cheddar cheese and thus eliminated the need for cheddaring. This observation was particularly significant for the complete mechanization of Cheddar cheese manufacture. Trials in New Zealand (Robertson, 1965a) quickly confirmed the Australian conclusions. Maximum reduction in openness was achieved with the combined use of vacuum pressing of granular curd and a homofermentative starter (Hoglund etal . , 1972a). Presumably, air can be removed more readily by vacuum from granular curd than from the closer textured cheddared curd. The technique used in the 'block-former' system of filling hoops under a partial vacuum is particularly effective in achieving a close t o v t , l r o ( R v , ~ , " b , z , o l l 1 0 ~ 1 " T a m i m o an,-] 1 a,xT ")(1(11) L1~.x~ L ~ t - ~. \ , JL J~J - - , l...~J V -- .

Mechanical-openness is rarely found in cheese made by the 'block-former' system, if the recommended operating procedures are practised in the block- former, e.g., filling time, residence time, pressing time and appropriate vacuum. However, slit-openness does develop if gas-producing organisms are present.

A factor that formerly restricted the size of Cheddar cheese blocks was the tendency for large cheeses to show severe mechanical-openness. With the aid of vacuum pressing, it has been found quite practicable to form curd into very large blocks (Robertson, 1967) which by extrusion into cutting equipment can be sub- divided into 20 kg blocks.

Rapid cooling An unwelcome side effect of large blocks is that a temperature gradient is set up within the block because of the relatively slow cooling of the block interior. This, and the demands of the marketplace for evenly ripened blocks of cheese, has necessitated the introduction of rapid cooling of blocks by placing them in open stacks in well-ventilated areas of the cool room or in especially designed cooling devices. Generally, the core temperature of each block is cooled to below 18 ~ in 24 h to keep the growth of the non-starter


lactic acid bacteria to a minimum (Fryer, 1982). Fur- ther details on the rapid cooling process and equipment are provided in 'General Aspects of Cheese Technology', Volume 2.

Developments in cheese marketing, coupled with increasing consumer standards, have resulted in a demand for cheese of greater uniformity of composition than in the past. Such uniformity is best achieved by a grading system based on compositional analysis, because the relationship between the composition and the quality of Cheddar cheese is now well established (Robertson, 1966; Lyall, 1968; O'Connor, 1971; Gilles and Lawrence, 1973; Fox, 1975; Pearce and Gilles, 1979). Lyall (1968) briefly reported on a procedure for evaluating chemical analyses of cheese, points being assigned on the basis of composition. However, the only scheme in commercial use for assessing Cheddar cheese quality by compositional analysis appears to be that proposed by Gilles and Lawrence (1973). Sug- gested ranges of MNFS, S/M, fat-in-dry matter (FDM) and pH for both first and second grade cheeses are given in Fig. 8. All New Zealand export Cheddar cheese is subject to compositional grading to ensure that it meets the appropriate specification. In addition, a sensory flavour assessment is carried out to ensure that the cheese is free from flavour defects (Lawrence and Gilles, 1980). Burton (1989) concluded that grad-

ing on the basis of composition may be a satisfactory method for deciding which cheese should be allowed to mature for the British market and which should be sold more quickly.

Any grading system based on compositional analysis will be relatively complex because a further factor, the rate and extent of acid production at the vat stage, must also be considered (Lawrence etal . , 1984; Lawrence and Gilles, 1986). The point in the process at which the curd is drained from the whey is the key stage in the manufacture of Cheddar cheese because it controls to a large extent its mineral content, the amount of residual chymosin in the cheese, the final pH and the moisture/casein ratio (Lawrence et al., 1984). All these factors influence the rate of proteolysis in the cheese. A relationship has also been found between the calcium content of the cheese, the concentration of residual chymosin and protein breakdown during ripening (Lawrence et al., 1983) and between the rate of acid development in the early stages of manufacture and proteolysis in the cheese (O'Keeffe et al., 1975). The calcium level is therefore an index of the extent of acid production up to the draining stage and also offers a rough indication of the rate of proteolysis that is likely to occur during ripening. Significant differences in the calcium content of Cheddar cheese would suggest differences in the proportions of residual chymosin in the cheese and thus differences in the rate of proteolysis and the development of flavour.

However, variations in calcium content have a much smaller effect on Cheddar cheese quality than MNFS, S/M and pH. It is important to recognize that these three parameters are interrelated (Lawrence and Gilles, 1986) and must be controlled as a group to ensure first-grade cheese. Nevertheless, the effect of each of these factors will, as far as possible, be examined separately.

Suggested ranges of salt-in-moisture (S/M), moisture in the non-fat substance (MNFS), fat-in-dry matter (FDM) and pH for first grade (shaded) and second grade Cheddar cheese. Analyses 14 days after manufacture.

Effect of MNFS

There is considerable circumstantial evidence that the main factor in the production of the characteristic flavour of hard and semi-hard cheese varieties is the breakdown of casein. This is supported by the finding that the ratios of moisture to casein and of salt to moisture are critical factors in cheese quality (Gilles and Lawrence, 1973; Lawrence and Gilles, 1986) because both parameters affect the rate of proteolysis in cheese (Thomas and Pearce, 1981). Traditionally, cheesemakers describe cheese in terms of its absolute moisture content but the ratio of moisture to casein is much more important because it is the relative hydration of the casein in the cheese that influences the


course of the ripening process (Lawrence and Gilles, 1980). However, it is difficult to measure the casein content of cheese accurately and most commercial plants analyse for only fat and moisture. Therefore, a practical compromise is to determine the ratio of moisture to non-fat substance rather than measure the moisture/casein ratio. The non-fat substance is not the same as the casein in the cheese but is equal to the moisture plus the solids-not-fat. Approximately 85% of the solids-not-fat consist of casein. Therefore, there is a relationship between the moisture/casein ratio and the MNFS.

The level of MNFS in cheese gives a much better indication of potential cheese quality than the moisture content of the cheese in the same way as the S/M ratio is a more reliable guide to potential cheese quality than is the salt content of the cheese per se (Lawrence and Gilles, 1980). In large mechanized cheese plants, a significant relationship exists between the FDM and MNFS values for a cheese (Lawrence and Gilles, 1986), probably as a result of the relative inflexibility of the procedures available for the control of moisture. This is of commercial interest because changing the FDM is an effective way of controlling the MNFS in the cheese as the composition of the milk changes throughout the season. The actual MNFS percentage for which a cheesemaker should aim depends on the storage temperature used and

Experience has shown that if Cheddar cheese is to be

stored at 10 ~ and the cheese is to be consumed after 6-7 months, then the MNFS of the cheese should be about 53%. The higher the MNFS percentage, the faster is the rate of breakdown. Thus, if one antici- pates that the cheese will be consumed after 3-4 months, the MNFS percentage can be increased to about 56%. However, the higher the MNFS, the more rapidly Cheddar cheese will deteriorate in quality after reaching its optimum. The same is true for a Cheddar cheese with a relatively low S/M, i.e., less than 4%, or with a high acid content. Such cheeses tend to develop gas and sulphide-type off-flavours after they have reached maturity.

Effect of pH

Every cheese variety has a characteristic pH range (Lawrence et al., 1984), within which the quality of the cheese is dependent upon both its composition and the way in which it is manufactured (Lawrence et al., 1983). The pH value is important in that it provides an indication of the extent of acid production throughout the cheesemaking process. In normal manufacture, the curd pH/titratable acidity at salting is

a key factor in determining the pH of dry-salted cheese (Fig. 9). However, the salting pH/titratable acidity is to a large extent controlled, in turn, by the pH/titratable acidity developed at draining (Lawrence and Gilles, 1982). The potential for a further decrease in pH after salting depends upon the residual lactose in the curd and its buffering capacity. The residual lactose will be determined by the rate at which an inhibitory level of NaC1 is absorbed by the cheese curd and the salt toler- ance of the starter strains used. The buffering capacity is largely determined by the concentrations of protein and phosphate present, and to a much lesser extent by ions such as calcium. The concentrations of phosphate and calcium retained in the cheese are influenced mainly by the rate of acidification prior to the separation of the whey from the curd. The buffering capacity is also influenced by seasonal, regional and lactational factors.

Given reliable starter activity at the vat stage, the actual pH reached in dry-salted cheeses is determined by the S/M value because this controls the extent of starter activity after salting, the rate of lactose utiliza- tion in the salted curd and thus the pH reached. An S/M concentration of 6% will inhibit the activity of all Lactococcus lactis subsp, crernoris strains, the starter organisms of choice for Cheddar manufacture (Lawrence and Heap, 1986). The proportion of residual lactose that remains unmetabolized in such o h . . . . . . . ;11 l~,z, h ; , ~ . . . . . . [ ' t o . "3 . - ~ n . t ~ . - . ( T . . . . . . . . d l~..ll~..l...a~,.. V V t t l U ~ 1 1 1 ~ ) 1 1 ~ . . V ~ . . . l l ClLItK. . I / - I I I U I I t l I / D ~, I U t l l l ~ . . . l ~ t t l U

Thomas, 1980). However, in a cheese with S/M of 4.5%, the starter will not be inhibited completely and the lactose will be metabolized rapidly. This explains why the pH of 1-day Cheddar cheese may range from 5.3 (which is about the pH of the curd at salting) to pH 4.9. In general, the higher the pH, the greater

I

Casein and mineral content

of curd

Buffering capacity

Milk composition I

II Starter

percentage

~ Curd acidity/pH at draining

<.2 Curd acidity/pH ___1",~ Starter

at salting - - - ~ activity

,,,. . . . . . . . . Time between , cutting and ",[ . . . . . . . . draining

Residual lactose in

salted curd

The main factors that de termine the pH of Cheddar cheese.


is the amount of lactose left unmetabolized. Under normal circumstances, this residual lactose does not affect the quality of Cheddar cheese at maturity (Gilles and Lawrence, unpublished results).

The importance of measuring the pH at day 1 has been generally overlooked in the past, probably because it is relatively difficult to measure the pH of cheese accurately. This has led to a lack of appreci- ation of the significance of relatively small changes in pH. In addition, a pH value per se is sometimes difficult to interpret unless considered in conjunction with the calcium level in the cheese (Lawrence and Gilles, 1980), as well as the pH/titratable acidity at draining.

Effect of S/M

The main factors that determine the S/M percentage of Cheddar cheese are summarized in Fig. 7. In young Cheddar cheese, the S/M ratio is the major influence controlling water activity. This in turn determines the rate of bacterial growth and enzyme activity in the cheese, specifically the proteolytic activity of chymosin (Fox and Walley, 1971; Pearce, 1982; Fox, 1987), plasmin (Richardson and Pearce, 1981) and starter proteinases (Martley and Lawrence, 1972). If the S/M value is low (<4.5%), the starter numbers will reach a high level in the cheese and the chance of off-flavours due to the starter bacteria is greatly increased (Lowrie and Lawrence, 1972; Breheny et al., 1975). For this reason, cheesemakers normally aim for an S/M value in Cheddar cheese between 4.5 and 5 .5% (Lawrence and Gilles, 1980, 1982). Within this S/M range, the rate of metabolism of the lactose is controlled by a second factor, the temperature of the cheese during the first few days of ripening, because this controls the rate of growth of non-starter bacteria such as lactobacilli and pediococci (Fryer, 1982). Although non- starter bacteria grow on energy sources other than lactose in cheese, undoubtedly the presence of lactose encourages their rapid growth. This tends to result in a more heterolactic metabolism of lactose, usually with the production of acetate, ethanol and carbon dioxide, and may lead to flavour and textural defects. Clearly, the initial number of non-starter bacteria in the salted curd should be controlled by hygiene during manufacture. Thereafter, their rate of growth, particularly after the first few days of ripening, should be kept to a minimum and this is largely controlled by the temperature of the cheese (Fryer, 1982). For this reason, large mechanized Cheddar cheese plants in New Zealand and Australia have incorporated rapid cooling systems, which reduce the core temperature of the 20 kg cheese blocks to less than 18 ~ within 24 h of manufacture.

Compositional ranges were introduced into the grading system to reduce variability within a day's manufacture, especially with respect to S/M. This measure has helped to reduce the previous variability. The rate of ripening will differ but all of the cheese is likely to be acceptable as long as its composition is within the required compositional range. For instance, variations in the moisture content and acidity of the curd before salting, in the accuracy of salt delivery by salting equipment and in the dimensions and structure of the milled curd will all result in considerable variation in salt uptake (Lawrence and Gilles, 1982). Despite improved understanding of salt diffusion (Baldwin and Wiles, 1996; Wiles and Baldwin, 1996) in large mechanized Cheddar cheese plants, as well as new improved ideas and equipment for salting cheese curds, inherent variation of S/M still exists.

The effect of manufacturing Cheddar cheese with reduced sodium and S/M levels, on cheese quality, is discussed in 'Salt in Cheese: Physical, Chemical and Biological Aspects', Volume 1.

Effect of FDM

The FDM in Cheddar cheese is less important than MNFS, S/M or pH, in that it normally influences cheese quality only indirectly through its effect on MNFS (Whitehead, 1948). Nevertheless, the FDM has more relevance to the cheesemaker than the fat content per se because moisture is volatile and legal limits for fat are usually specified in terms of FDM. Use of FDM has the further advantage that it can be controlled directly by altering the casein/fat ratio of the milk.

Most consumers of Cheddar cheese consider texture and flavour to be its most important attributes (McEwan et al., 1989; Jack et al., 1993). On the other hand, Cheddar purchased for repackaging also needs to withstand cutting, slicing and moulding. Hence the rheological properties are important (Gunasekaran and Ak, 2003). Cheddar destined to be used as a func- tional ingredient has the rather different requirements to give particular textural (and flavour) characteristics to the final product ('Rheology and Texture of Cheese', Volume 1).

The desirable textural characteristics of a Cheddar cheese are different for different consumers, and this usually involves personal assessments of breakdown in the mouth, evenness of dissolution (melting), amount of chewing required, gritty remnants, etc. Traditionally, the textural properties of cheese sold for immediate


human consumption are determined by trained graders. For texture research, such assessments are often made by consumer panels, and there have been many studies using either rheological parameters or the results from specific texture-measuring devices. Such studies have shown that perceived texture correlates moderately well with the indices investigated and developed by Szczesniak (1968, 1987) and discussed by Fox et al. (2000) and Gunasekaran and Ak (2003), and less well with the traditional rheological measures (Breuil and Meullenet, 2001). Nevertheless, there is still no clear method for discerning instrumentally which blocks of Cheddar cheese have acceptable textural properties.

The complex interrelationships between the parameters that affect cheese texture make it almost impossible to design simple experiments in which the effect of a single parameter, such as fat content, can be examined in isolation. The wide-ranging experiments carried out by the New Zealand group in the 1970s and 1980s laid the basis for a good understanding of the factors under- lying the production of Cheddar cheese of appropriate texture and flavour throughout the maturation cycle (Lawrence et al., 1993). Nevertheless, some recent studies demonstrate that fat content (Fenelon and Guinee, 2000) and pH (Pastorino et al., 2003) can affect the rheological properties of Cheddar cheese. However, by using modern statistical approaches, it is now possible to segregate the effects of several parameters, although each experiment needs to be very large and use many cheese samples (C.J. Coker, T.M. Dodds, S.P Gregory, K.A. Johnston and L.K. Creamer, unpublished results, 2000).

Cheddar cheese has a texture that is intermediate (Fig. 10) between those of the relatively high pH cheeses, which flow readily when a force is applied, and the low pH cheeses which tend to deform, by shattering, only at their yield point. Scanning electron

microscopy has established that cheese consists of a continuous protein matrix but that this matrix is clearly different in the various cheese types (Hall and Creamer, 1972). The structural units in the protein matrix of Gouda are essentially in the same globular form (10-15 nm in diameter) as in the original milk. In contrast, the protein aggregates in Cheshire are much smaller (3-4 nm) and are apparently in the form of strands or chains, i.e., the original sub-micellar protein aggregates appear to have lost much of their iden- tity. Cheddar is intermediate between Gouda and Cheshire, i.e., much of the protein in Cheddar is in the form of smaller particles than in Gouda (Fig. 10). As the pH decreases towards that of the isoelectric point of para-casein (approximately 4.5), the protein assumes an increasingly compact conformation and the cheese becomes shorter in texture and fractures at a smaller deformation (Creamer and Olson, 1982; Walstra and van Vliet, 1982). The texture of Cheddar cheese has a wider range of consumer acceptability than the texture of other varieties as a consequence of the intermediate position of Cheddar in the cheese spectrum.

The high moisture and relatively high pH (5.2) of American Cheddar resulted traditionally in a more cohesive and waxy texture (Kosikowski and Mistry, 1997) than that of traditional English and New Zealand Cheddar. In North America, a relatively low 1 . . . . 1 ,,r ~,.~A ..... developed ~,, ,~, . . . . . ~ up ,,- ,~,o ~1,_ ing stage (less than 0.65% titratable acidity). In contrast, English cheesemakers strove for a high salting acidity (about 0.85%) with consequently a low final pH (about 4.9). New Zealand cheesemakers aimed for a final pH of 5.0 and a moisture content of about 35%, in contrast to the 38-39% moisture level found in both American and English Cheddar. In recent times, however, most Cheddar-producing countries have tended

Diagrammatic representation of the effect of the pH on the microstructure and texture of cheese.


towards the American style of 'sweet' Cheddar cheese with a final pH between 5.1 and 5.3 now being common.

Effect of pH, calcium and salt

Although the mineral content plays an important role in establishing the characteristic structure (Lawrence et al., 1983, 1984), the texture of Cheddar cheese appears to be more dependent upon pH than on any other factor (Lawrence et al., 1987). For the same calcium content, the texture at 35 days can vary from curdy (pH >5.3) to waxy (pH 5.3-5.1) to mealy (pH <5.1). Trials in New Zealand have shown that, for any given pH value, the concentration of calcium in Cheddar can vary over a range of ___ 15 mmoles/kg with only a slight effect on the texture (Lawrence et al., 1993), although there is a general tendency for the cheese to become less firm as the calcium content decreases.

However, the dominant effect of pH on texture can be modified by other compositional factors, particularly the levels of moisture, salt and calcium. Between pH 5.5 and 5.1, much of the colloidal calcium phosphate and a considerable part of the casein are dissociated from the sub-micelles (Roefs et al., 1985). These changes in the size and characteristics of the sub-micelles significantly increase their ability to absorb water (Tarodo de la Fuente and Alais, 1975; Snoeren et al., 1984; Creamer, 1985; Roefs et al., 1985), casein hydration reaching a maximum at about pH 5.35. More relevantly, Creamer (1985) found that casein hydration in renneted milk increased greatly in the presence of NaC1 between pH 5.0 and 5.4. Furthermore, at any given pH, the extent of solubilization of the micelles by the NaC1 decreased as the calcium concentration in the solution increased. This finding is in agreement with the effects of calcium in brine on the solubilization of the rind of Gouda-type cheese (Guerts et al., 1972). It also explains the observations that a higher Ca2+/Na + ratio results in a firmer cheese (Walstra and van Vliet, 1982), and that Cheddar cheese made from milk to which calcium has been added has a reduced protein breakdown and is of poorer quality (Babel, 1948; Ernstrom et al., 1958). The high level of calcium in buffalo milk (Rajput et al., 1983) may also account for the difficulty in manufacturing Cheddar cheese from buffalo milk. The extent of proteolysis is low (Neogi and Jude, 1978), presumably because the degree of solubilization of the casein micelles by the NaC1 is reduced. As a result, Cheddar cheese needs to be stored for a long period before its characteristic texture and flavour develop.

Therefore, it is not surprising that the texture of Cheddar cheese changes markedly as the pH varies between 5.4 and 4.9. A wide range of casein aggregates is present and differences in the sodium and calcium ion concentrations, as well as the proportion of water to casein, markedly affect the extent of swelling of the sub-micelles (Fig. 10). Salt also has a more direct effect on the texture of Cheddar cheese; excessive salting (i.e., an S/M > ~6%) produces a firm-textured cheese which is drier and ripens at a slow rate (Van Slyke and Price, 1952), whereas under-salting (i.e., an S/M < ~4%) results in a pasty cheese with abnormal ripening and flavour characteristics. Such factors as enzyme activity and the conformation of ors1- and [3-caseins in salt solu- tions (Fox and Walley, 1971), solubility of protein breakdown products, hydration of the protein network (Guerts et al., 1974) and interactions of calcium with the para-caseinate complex in cheese (Guerts et al., 1972) are all influenced by salt concentration.

Effect of protein, fat and moisture

In dry-salted cheeses, water, fat and casein are present in roughly equal proportions by weight, together with small amounts of NaC1 and lactic acid. As protein is considerably more dense than either water or fat, it occupies only about one-sixth of the total volume. Nevertheless, the protein matrix is largely responsible for the rigid form of the cheese. Any modification of the nature or the amount of the protein in the cheese will modify its texture. Thus, reduced-fat Cheddar (17% fat) is considerably more firm and more elastic than full-fat Cheddar (35% fat), even when the level of MNFS in the cheese are the same (Emmons et al., 1980). This difference was explained by the presence in the reduced-fat cheese of about 30% more protein matrix, which must be cut or deformed in texture assessments, but such a large reduction in fat must also affect the texture of the cheese.

Fat in cheese exists as physically distinct globules, dispersed in the aqueous protein matrix (Kimber et al., 1974). In general, increasing the fat content results in a slightly softer cheese (Bryant et al., 1995), as does an increase in moisture content, because the protein framework is weakened as the volume fraction of protein molecules decreases. However, relatively large variations in the fat content are necessary before the texture of the cheese is affected significantly (Lawrence and Gilles, 1980). Commercial cheese with a high FDM usually has a high MNFS (Lawrence and Gilles, 1986) and this causes a decrease in firmness. An inverse relationship between the fat content and cheese hardness has been reported (Whitehead, 1948; Baron, 1949; Fenelon and Guinee, 2000).


Effect of ripening

Considerable changes in texture occur during ripening as a consequence of proteolysis (Hort and Le Grys, 2000, 2001). The rubbery texture of 'green' cheese changes relatively rapidly as the framework of Otsl-casein molecules is cleaved by the residual coagulant (Creamer and Olson, 1982; Johnston et al., 1994; Watkinson et al., 2001). A group of Cheddar cheeses examined over a period of nearly a year increased in hardness and decreased in elasticity with the age of the cheese, the greatest changes occurring during the first 30 days (Baron, 1949). Watkinson et al. (1997) measured proteolysis of ors1- and [3-caseins, and the strain at fracture (a measure of shortness (Gunasekaran and Ak, 2003)) as a function of ripening time. These results showed that the strain at fracture increased initially, probably as curd fusion continued, and then decreased continuously for the 400 days of the experiment. In part, this latter rheological (or textural) change is caused by the loss of structural elements, but another feature of proteolysis is probably important (Creamer and Olson, 1982): as each peptide bond is cleaved a molecule of water is incorporated into the resulting polypeptides and, in addition, two new ionic groups are generated and each of which will compete for the available water in the system. Thus, the water previously available for solvation of the protein chains becomes tied up by the new ionic groups, making the cheese more firm and less easily deformed. This change, in combination with the loss of an extensive protein network, gives the observed effect.

Clearly, the change in texture during ripening depends upon the extent of proteolysis, which, for any individual cheese, is determined by the duration and temperature of maturation. The main factor that influences the rate of proteolysis appears to be S/M (Fox and Walley, 1971; Pearce, 1982; Fox, 1987). A direct relationship between S/M and residual protein was established whereas the correlation between moisture and residual protein was relatively weak. A cheese with a low S/M value has a higher rate of proteolysis and is corre- spondingly softer in texture than a cheese with a high S/M. The concentrations of residual rennet and plasmin in the cheese, together with the starter and non-starter proteinases present, are the important factors that determine the rate of proteolysis (Lawrence etal . , 1983; C.J. Coker, T.M. Dodds, S.P. Gregory, K.A. Johnston and L.K. Creamer, unpublished results, 2000).

Cheese ripening is essentially the slow controlled decom- position of a rennet-induced coagulum of the con- stituents of milk to produce flavour (taste and aroma)

and textural changes. The final targeted flavour profiles and textures of ripened Cheddar and related dry-salted cheese varieties are variable as defined by different end- customer requirements and traditional cultural flavour expectations. At the young end of the age range is cheese used solely as a source of intact casein for processed cheese, which has minimal flavour and textural change from the fresh curd. A low coagulant concentration, a low storage temperature, high S/M, short storage time or combinations of these are the main parameters used to achieve this end-use. At the other extreme are the strong flavoured Cheddar cheeses ripened for 12-24 months or more. During ripening, there are many changes and the ripening processes responsible are understood in general terms but many of the details are still being investigated.

A vocabulary of sensory attributes has been developed to describe Cheddar (Muir and Hunter, 1992), and has been modified to include five odour, ten flavour and five textural attributes (Muir et al., 1995). Using this vocabulary with an experienced panel in combination with data analysis, the similarities and differences between Cheddar and 13 other hard cheeses popular in the United Kingdom have been described (Muir et al., 1995). The medium and vintage Cheddars stand out in a number of respects. In a similar analysis of 34 different Cheddars, a diversity of flavours was shown (Muir e t a l . , 1997). Cheddars made from raw milk were more intensely flavoured ,,,,,a ~,,,4 ,,t,,~,,,~l n . . . . . . . . . . . ~,~ farmhouse cheeses

I t , . . . . . .

showing wide variations in composition and being associated with atypical flavour and texture.

There is a significant correlation between the levels of proteolysis products and the extent of flavour development. Hydrolysis of the casein network, specifically e~sl-casein, by the coagulant appears to be responsible for the initial changes in the coagulum matrix (Creamer and Olson, 1982). The level of chymosin retained in the curd is pH dependent (Lawrence et al.,

1983; Creamer et al., 1985). In fresh milk, plasmin, the indigenous alkaline milk proteinase, is associated with the casein micelles but it dissociates at low pH (Richardson and Pearce, 1981" Farkye and Fox, 1990). The activity of plasmin in cheese is reported to be dependent on cooking temperature (Farkye and Fox, 1990) as well as on pH and the salt and moisture contents of the cheese (Richardson and Pearce, 1981" Farkye and Fox, 1990). The role of plasmin in Ched- dar cheese flavour has yet to be elucidated but it has been reported that the rate and extent of characteristic flavour development in Cheddar cheese slurries appeared to be related directly only to the degradation of [g-casein (Harper et al., 1971). Therefore, plasmin may well prove to be an enzyme of considerable importance in the development of cheese flavour.


As the original casein network is broken down, ideally a desired balance of flavour and aroma compounds is formed. However, the precise nature of the reactions that produce flavour compounds and the way in which their relative rates are controlled are poorly understood. This has been due firstly to the lack of knowledge of compounds that impart typical flavour to Cheddar cheese, and secondly to the complexity of the cheese microflora as the potential producers of flavour compounds. Any organism that grows in the cheese, whether starter, adventitious non-starter lactic acid bacteria (NSLAB) or adjunct culture and any active enzyme that may be present, such as chymosin or plasmin, will have an influence on the subsequent cheese flavour (Fig. 11). Research in New Zealand has shown that if the growth of starter and NSLAB is limited (Fryer, 1982; Lawrence et al., 1983) and if as little chymosin as possible is used (Lawrence and Gilles, 1971; Lawrence et al., 1972), the flavour that develops in Cheddar cheese is likely to be acceptable to most consumers.

This section is an attempt by the present authors to summarize what they consider to be relevant to flavour development in Cheddar. Since the last version of this section (Lawrence et al., 1993), more details have been published; however, the last word on the flavour of Cheddar cheese is still to come. For more

Basic structure for Cheddar (pH and mineral content)

Ripening conditions within cheese (Moisture-in-casein; salt-in-moisture; lactose; temperature)

Residual rennet, plasmin and

starter activity

Non-starter activity

" " " " " ,, ~ S S S S S s

i

Acceptable Cheddar flavour Off-flavours

The main factors that determine the development of flavour in Cheddar cheese.

details on the biochemistry of cheese ripening, refer to 'Biochemistry of Cheese Ripening: Introduction and Overview', 'Metabolism of Residual Lactose and of Lactate and Citrate', 'Lipolysis and Catabolism of Fatty Acids in Cheese', 'Proteolysis in Cheese during Ripen- ing', 'Catabolism of Amino Acids in Cheese during Ripening', Volume 1.

Effect of milk-fat

It is well accepted that Cheddar cheese made from skim milk does not develop a characteristic flavour. Cheese with an FDM greater than 50% developed a typical flavour whereas cheese with an FDM less than 50% did not (Ohren and Tuckey, 1969). In this study, when a series of batches of cheese were made from milk of increasing fat content (from 0 to 4.5%), the quality of the flavour improved as the fat content increased. However, if the fat content was increased above a certain limit, the flavour was not further improved. Substituting vegetable or mineral oil for milk-fat still resulted in a degree of Cheddar flavour (Foda et al., 1974). This suggests that the water-fat interface in cheese is important and that the flavour components are dissolved and retained in the fat.

Clearly, although milk proteins and lactose are the most likely sources of many of the flavour precursors in Cheddar cheese, the fat plays an important but not yet defined role; in part, the lack of understanding is due to the more limited fat modifications. The extent of lipolysis has been calculated to vary between 0.5 and 1.6% over time in good quality Cheddar (Perret, 1978). A number of fatty acids, keto acids, methyl ketones, esters and lactones in Cheddar are likely to have been derived from milk-fat; some are at concentrations to impact on flavour, but others contribute only to a background flavour (Urbach, 1995; McSweeney and Sousa, 2000). The residual activity after pasteurization of the indigenous milk lipase and the relatively low lipase/esterase activities of the starter and NSLAB are likely to be important in the hydrolysis of milk-fat to free fatty acids because of their flavour potency. The quality of the milk is probably a factor in excessive lipolysis in off-flavoured Cheddar (Perret, 1978). The catabolism of free fatty acids to other flavour compounds, by implication of their presence, occurs but the mechanisms are ill-defined.

Effect of proteolysis

As described earlier, the consequence of proteolysis of casein represents the most important biochemical ripening event in Cheddar, causing major texture changes and in addition making important contributions to both aroma and taste (Fox, 1989; Fox and


McSweeney, 1996). A further consequence of proteolysis may be the release of flavour components that were previously bound to the protein (McGugan et al., 1979). The products of proteolysis include small- and intermediate-sized peptides and free amino acids and contribute at least to a background flavour (McSweeney and Sousa, 2000), or make a significant contribution to flavour intensity. It has been suggested (McGugan et al., 1979; Aston and Creamer, 1986) that the importance of low levels of such non-volatile compounds as peptides, amino acids and salts has been under-rated in the past. This view is supported by the highly significant correlations found between the levels of proteolysis products and the extent of flavour development (Aston et al., 1983). The level of phos- photungstic acid-soluble amino nitrogen was found to be a reliable indicator of flavour development. Above certain limits, however, the level of peptides results in bitterness. Cheddar cheeses made using temperature- insensitive starter strains were found to become bitter because large numbers of starter cells contributed excessive levels of proteinases. These released bitter- tasting peptides from high molecular weight peptides that had been produced mainly as a result of chymosin action (Lowrie and Lawrence, 1972). The subject of bitterness, the single most common defect in Cheddar cheese, has been extensively reviewed (Crawford, 1977; Fox, 1989).

for reactions that produce a range of flavour compounds (McSweeney and Sousa, 2000). Recent studies using gas chromatography-olfactometry and related techniques have identified key aroma components of Cheddar cheese (O'Riordan and Delahunty, 2001; Zehentbauer and Reineccius, 2002). Some of these (dimethyl sulphide, methional, dimethyl trisulphide and 3-methylbutanal) are likely to originate from amino acids (Urbach, 1995).

Several reports strongly implicate the volatile sulphur compounds, specifically methanethiol, in Ched- dar cheese flavour (Green and Manning, 1982; Lindsay and Rippe, 1986), but an Australian report (Aston and Douglas, 1983) concluded that none of these sulphur compounds is a reliable indicator of flavour development. However, it is conceivable that, although the volatiles do not make a measurable contribution to the intensity of Cheddar flavour, they may still be an essential factor in the quality of the flavour (McGugan et al., 1979). This is supported by the finding (Manning et al., 1983) that the quality of blocks of Cheddar cheese decreased, and off-flavours increased, with a decrease in block size. Headspace analysis showed that the concentrations of HzS and CH3SH, compounds that are extremely susceptible to oxidation, decreased as the

quality of the cheese decreased. Some amino acids such as phenylalanine and the branched amino acids yield Strecker degradation products, which in excess cause unclean flavour defects in Cheddar (Dunn and Lindsay, 1985).

Role of starter

The absence of any Cheddar flavour in glucono lactone- acidified cheese and the development of typical, balanced Cheddar flavour in starter-only cheese (Reiter et al., 1966) established that Cheddar starter, normally Lactococcus lactis subsp, cremoris as discussed in 'Effect of milk composition and starter culture', has a role in the development of cheese flavour. However, the exact role has been much more difficult to determine. An important indirect role of the starter is considered to be providing a suitable environment that allows the development of characteristic cheese flavour (Lowrie et al., 1974). Starter activity results in the required redox potential, pH and moisture content in the cheese that allows enzyme activity to proceed favourably. In addition, the temperature during manufacture and the S/M must be controlled to ensure that the net metabolic activity of the starter organisms is low (Lawrence et al., 1972; Lowrie et al., 1974) but nevertheless adequate to allow the required pH at day 1 to be reached. Should the starter reach too high a

. . . . . | ~ t l . . . . . . . . . . i . r o t ~ l r , ~ , ~ n . . . . . . . r l o ~ o n t . . . . . h ~lc L a U ~ V O t L ~ L L U I L UA ~tJtL V L V l , . . L U ~ . . n L ~ . . P i l ~ , L L O t V I J L . ~ L q.at~.L~.~.1.00t.X'~Li CLO

bitterness, which mask or detract from cheese flavour, are produced. A reduction in unpleasant flavour is associated with improved perception of the Cheddar flavour (Lowrie and Lawrence, 1972; Lowrie et al., 1974).

The increase in the use of direct vat inoculum (DVI) cultures in Europe for the manufacture of cheese has led to greater usage of Lc. lactis subsp. lactis strains of starter. Because these strains have a greater tendency than Lc. lactis subsp, cremoris strains to produce bitterness in cheese, bitterness is more common with the use of DVI cultures than with bulk cheese starter (Heap, personal observation).

During, or soon after, the manufacture of Cheddar curd, the starter viability decreases and is <1% by 3 months (Martley and Lawrence, 1972). The decrease in starter viability is generally an indication of starter autolysis, which is considered to have important conse- quences for the control of bitterness, as discussed in 'Effect of proteolysis', and for other ripening developments in Cheddar (Wilkinson et al., 1994a,b; Crow et al., 1995). Starter autolysis in Cheddar is influenced by the choice of starter strains (Martley and Lawrence, 1972), and the extent and the rapidity of autolysis are modified by manufacturing conditions, particularly


cook temperature, pH and salting, and the composition of the final cheese. A balance between intact and auto- lysed starter cells is important for good quality Ched- dar flavour. Sufficient intact cells are needed for lactose removal and potentially for other physiological reactions such as oxygen removal (Crow etal . , 1995). There is indirect evidence that starter autolysis results in higher concentrations of small peptides and free amino acids (Wilkinson et al., 1994a,b). The intracellu- lar peptidases are released by autolysis before they can be very active in cheese as the peptide transport systems are probably inactive in the stressed starter cells (Crow et al., 1993).

Role of s tar ter e n z y m e s It is likely that a number of starter enzymes have a direct role in flavour development (Lawrence and Thomas, 1979; Law and Wigmore, 1983; Farkye et al., 1990).

The starter proteinase is a cell-associated endopep- tidase (lactocepin), which has been studied extensively, biochemically and genetically (Reid and Coolbear, 1998) and makes important contributions to proteolysis in cheese ripening (see reviews, Pritchard and Coolbear, 1993 and Kunji et al., 1996). Proteinase- negative starters have been used in different ratios with normal starter strains to demonstrate that a lower level of this enzyme during Cheddar ripening can reduce the development of bitter flavours (Mills and Thomas, 1980). Using proteinase-negative starters, Lane and Fox (1997) have shown that the absence of starter proteinase during cheese ripening gives rise to decreased levels of small peptides and amino acids, and a poorer quality Cheddar. Therefore, a balance of proteinase activity is often important to Cheddar flavour. There are also different starter proteinases (mainly types I and III), and their specificity differs (Broadbent et al., 1998) and/or their stability in cheese differs (Reid and Coolbear, 1999), factors that contribute to the starters' influence on proteolysis and bitterness.

The other proteolytic activities of starters that are important for proteolysis in Cheddar are the peptidases (see reviews, Pritchard and Coolbear, 1993 and Kunji et al., 1996). The early appearance of free amino acids in Cheddar is due mainly to the starter peptidases (O'Keeffe et al., 1976). The mesophilic starters have about 15 peptidases with different specificities (McSweeney and Sousa, 2000); a number have been shown to be intracellularly located and the activities vary between strains (Crow et al., 1994). Although the collective importance of the peptidases in Cheddar proteolysis is reasonably well established, the individual roles of the peptidases are not clear. Genetic modi-

fication of the peptidase expression in the starter may clarify their roles (Kok and Venema, 1995). For example, Cheddar flavour was not accelerated using a starter that overproduced the general aminopeptidase, PepN (McGarry et al., 1994). This may not be surprising if the suggestion by Fox and Wallace (1997) is correct, that production of amino acids in cheese ripening is not rate limiting.

To date, there is no strong evidence to suggest that the lactococcal starters have a true lipase that is cap- able of hydrolysing the milk triglycerides. However, they have esterase activity that acts on milk mono- and di-glycerides, with the short chain fatty acids being released preferentially (Holland et al., 2002). There is evidence that the starter esterase can also produce short-chain fatty acid esters (Nardi e ta l . , 2002). Starter strains have a range of esterase activity with a significant proportion located on the cell surface (Crow et al., 1994). Both the hydrolysis and ester syn- thesis reactions of the starter esterase probably play a role in Cheddar flavour but more investigations are needed to define the extent to which this impacts on Cheddar flavour.

Role of other starter activities In the young Cheddar curd, starters ferment the remaining lactose to lactic acid and possibly to other minor fermentation products. Some starters will also ferment citric acid to products including diacetyl, acetate and carbon dioxide. These products can contribute to flavour (Lawrence and Thomas, 1979) and the fermentations are dependent on intact viable cells (Crow et al., 1993). Other ripening reactions by starter that may be important in cheese are the modifications of amino acids and fatty acids (McSweeney and Sousa, 2000). It has been suggested that the enzymatic or chemical modification of amino acids is a rate-limiting factor in cheese ripening (Fox and McSweeney, 1996).

Starters have a wide range of abilities to metabolize amino acids. This includes converting arginine to ornithine (Crow and Thomas, 1982), leucine, methio- nine and phenylalanine to their corresponding aldehydes (MacLeod and Morgan, 1958) and a range of amino acids to their corresponding organic acids (Nakae and Elliot, 1965). Many of the amino acid con- versions by starters and other dairy bacteria rely on transamination reactions that are believed to be rate limited by the availability of the ot-ketoglutarate (Tanous et al., 2002). Enhancement of amino acid metabolism increased the aroma (Banks et al., 2001) and the flavour maturity (Shakeel-Ur-Rehman and Fox, 2002) in Cheddar made with added ot-ketoglutarate. Despite all this information, it is still not clear what con- stitutes a proper balance of amino acid transformations


by starter with respect to a balanced Cheddar flavour. In mature commercial Cheddar, the availability of amino acids increases with time when the adventitious microflora are likely to contribute. In addition, at this later stage of ripening in good quality Cheddar, there is normally significant starter autolysis, which could affect the ability and the way the starters metabolize amino acids.

It is clear that starters, because of the initial high biomass in young curd and associated ripening enzymes and fermentative abilities, will contribute to Cheddar flavour. In the past and currently, the choice of starter has been dictated more by its importance to commercial curd manufacture (particularly reliable acid production and associated phage resistance) than by its ripening properties, which have been focused mainly on minimizing flavour defects, such as bitterness. With the increasing understanding of Cheddar ripening and the role of the starter, it will be possible to use more lactococcal strains with specialized ripening attributes. Such strains can be used either as starters or as flavour adjuncts if their starter properties are compromised.

Role of non-starter lactic acid bacteria

Cheddar contains a heterogeneous adventitious microflora originating from the milk and/or the manufacturing environment (Peterson and Marshall, 1990; Martiey and Crow, i993), in Cheddar, the main microflora identified are mesophilic lactobacilli and occasionally pediococci (Jordan and Cogan, 1993" Crow et al., 2001), commonly referred to as NSLAB. The most common species are Lactobacillus paracasei, Lb. casei, Lb. rhamnosus, Lb. plantarum and Lb. curvatus. Strains of heterofermentative lactobacilli (Lb. brevis and Lb. fer- mentum) are identified occasionally. The common species vary between countries (Fox et al., 1998) and the species and strains can vary between factories, within a factory and within a block of cheese during ripening (Crow et al., 2002).

Cheddar cheese made under controlled bacteriological conditions and containing only starter streptococci develops balanced, typical flavour (Reiter et al., 1966) but it is intriguing that cheeses made in open vats develop such flavour more rapidly (Reiter et al., 1967 Law etal . , 1976, 1979). This suggests that NSLAB present as a result of post-pasteurization con- tamination are beneficial. Nevertheless, there have been reports that conclude that NSLAB have little effect on normal Cheddar cheese flavour development (Law and Sharpe, 1977, 1978).

The role of NSLAB in contributing positively to Cheddar cheese flavour has yet to be elucidated (Peterson and Marshall, 1990; Martley and Crow,

1993). In general terms, the numbers and types of dominant NSLAB are important (Crow et al., 2002). These factors are influenced by milk quality, factory hygiene, the rate of cooling of the cheese, the ripening temperature and the cheese composition (Lane et al., 1997; Fox et al., 1998). The inherent variability of the initial NSLAB strains makes consistent control of ripening by NSLAB a challenge. The rate of cooling of the cheese, after pressing the curd, appears to be a significant factor in controlling the cheese flora (Fryer, 1982) and appears to offer the easiest method of controlling cheese flavour (Miah et al., 1974). Recent evidence suggests that selected NSLAB can be used as adjunct cultures to provide an important additional tool in controlling Cheddar flavour (see 'Role of adjuncts').

The NSLAB have a diversity of metabolic and enzyme activities. Different NSLAB strains have a wide range of proteinase (Broome et al., 1991a), peptidase (Broome et al., 1991b) and esterase (Williams and Banks, 1997) activities and types, can catabolise a range of amino acids (Christensen et al., 1999) and can produce esters (Liu et al., 1998). Different NSLAB are likely to grow on different energy sources in cheese (Fox et al., 1998) and influence the redox potential in different ways (Thomas et al., 1985). The balance of all these activities is probably important to good quality Cheddar ripening.

The prolonged presence of high numbers of some NSLAB species in Cheddar has been associated with lilJc,titt ~ ucL~+t~ ~u~+l, a~ un-LLavuuL~, ~.t~ atlu ~+ty~tal~ (Crow et al., 2001). Off-flavours can be produced by Lb. brevis and Lb. plantarum (Puchades et al., 1989). Slits have been attributed to heterofermentative lactobacilli (Laleye et al., 1987) and the formation of white spots of calcium lactate pentahydrate crystals has been associated with the racemizing activity of certain NSLAB (Thomas and Crow, 1983; Johnson et al., 1986, 1989; Dybing et al., 1988 Bhowmik et al., 1990). In Cheddar, the total lactate (usually the L(+) isomer) is at a concentration close to crystallizing out. Crystallization of calcium lactate on the surface of Cheddar cheese is a common and troublesome defect (Pearce et al., 1973). A number of lactobacilli isolates and all pediococci isolates can convert the L(+) isomer of lactate to the D(--) isomer in Cheddar such that an equilibrium is eventu- ally reached where there is an equal mixture (a racemic mixture) of both isomers. As the racemic mixture of lactate is more insoluble than the separate isomers, there is a higher possibility of lactate crystallization in Cheddar containing a racemic mixture of lactate.

Role of adjuncts

Cheddar-type varieties traditionally have starter cultures as the only dairy microorganisms deliberately added to the milk. Adjuncts, cultures that are added deliberately


for features other than for acid production, and used in some other cheese types (e.g., propionibacteria in Swiss- type cheese), have generally not been used for Cheddar. There has been an increasing interest in the use of adjuncts for Cheddar, usually for flavour acceleration but also for flavour consistency or for contributing to unique flavour profiles (Fox et al., 1998). A number of cultures with putative health attributes are also being investigated as adjuncts to produce probiotic cheese, including Cheddar (Ross et al., 2002). Much of the published work has concentrated on the use of NSLAB as adjuncts. Other adjuncts studied include attenuated thermophilic lactobacilli to accelerate ripening (Wilkinson, 1993). Addi- tion of non-attenuated thermophilic lactobacilli, particularly Lb. helveticus, has been shown to accelerate ripening, reduce bitterness and provide a different flavour profile (Fox et al., 1998). Some other adjuncts studied in less detail include smear bacteria, Enterococ- cus, Pseudomonas and yeast (Crow et al., 2002).

There is some commercial use of NSLAB adjuncts, but time and economics will determine if their use is sustained (Crow et al., 2002). Earlier work (Lane and Hammer, 1935; Reiter et al., 1967) has been followed by a recent increased effort in studying their use (Puchades et al., 1989; Broome et al., 1990; McSweeney et al., 1994; Lynch et al., 1996; Muir et al., 1996; Crow et al., 2001). In these studies, the NSLAB adjuncts often improved the flavour intensity. Although the desirable ripening mechanisms for suitable adjuncts are not known, some analysis shows that flavour improvement is associated with an increase in the concentration of amino acids and small peptides and that the volatiles are produced in a different ratio (Fox et al., 1998). Some strains tested produced flavour defects (e.g., Lee et al., 1990).

The dynamics of the interactions between adventitious and adjunct NSLAB growing in Cheddar are not fully understood (Fox et al., 1998). For use in New Zealand Cheddar, successful NSLAB adjuncts are care- fully selected from good quality cheese; to achieve com- petition against the range of adventitious NSLAB and to provide a balance of cheese ripening attributes, an adjunct is made up from more than one strain (Crow et al., 2002). Provided that the milk quality and the factory hygiene are high (i.e., a low level of adventitious NSLAB), the adjunct strains can overgrow the adventitious NSLAB and be the main population of NSLAB throughout ripening, thus providing consistency to mature Cheddar flavour development.

There is no one standard for measuring cheese quality. Young Cheddar cheese is judged on the basis of whether it has properties characteristic of its variety.

Compositional analysis provides an objective method for detecting atypical cheese and is to be preferred to subjective grading methods. In the case of mature cheese, quality assessment is largely a matter of specific market preference, with consumers in different countries differing considerably in their requirements with respect to cheese flavour. Cheddar cheese flavour requirements is specific to country, ethnicity and end- application.

Whereas sensory profiling of cheese provides a powerful tool for quality assurance and new product development, grading is a highly efficient method of identifying out-of-specification cheese early in the ripening period (Muir, 2002).

Grading of cheese encompasses sensory evaluation and functionality tests on the finished product, and is carried out in tandem with chemical and microbiological analyses as part of the manufacturer's quality assurance programme. Grading is carried out as a series of checks during ripening to determine whether or not the manufacturer has achieved what he initially set out to achieve (S.P. Gregory, personal communication).

Although the assignment of a grade to a consign- ment of cheese may be improperly influenced by the sample because differences may exist between blocks of cheese made from the same vat of milk, it has thus far been the most practical way of grading. Flavour defects, such as fruitiness and sulphide off-flavours, have sometimes been located in particular areas within a cheese (Gilles and Lawrence, unpublished results). Such lack of uniform flavour usually results from variations in S/M (Lawrence and Gilles, 1982). Differences between cheeses have also been attributed to an uneven cooling of cheese blocks stacked closely on pallets while the cheese is still warm (Conochie and Sutherland, 1965b). It is therefore possible that a grade score is highly biased if the assessment of a whole vat depends on a single randomly drawn sample (Sutherland, 1977).

The texture of Cheddar cheese changes dramatic- ally during the first few days of ripening. The simplest explanation for this observation is that the cheese microstructure consists of an extensive network of otsl-casein and that cleavage by chymosin (or rennet substitute) of just a few peptide bonds of otsl-casein greatly weakens this network (Creamer and Olson, 1982). This results in a relatively large change in the force necessary for deformation. It is differences in this force that a grader attempts to assess when he rubs down a plug of cheese between his thumb and forefin- ger. From this assessment of the texture, after the cheese has been allowed to ripen for about 30 days, the grader proceeds to predict what the quality of the cheese will be after it has matured (Lawrence et al., 1983).


Based on the grade or quality attributes, the cheese is identified as one with potential for long-hold (mature Cheddar), medium-hold or short-hold (mild Cheddar). Bitterness is the most common flavour defect detected in Cheddar cheese.

Therefore, the sensory method of prediction traditionally used by graders has some validity because the rate of change in cheese texture during the first few days of ripening is determined by the same factors, i.e., the pH at day 1, the salt/moisture ratio and the moisture/casein ratio, that also influence the quality of the cheese at maturity (Fig. 12). Experi- ence has long shown that a Cheddar cheese with an atypical texture seldom, if ever, develops a characteristic flavour but unfortunately the reverse is not true. A good-textured cheese does not always develop an acceptable flavour, because off-flavours can still be produced if unsuitable manufacturing and ripening procedures are used (Lawrence et al., 1983). Detection of atypical cheese can be achieved more directly and objectively by compositional analysis.

Low-fat Cheddar cheese

Although a relatively minor product, low-fat Cheddar e ] ~ o o c o i c i m n n r t ~ n t i n t n , , - l , ' J , r ~ c h o a ] t h _ e n n c e i n , l C c n e i _ ~s L O L I L L ~ / U I t R I L L 1 1 1 LU~.~L(,~k~ ~.~ l l W l , . ~ ( ~ k l t J k l - - t . . . U L l ~ J ~ . ~ l U I..~kO ~3DVlk..,l--

ety, where consumers have become more concerned about the amount of fat in their diet. However, it has been difficult to produce a low-fat Cheddar cheese with the same flavour and texture characteristics as

those of a full-fat cheese (Guinee and Law, 2002). The flavour and acceptability at 3 months decrease with decreasing fat content (Banks et al., 1989). Cheddar cheeses containing only 15-30% fat are noticeably more firm and less smooth, when young, than full-fat cheese. The differences in texture, although marked in the early stages of ripening, apparently narrow after the cheese has matured for 1 or 2 months (Olson, 1984a).

There has been some success at improving the quality of low-fat Cheddar cheese (Guinee and Law, 2002), but there is still a poor consumer perception of lower fat cheeses, judging by their relatively low consumption. The approaches taken to improve both the flavour and the texture have been reviewed (Guinee and Law, 2002).

A novel idea for improving the body and texture of low-fat Cheddar cheese was through the use of sweet ultrafiltered buttermilk (Mistry et al., 1996). However, the differences in texture, seen at 4 weeks, between cheeses made with and without buttermilk, were smaller after 24 weeks.

By using a combination of manufacturing changes and novel starter and starter adjuncts, Johnson et al. (1998) claim to have achieved a cheese with more acceptable texture, but with a flavour, although improved, that is not identical to that of Cheddar.

Guinee et al. (1999) claim to have developed a half-

ity by modification of the cheesemaking procedure, including increasing the pasteurization temperature, and using selected starter cultures and bacterial culture adjuncts. The addition of fat mimetics to the milk is

Acid production s ~ at draining pH change and mineral los Basic structure

i Quality at maturity

Proteinase activity (salt-in-moisture

moisture-in-casein)

/ " i

t _ _ _ _

i , q , Texture and s t i

�9 ." Sensory evaluation flavour at ". ' ' grading

i

An explanation for the general validity of traditional sensory testing of Cheddar cheese.


also a method that is claimed to improve the quality of reduced fat cheese (Guinee and Law, 2002).

Stirred curd or granular cheese

As discussed earlier, Granular cheese preceded, histor- ically, the manufacture of traditional Cheddar cheese. It is made as for normal Cheddar cheese except that the curd fusion or cheddaring step is omitted. There- fore, more acid has to be developed at the vat stage to compensate for the shorter total manufacturing time. Starter systems are available that allow very acceptable Granular cheese to be made. Maintaining curd in the granular form, without the need for milling prior to salting, has obvious attractions. However, there is a tendency for the curd to mat after drying unless it is agitated, and continued stirring may lead to higher fat losses. Moisture expulsion is also faster than during cheddaring.

The salted curd particles take some time to fuse together, the rate of bonding depending largely upon the pH of the curds at salting. However, there are advantages in mechanized cheesemaking systems in having the curds in a granular form. The salt readily mixes with the curd, and the salted granules flow and can be hooped easily. Stirred curd cheesemaking is now widely used in the manufacture of 'barrel' (bulk pressed) cheese, although variations in moisture level may occur as a consequence of different temperatures within the block (Olson, 1984b; Reinbold and Ernstrom, 1988).

The pressing of granular curd gives rise to open- textured cheese as a result of air being entrapped within the cheese (Czulak and Hammond, 1956). However, this has been overcome by the development and widespread use of methods of pressing the curd under vacuum (Brockwell, 1981; Tamime and Law, 2001). Granular cheese resembles Cheddar cheese in composition but it matures somewhat differently because of the relatively low acidity at which the curds are salted. Curds hooped in the granular form give a texture at 14 days which, although completely close, is just perceptibly different from that of normal Cheddar cheese (Czulak, 1962). However, this difference in texture diminishes as the cheese matures.

Washed curd varieties

There has been a substantial increase in recent years in the consumption of washed-curd varieties of Cheddar cheese (Olson, 1981). Varieties such as Colby and Monterey are milder in flavour and have a more plastic texture than Cheddar. They are relatively high-moisture cheeses (39-40%) and ripen rapidly.

It has also become popular in New Zealand and Australia to manufacture dry-salted Gouda-style, Edam-

style and other washed-curd cheese types using a method similar to that outlined for traditional Colby and Monterey.

Colby and Monterey The recent improvements in the production of granular Cheddar for processing are also indirectly responsible for the production of Colby and Monterey because these varieties are in fact washed-curd, granular cheeses. Traditionally, whey is drained off until the curds on the bottom of the vat are just breaking the surface and cold water is added to reduce the temperature of the curds/whey to about 27 ~ (Fig. 13). The moisture content of the cheese can be controlled by the temperature of the curds/whey/water mix. The moisture content decreases as the temperature is increased between 26 and 34 ~ The pH of the cheese is determined by the proportions of both whey removed and water added. The length of time the water is in contact with the curd is also important because this determines the level of residual lactose. As salt penetration into the interior of the granular curd particles is rapid, no pH gradient occurs and seaminess is not a problem.

Manufacture as for Cheddar up to running stage

Proportion of whey removed and water added

Contact time

Residual lactose

Temperature of curd/whey/water

mixture

pH Moisture content

Typical texture of Colby

The main factors that determine the characteristic texture of Colby cheese.


The calcium content of Colby tends to be slightly lower than that of Cheddar because of the higher percentage of starter used and a further small loss of calcium at the washing stage. However, as discussed previously, it is the pH that determines to a large extent the texture of a cheese. The addition of water results in a small increase (about 0.1-0.2 units) in the pH of the finished cheese but this is sufficient to give the cheese a more plastic texture than Cheddar. Recent trials (Creamer et al., 1988) have shown that the calcium content of Colby cheese can vary between 120 and 180 mmoles/kg cheese without influencing significantly the texture of the cheese as long as the pH is greater than about 5.2. The characteristic texture of Colby cheese is thus influenced almost entirely by its pH and moisture level (Fig. 13). Traditionally, Colby had a mechanically open texture but the use of short- time pressing systems (Wegner, 1979; Brockwell, 1981; Tamime and Law, 2001), in which the curd is transported to be pressed under a partial vacuum, results in a texture that is as close as that of Gouda- type cheese varieties. Monterey cheese has many similarities to Colby but is usually softer (Kosikowski and Mistry, 1997).

The editorial assistance of Ms Claire Woodhall and the advice of bar Philip X,u are gratefully acknowledged.

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