J Sci Food Agric 1998, 78, 196È200

Assessment of the Thermal Treatment of OrangeJuice during Continuous Microwave andConventional HeatingMar Villamiel, M Dolores del Castillo, Cristina San Mart•�n and Nieves Corzo*

Instituto de Fermentaciones Industriales (CSIC), C/ Juan de la Cierva, 3, 28006 Madrid, Spain

(Received 28 July 1997 ; revised version received 21 November 1997 ; accepted 4 February 1998)

Abstract : The e†ect of continuous-Ñow microwave heat treatment of orange juicewas evaluated by using parameters of quality (pectin methylesterase (PME) inac-tivation and ascorbic acid, free amino acids, carbohydrates and hydroxymethylfurfural (HMF) contents, as well as degree of browning). Results were comparedwith those obtained using a conventional tubular heat exchanger having thesame heating and cooling phases. The continuous microwave process proved tobe an e†ective system for PME inactivation without the inclusion of a holdingphase. In general, no di†erences were found between the two systems used in themajority of the indicators studied, except that a slight decrease was observed inthe most reactive amino acids in the case of conventionally heated orange juice.This fact could be due to overheating of the juice at the tubular heat exchanger.

1998 Society of Chemical Industry(

J Sci Food Agric 78, 196È200 (1998)

Key words : microwave ; conventional heating ; quality parameters ; orange juice

INTRODUCTION

One of the principal reasons for pasteurising citrus juiceis the inactivation of pectin methylesterase (PME), theenzyme responsible for the loss of cloudiness, which is avery important quality attribute for the consumers(Varsel 1980). Nevertheless, process conditions of juicescan a†ect some quality parameters such as ascorbic-acid content (Saguy et al 1978). Ascorbic-acid degrada-tion can lead to non-enzymatic browning (NEB) ;therefore, ascorbic-acid loss is not only important nutri-tionally, but also its degradation is related to Ñavourand colour changes (Handwerk and Coleman 1988 ; Leeand Nagy 1988 ; Nagy et al 1989). Other parametersinclude sucrose loss due to hydrolysis (Curl and Talburt1961) and hydroxymethyl furfural (HMF) formation(Nagy 1980).

* To whom correspondence should be addressed.Contract/grant sponsor : Comision Interministerial de Cienciay Tecnolog•� a.Contract/grant number : ALI 94-0872.

During recent years, microwave heating and otheremerging technologies have received considerable atten-tion due to the consumer interest in minimally pro-cessed products. Microwave treatment has well-knownadvantages over conventional heating processes, partic-ularly because of the fact that foods are heated directlyand rapidly without contact with hot surfaces (Youngand Jolly 1990).

Since the Ðrst reported study on the use of microwaveenergy to pasteurise orange juice (Copson 1954), someauthors have investigated the e†ect of this treatment onorange and other citrus juices (Nikdel et al 1993 ;Abd-El-Al et al 1994). Thus, using a continuous micro-wave system and a conventional plate heat exchanger,Nikdel et al (1993) performed di†erent treatments andpointed out the e†ectiveness of both systems to inac-tivate PME and bacteria in freshly squeezed orangejuice. However, conventional heat pasteurisation causedo†-Ñavour due to overheating of the juice at the heat-exchange surface. Also, Abd-El-Al et al (1994) foundsmaller ascorbic-acid losses and browning during batch

1961998 Society of Chemical Industry. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain(

T hermal treatment of orange juice 197

microwave treatment of orange juice than thoseobtained by batch conventional processes.

The aim of this work was to assess the e†ect of con-tinuous microwave treatment of orange juice on qualityparameters and to compare it with a conventionalheating system under similar processing conditions.

EXPERIMENTAL

Material

Freshly squeezed juice was prepared in the laboratoryfrom 10 kg of oranges (Navel variety) using a domesticjuicer Braun Citromatic MPZ2 (Braun Espan8 ola,Barcelona, Spain). The extracted juice was Ðlteredthrough a double layer of gauze to remove seeds, albedofragments and to reduce pulp content. The pH of theorange juice was measured before and after thermaltreatments with a pH-meter MicropH2001 (CrisonInstruments, Barcelona, Spain).

Microwave and conventional heating systems

A continuous-Ñow microwave heating system was setup using a 2450 MHz MDS-2000 oven (CEM Corpora-tion, Buckingham, UK). The measured power outputwas 525 W, as determined by the water load method(Copson 1975). The oven was set to operate at 100%power.

A TeÑon coil (2É0 m long, 5 mm internal diameter)was introduced into the oven cavity through two holesof 7 mm diameter drilled into the lower left-hand side ofthe oven. Previous experiments were conducted withdistilled water in order to select the conditions for con-tinuous microwave treatment that provided a gooddegree of uniformity in the outlet temperatures(Villamiel et al 1996). Inlet and outlet temperatures werecontinuously monitored using digital thermometersDigitem D 2000 (TFA-Dostmann, Germany) positionedjust outside the cavity. The orange juice, initially at

20¡C, was pumped through the system using a VariableSpeed Peristaltic Tubing Pump XX8000230 (Millipore,Bedford, MA, USA). The process conditions wereadjusted by varying the Ñow rate (Table 1). The orangejuice leaving the oven was then passed through a 4É5 mlong and 2É16 mm diameter stainless-steel tubeimmersed in an ice/water-bath to cool the microwavedorange juice rapidly. Before samples were taken foranalysis, sufficient orange juice was run to achievesteady-state heating conditions.

The tubular conventional heating system was set upas the microwave system, except for the fact that theheating section was replaced by a 2É5 m long and4É5 mm diameter stainless-steel coil (with the sameinternal volume as the TeÑon coil), and a wall thicknessof 0É93 mm, immersed in a temperature-controlled glyc-erol bath. The temperature of the glycerol bath wasadjusted to provide the same outlet temperature as themicrowave oven under the same Ñow conditions (Table1), thus ensuring equal heat input.

All heat-treatment assays were performed in dupli-cate.

Chemical analyses

PME activity was determined according to the methodof Rouse and Atkins (1952) by estimating the free car-boxyl groups formed in pectin as a result of enzymeaction.

The content of ascorbic acid was determined byHPLC as previously described (Rapisarda and Inteli-sano 1996).

The samples were centrifuged at 6940] g for 20 minand diluted with borate bu†er 0É4 M (pH 10É3), fordetermination by reverse phase HPLC of the OPA-amino acid derivatives following the method describedby Gonza� lez de Llano et al (1991).

Carbohydrates (fructose, glucose, sucrose and myo-inositol) were determined by gas chromatography(Villamiel et al 1997).

HMF was determined by measuring the absorbanceat 550 nm according to the method reported by IFFJP(1972).

TABLE 1Orange juice heating conditions used in continuous microwave and conventional systems

Flow Microwave Conventional(ml min~1)[residence T rial Outlet temperature* T rial Outlet temperature Bath temperaturetime, (s)] (¡C) (¡C) (¡C)

98É0 [24] A 89É7 C 89É8 95É087É2 [27] B 96É4 D 96É5 102É0

* Average outlet temperatures of two experiments (relative SDO 1É0%). In all cases, the initial tem-perature of the orange juice was 20¡C.

198 M V illamiel et al

Browning was measured as the absorbance at 420 nmof the samples following the method of Meydav et al(1977).

RESULTS AND DISCUSSION

Table 2 shows the e†ect of heating on some qualityparameters studied in orange juice heated at tem-peratures close to 90 and 96¡C (Table 1) in bothcontinuous-Ñow microwave and conventional systems.The values of pH were as expected for juice obtainedfrom the Navel variety (Hasegawa et al 1982), and nochanges were observed during heat treatment.

PME activity of the raw orange juice was lower thanthe values reported by Snir et al (1996) for the Navelvariety harvested at di†erent times. This could be due tothe fact that the orange juice was Ðltered, as it is well-known that PME activity is lower in juice with lowpulp content (Rouse et al 1954). Regarding the e†ect ofthe heat treatment, the inactivation of PME was veryhigh and similar in both the microwave system and con-ventional tubular heat exchanger used. The PME inac-tivation obtained was similar to that reported by Nikdelet al (1993), who heated orange juice in a continuousmicrowave system with the inclusion of a holding phase

(10 and 15 s) to maintain the temperature after the oven(90É5È97¡C) and in a conventional plate heat exchangerunder similar conditions. Although 2É5% of originalenzyme activity remained in one of the microwave heattreatments, the cloudiness of orange juice could bemaintained for a few months (Rothschild and Karsenty1974).

The content of ascorbic acid in the raw orange juicewas within the range found by Park et al (1983) forNavel single-strength orange juice. Ascorbic-acid con-centration remained unchanged after both thermaltreatments, so the severity and type of thermal pro-cessing did not a†ect the ascorbic-acid content. Abd-El-Al et al (1994) observed smaller ascorbic-acid losses inorange juice heated in a batch microwave system thanin a hot water-bath.

The carbohydrate content was not a†ected by theprocessing. Fructose, glucose and sucrose values werevery similar to those found by Park et al (1983) forNavel single-strength orange juice. Myo-inositol contentwas close to values reported by Krehl and Cowgil (1950).

No presence of HMF was detected in any of theorange juices analysed, probably due to the mildheating conditions. This result is in agreement with theascorbic acid and carbohydrate data since it is knownthat HMF can be an end product of ascorbic aciddecomposition or carbohydrate breakdown (Nagy1980 ; Lee and Nagy 1988 ; Nagy et al 1989).

TABLE 2E†ect of heat treatments on di†erent constituents of orange juice heatedunder similar conditions in continuous microwave and conventional systemsa

Analysis Control Microwave Conventional

A B C D

pH 3É61 3É66 3É62 3É65 3É62(0É00) (0É01) (0É00) (0É01) (0É01)

PME Èb 97É50 98É80 98É90 98É90(% inactivation) (0É00) (0É21) (0É00) (0É00)Ascorbic acid 0É58 0É58 0É57 0É58 0É57

(g litre~1) (0É02) (0É01) (0É01) (0É00) (0É02)Fructose 26É85 NDc 27É10 ND 26É95(g litre~1) (1É48) (0É86) (0É86)Glucose 25É91 ND 26É86 ND 26É49

(g litre~1) (1É05) (0É66) (1É50)Sucrose 54É06 ND 54É49 ND 55É04

(g litre~1) (1É33) (0É65) (0É10)Myo-inositol 1É85 ND 1É92 ND 1É87

(g litre~1) (0É07) (0É04) (0É08)HMF 0É021 0É018 0É016 0É020 0É020

(*A550) (0É002) (0É002) (0É001) (0É002) (0É001)Browning 0É168 ND 0É197 ND 0É203(*A420) (0É000) (0É007) (0É002)

a Mean values, with SD in parentheses (n \ 2).b PME activity of the raw orange juice was 1É40 pectin methylesterase activ-ity unit (PEU) ml~1.c Not determined.

T hermal treatment of orange juice 199

A slight development of colour was noted in thesamples after the heat treatments. This result could be aconsequence of cloud modiÐcation caused during theprocessing (Curl and Talburt 1961). The present authorsdid not observe any inÑuence of the intensity and typeof heat treatment. Abd-El-Al et al (1994) reportedsmaller browning development during batch microwavetreatment of orange juice as compared to that arisingduring batch conventional heating. This could be attrib-uted to higher rates of heat penetration in the case ofmicrowave procedures which reduces the treatmenttime.

In general, amino-acid values (Table 3) were veryclose to the data reported by Park et al (1983) for Navelsingle-strength orange juice. The little di†erencesobserved might be due to geographical and farmingpractices (Wallrauch and Faethe 1988), as well as thesize of the sample and the methodology used to deter-mine amino-acid content. Taking into account the rela-tive standard deviation of the methodology (6%), nodi†erences were found between control and microwave-treated orange juice. However, conventionally heatedjuice showed a slight decrease in some amino acidswhich have been previously reported as the most reac-tive in orange juice (Wolfrom et al 1974 ; del Castilloet al 1998). Since, in the present work, the level of heattreatment was the same in both microwave and conven-tional systems, the little di†erences observed could be

attributable to better heat distribution and the lack ofhot surfaces contacting the orange juice in the case ofthe microwave unit.

All these results indicate that continuous microwaveheating can be an e†ective method for PME inac-tivation without the inclusion of a holding phase.Changes of orange juice constituents were very similarto those obtained with a conventional system havingsimilar heating and cooling phases. So, continuousmicrowave heating compares favourably with conven-tional heating at PME inactivation temperatures.

ACKNOWLEDGEMENTS

This work has been supported by the Comision Inter-ministerial de Ciencia y Tecnologi� a (Project ALI 94-0872). One of the authors (MV) acknowledges InstitutoDanone for a fellowship.

REFERENCES

Abd-El-Al M G, Abd-El-Fadeel M G, El-Samahy S K, AskarA 1994 Application of microwave energy in the heat treat-ment of fruit juices, concentrates and pulps. Fruit Proc 4307È312.

TABLE 3E†ect of heat treatments on amino acid fraction of orange juice heatedunder similar conditions in continuous microwave and conventional sys-

temsa

Amino acid Control Microwave Conventional(mmol litre~1) (B) (D)

Alanine 1É118 (0É021) 1É108 (0É035) 1É008 (0É035)c-Amino butyric acid 2É171 (0É002) 2É021 (0É020) 1É791 (0É089)Arginine 6É036 (0É054) 5É676 (0É039) 5É355 (0É268)Asparagine 5É318 (0É110) 5É199 (0É058) 4É936 (0É260)Aspartic acid 1É167 (0É010) 1É144 (0É012) 1É068 (0É066)Glutamic acid 1É055 (0É007) 1É026 (0É006) 0É964 (0É040)Glutamine 0É680 (0É002) 0É661 (0É024) 0É640 (0É032)Glycine 0É289 (0É007) 0É285 (0É001) 0É267 (0É017)Hystidine 0É273 (0É002) 0É244 (0É009) 0É246 (0É009)Isoleucine 0É163 (0É012) 0É149 (0É007) 0É157 (0É009)Leucine 0É178 (0É009) 0É166 (0É012) 0É164 (0É010)Lysine 0É520 (0É010) 0É520 (0É010) 0É429 (0É017)Metionine 0É111 (0É010) 0É099 (0É003) 0É104 (0É001)Ornitine 0É190 (0É004) 0É208 (0É022) 0É202 (0É039)Phenylalanine 0É227 (0É012) 0É229 (0É005) 0É229 (0É018)Serine 2É035 (0É014) 2É003 (0É031) 1É885 (0É087)Threonine 0É250 (0É010) 0É240 (0É005) 0É250 (0É011)Tryptophan 0É151 (0É001) 0É135 (0É001) 0É150 (0É007)Tyrosine 0É165 (0É005) 0É167 (0É003) 0É157 (0É004)Valine 0É230 (0É005) 0É214 (0É006) 0É207 (0É001)

a Mean values, with SD in parentheses (n \ 2).

200 M V illamiel et al

Copson D A 1954 Microwave irradiation of orange juice con-centrate for enzyme inactivation. Food T echnol 9 397È399.

Copson D A 1975 In : Microwave Heating. AVI, Westport,CT, USA, 2nd edn, pp 615.

Curl A L, Talburt W F 1961 Deterioration in storage. In :Fruit and V egetable Juice : processing technology, eds Tress-ler D K & Joslyn M A. AVI, Westport, CT, USA, pp 410È446.

del Castillo D, Corzo N, Polo M C, Pueyo E, Olano A 1998Changes in the amino acid composition of dehydratedorange juice during accelerated nonenzymatic browning. JAgric Food Chem 46 277È280.

Gonza� lez de Llano D, Polo M C, Ramos M 1991 Production,isolation and identiÐcation of low molecular mass peptidesfrom blue cheese by high performance liquid chromatog-raphy. J Dairy Res 58 363È372.

Handwerk R L, Coleman R L 1988 Approaches to the citrusbrowning problem: a review. J Agric Food Chem 36 231È236.

Hasegawa S, Patel M N, Snyder R C 1982 Reduction oflimonin bitterness in Navel orange juice serum with bac-terial cell immobilized in acrilamide gel. J Agric Food Chem30 509È511.

IFFJP 1972 Analyses, 3rd edn. International Federation ofFruit Juice Producers, p 12.

Krehl W A, Cowgill G R 1950 Vitamin content of citrus pro-ducts. Food Res 15 179È191.

Lee H S, Nagy S 1988 Quality changes and nonenzymaticbrowning intermediates in grapefruit juice during storage. JFood Sci 53 168È172.

Meydav S, Saguy I, Kopelman I J 1977 Browning determi-nation in citrus products. J Agric Food Chem 25 602È604.

Nagy S 1980 Vitamin C content of citrus fruit and their pro-ducts : a review. J Agric Food Chem 28 8È18.

Nagy S, Rouse† R L, Lee H S 1989 Thermally degraded Ña-vours in citrus juice products. In : T hermal Generation ofAromas (ACS Symp Ser 409) eds Parliment T H, McGorrinR J & Ho C. American Chemical Society, Washington, DC,USA, pp 331È345.

Nikdel S, Chen C S, Parish M E, Mackellar D G, Friedrich LM 1993 Pasteurization of citrus juice with microwaveenergy in a continuous-Ñow unit. J Agric Food Chem 412116È2119.

Park G L, Byers J L, Pritz C M, Nelson D B, Navarro J L,

Smolensky D C, Vandercook C E 1983 Characteristics ofCalifornia Navel orange juice and pulpwash. J Food Sci 48627È632, 651.

Rapisarda P, Intelisano S 1996 Sample preparation forvitamin C analysis of pigmented orange juices. Ital J FoodSci 8 251È256.

Rothschild G, Karsenty A 1974 Cloud loss during storage ofpasteurized citrus juices and concentrates. J Food Sci 391037È1041.

Rouse A H, Atkins C D 1952 Heat inactivation of pecti-nesterase in citrus juice. J Food T echnol 6 291È294.

Rouse A H, Atkins C D, Huggart R L 1954 E†ect of pulpquantity on chemical and physical properties of citrus juicesand concentrates. Food T echnol 8 431È435.

Saguy I, Kopelman I J, Mizrahi S 1978 Simulation of ascorbicacid stability during heat processing and concentration ofgrapefruit juice. J Food Proc Engng 2 213È225.

Snir R, Koehler P E, Sims K A, Wicker L 1996 Total andthermostable pectinesterases in citrus juices. J Food Sci 61379È382.

Varsel C 1980 Citrus juice processing related to quality andnutrition. In : Citrus Nutrition and Quality (ACS Symp Ser143), eds Nagy S & Attaway, J A. American ChemicalSociety, Washington, DC USA, p 225.

Villamiel M, Lo� pez-Fandin8 o R, Corzo N, Marti� nez-Castro I,Olano A 1996 E†ects of continuous-Ñow microwave treat-ment on chemical and microbiological characteristics ofmilk. Z L ebensm Unters Forsch 202 15È18.

Villamiel M, Marti� nez-Castro I, Olano A, Corzo N 1998Quantitative determination of carbohydrates in orangejuice by gas chromatography. Z L ebensm Unters Forsch A206 48È51.

Wallrauch S, Faethe W 1988 Germany : RSK valuesÈguidelines and tolerances for speciÐed constituents in fruitjuice. In : Adulteration of Fruit Juice Beverages, eds Nagy S,Attaway J A & Rhodes M E. Marcel Dekker, New York,USA, pp 405È470.

Wolfrom M L, Kashimura N, Horton D 1974 Factors a†ect-ing the Maillard browning between sugars and amino acids :studies on the nonenzymatic browning of dehydratedorange juice. Food Chem 22 796È800.

Young G S, Jolly P G 1990 Microwaves : the potential for usein dairy processing. Aust J Dairy T echnol 45 34È37.