Lebensm.-Wiss. u.-Technol., 34, 380}383 (2001)

Thermal Kinetics of Colour Degradation and StorageCharacteristics of Onion Paste

J. Ahmed and U. S. Shivhare*

Department of Food Science & Technology, Guru Nanak Dev University, Amritsar-143005 (India)(Received February 29, 2000, accepted February 13, 2001)

The kinetics of colour degradation at selected temperatures and ewects of packaging materials and storage temperatures on colour ofonion paste were determined. Colour change during thermal processing of onion paste followed xrst-order reaction kinetics. Thetristimulus colour value combination La/b adequately described the colour degradation. Dependence of the rate constant during thermalprocessing of onion paste obeyed the Arrhenius relationship. The activation energy for onion paste was estimated to be 16.2 kJ/mol. Thetotal colour of onion paste was signixcantly awected (P40.05) by packaging materials, temperature and duration of storage. The pastewas more stable at low temperature (5 3C) than at higher temperatures (25 3C) with respect to colour, and colour degradation wasminimum when the paste was packed in a high-density polyethylene pouch.

( 2001 Academic Press

Keywords: onion paste; kinetics; colour; packaging materials; storage

Introduction

Onion (Allium cepa L.) is valued for its therapeutic prop-erties and possesses a strong characteristic aroma and#avour which makes it an important ingredient duringfood processing (Augusti et al., 1996; Dron et al., 1997).The characteristic #avour of onions comes primarilyfrom volatile organic sulphur compounds released en-zymatically by the action of allinase on several naturallyoccurring amino-precursors during comminution. Theprimary reaction products are thiosul"nates, which areheat labile and dissociate on heating to form di- andtri-sul"des and other sulphur compounds that havebeen associated with cooked onion #avour (Block& O'Conner, 1974). Onion paste is one such product thatis convenient to use and could retain the original colourand #avour in a semi-solid form.Colour is an important attribute because it is usually the"rst property the consumer observes (Saenz et al., 1993).Various factors are responsible for the loss of colourduring processing of food products. These includenon-enzymatic and enzymatic browning and processconditions such as pH, acidity, packaging material andduration and temperature of storage. To optimize theprocess, it is important to determine the kinetic para-meters (reaction order, reaction rate constant, activation

*To whom correspondence should be addressed. E-mail: jasahmed@redi!mail.com

0023-6438/01/060380#04 $35.00/0( 2001 Academic Press All art

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energy) for colour change (Weemaes et al., 1999). Thecolour kinetics of food products is a complex phenom-enon and dependable models to predict experimentalcolour change, which can be used in engineering calcu-lations, are limited. Therefore experimental studies andapplication of various simpli"ed models to represent thebehaviour are required.Anthocyanin is the major pigment responsible for colourin red onions and the major compounds identi"edwere cynidin-3-glucoside, cyanidin-3-diglucoside andpeonidin glucoside (Kalra, 1987). Onions contain reduc-ing sugars and amino acids that enhance non-enzymaticbrowning during thermal processing (Berk, 1980; Shin &Bhowmick, 1994). Several authors have studied the col-our kinetics of food materials during thermal processingin terms of changes in Hunter tristimulus colour values ¸,a and b (Shin & Bhowmik, 1994; Berry, 1998; Kajunaet al., 1998; Nanke et al., 1999; Weemaes et al., 1999; Ahmedet al., 2000). Hence, if the kinetics of colour degradation isdetermined and the order of colour change is established,the total colour can be used to evaluate quality of foodmaterial during thermal processing. No information isavailable on thermal kinetics of colour degradation andstorage behaviour of onion paste.The present study was undertaken to investigate thekinetics of colour degradation of onion paste at selectedprocessing temperatures using the Hunter colour scalevalues (¸, a, b) and the e!ects of temperature and packag-ing material on colour during storage of onion paste.

doi:10.1006/fstl.2001.0771icles available online at http://www.idealibrary.com on

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lwt/vol. 34 (2001) No. 6

Materials and Methods

Preparation of pasteOnion bulbs (variety: Pusa Red) were obtained locally,peeled, cut and immediately processed into puree usinga laboratory size grinder. The puree was passed througha 14-mesh sieve to obtain the product of uniform consist-ency. The puree was held at room temperature(25$1 3C) for 1 h in a covered container to facilitateenzymatic action for colour and #avour development(Dron et al., 1997). Common salt (sodium chloride) wasadded (5, 7 and 10 g/100 g) to onion puree to increase itstotal soluble solids (TSS). Sensory analysis results in-dicated consumer's preference for the puree containing10% salt. The paste was therefore prepared by adding10% salt. Onion puree had a pH of 5.45 and "nal pH ofthe paste was adjusted to 3.9 by adding desired quantityof citric acid solution. It has been established that citricacid acts as an anti-oxidant and an acidi"ed food(pH (4.6) requires mild heat treatment to make it shelfstable (Garcia et al., 1999).

Thermal kineticsThermal kinetics of onion paste was studied by heating itat selected temperatures (60, 70, 80 and 90 3C) for a resi-dence time of 0}20 min. Approximately 200 g paste wasweighed and transferred into a 250 mL glass beaker andcovered with a lid. The beakers containing paste wereplaced in a constant temperature water-bath at selectedtemperatures ($1 3C) and periodically agitated to en-sure uniform temperature throughout the bulk of sample.The temperature of the sample at its geometric centrewas monitored using a thermometer. The beakers wereheated for 0, 5, 10, 15 and 20 min respectively after thepaste at its geometric centre attained the desired temper-ature. The samples were transferred to an ice water bathimmediately after the thermal treatment.

Colour measurementColour measurement was carried out using a Huntercolourimeter model D25 optical sensor (Hunter Associ-ates Laboratory Inc., Reston, VA, U.S.A.) on the basis ofthree colour values, namely ¸, a and b. The instrument(453/03 geometry, 103 observer) was calibrated againsta standard white reference tile (¸"90.55, a"!0.71,b"0.39). A glass cell containing the heat-treated pastewas placed above the light source and covered witha white plate and ¸, a, b values were recorded.

Model for computation of change in colourDegradation of colour pigments has been shown to fol-low the "rst-order reaction kinetics (Huang & Von Elbe,1985; Hutchings, 1994; Shin & Bhowmik, 1994; Toledo,1997; Ahmed et al., 2000). Therefore, it was reasoned thatthe colour change due to thermal treatment of onionpaste follows the "rst-order reaction kinetics, representedby Eqn [1]

ln (C/C0)"!k

)) t

)Eqn [1]

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with C measured Hunter colour value (¸, a, b) or a com-bination of these, dimensionless; C

0measured colour

value(s) at zero time, dimensionless; k)

rate constantduring heating (min~1), t

)heating time (min).

The Arrhenius equation to relate the dependence of therate constant with temperature is represented by Eqn [2]:

k)"k

)0exp (!E/RT) Eqn [2]

with k)0

frequency factor (min~1), E"activation energy(kJ/mol), R"universal gas constant (8.314 J/mol/K),T"absolute temperature (K).

Storage studiesThe paste was thermally processed at 80 3C for 15 minand packaged immediately in selected containers [glass,polyethylene terephthalate (PET) and high-density poly-ethylene (HDPE) pouch]. Storage studies were carriedout at room temperature (25$1 3C) and refrigerationtemperature (5$1 3C) respectively for 71 days. The sam-ples were analysed periodically for colour, TSS, pH andtitratable acidity.

Physico-chemical propertiesTotal soluble solids (3Brix) and pH were determinedusing a Refractrometer (Atago, Japan) at 20 3C and a pHmeter with glass electrode (Systronics, India) respectively.Titratable acidity was measured in terms of citric acid bytitrating the diluted paste against 0.1 N NaOH solutionusing phenolapthalein indicator (Rangana, 1986). So-dium chloride was determined by titration with silvernitrate (Rangana, 1986). Water activity (a

8) of the prod-

uct was determined following the method of Landrockand Proctor (1951). Each experiment was replicated twiceand the average values were used in the analysis.

Statistical analysisThe in#uence of time and temperature of storage oncolour was determined by paired samples t-test usingSPSS (1996) software. Signi"cance of di!erences was de-"ned at P40.05.

Results and Discussion

The pH and acidity of onion paste were 3.9 and 0.41%respectively. TSS and water activity values were 16.53Brix and 0.84 respectively. The paste contained 9.3%sodium chloride. TSS, titratable acidity and pH of onionpaste did not change signi"cantly (P'0.05) during stor-age. The initial Hunter colour ¸, a and b values of onionpaste were 39.48, 7.3 and 4.20 respectively.Degradation in colour of onion paste was observed dur-ing thermal processing. Red pigment decreased andproduct turned brown with time as re#ected by decreasein both a and ¸ values. There was a correspondingincrease in the b value. Therefore, di!erent combinationsof tristimulus ¸, a, b colour values were tested to describethe total colour change of onion paste. These combina-tions were subjected to linear regression with respect to

1

Table 1 Regression coe$cients of Eqn [1] for selectedcombinations of the Hunter colour scale values for onionpaste at 60 3C

Combination Correlation coe$cient Standard error

¸ab 0.832 0.0024¸a/b 0.980 0.0015¸/ab 0.967 0.0017b¸/a 0.911 0.0021

Fig. 1 Temporal variation of Hunter colour value ratio(¸a/b)/(¸

0a0@b0) of onion paste at selected temperatures (s)

60 3C, (n) 70 3C, (h) 80 3C, (e) 90 3C, (!) Eqn [1]

Fig. 2 The Arrhenius plot relating rate constant to processtemperature (!) Eqn [2]

Fig. 3 E!ect of packaging materials on total colour of onionpaste packed in (s) glass container, (n) PET container, (h)HDPE pouch, (!) Eqn [3] at 5 3C

lwt/vol. 34 (2001) No. 6

time as represented by Eqn [1] and the coe$cients weredetermined (Table 1). Correlation coe$cient and stan-dard error values were used as the basis to select thecombination which best described the "rst-order reactionfor the entire temperature range. It was found that ¸a/bwas the most appropriate combination, which describedclosely the "rst-order reaction kinetics of colour degrada-tion of onion paste (Fig. 1). The coe$cient of correlationvalues were between 0.980 and 0.997 while the standarderror values were less than 0.0016. Shin and Bhowmik(1994) have reported similar observations while workingon thermal processing of pea puree. They reasoned thatall the three parameters should be combined togetherand found ¸a/b as the optimum combination to describethe total colour degradation. While working on thermal

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processing of green chilli puree Ahmed et al. (2000) found¸]a]b to be the best combination for describing thetotal colour degradation.E!ect of temperature on the rate constant is shown inFig. 2. The dependence of the rate constant on temper-ature obeyed the Arrhenius relationship (Eqn [2]). Cor-relation coe$cient for the linear regression analysisequaled 0.99. The computed value of activation energywas 16.2 kJ/mol which is in range with the value reportedby Ahmed et al. (2000) (16.0 kJ/mol for lye-treated greenchilli puree).

Storage characteristicsColour of onion paste was signi"cantly a!ected(P40.05) by both packaging materials and conditionsduring storage. Change in colour of paste was minimumin HDPE pouch but maximum while it was stored inglass containers at both 5 and 25 3C. A typical behaviouris shown in Fig. 3. Air was excluded before sealing theHDPE pouches, while glass and PET bottles had headspace. Presence of head space air may therefore havecaused oxidation leading to colour change during stor-age. The product was stable at refrigerated temperature(5 3C) as compared to 25 3C (Fig. 4). This may be due tofaster rate of colour degradation at higher temperature(Sagar & Maini, 1997).Variation in colour of paste during storage was repre-sented by Eqn [3]

ln (¸a/b)/(¸0a0/b

0)"!k

4) t

4Eqn [3]

with k4rate constant during storage (day~1), t

4storage

time (days) (Table 2). The solid lines in Figs 3 and 4 rep-resent Eqn [3]. It is obvious from these "gures that Eqn[3] described adequately the variation of total colour ofonion paste with duration of storage.This study has veri"ed the previous "ndings (Bajaj et al.,1997; Berk, 1980) that red pigment (anthocyanin) de-grades (manifested by decrease in the Hunter a value) andnon-enzymatic browning takes place (decrease in the¸ and increase in the b values) during thermal processingand the extent of variation was governed by processingtemperature. Further, the total colour (¸a/b), expressedby incorporating all three Hunter colour (¸, a, b) values,

2

Table 2 Values of the coe$cients of Eqn [3]

Packaging material Storage temperature k4

Correlation coe$cient Standard error

Glass container 25 3C !0.0673 0.989 0.00225 3C !0.0175 0.991 0.0005

a 25 3C !0.0328 0.982 0.00145 3C !0.0069 0.994 0.0002

b 25 3C !0.0341 0.955 0.00225 3C !0.0060 0.963 0.0004

aPET container.bHDPE pouch.

Fig. 4 E!ect of storage temperature on total colour of onionpaste packed in HDPE pouch at (s) 25 3C, (n) 5 3C, (!)Eqn [3]

lwt/vol. 34 (2001) No. 6

represented well the degradation process in both thermalprocessing and storage. Therefore the tristimulus colourmeasurement may be incorporated for on-line qualitycontrol as advocated by Rocha et al. (1993). Results ofthis study indicated that onion paste should be stored inHDPE pouch at 5 3C (Fig. 4).

Conclusions

The kinetics of colour change of onion paste followed"rst-order reaction. The combination ¸a/b can be used topredict the degradation of total colour of onion paste.The rate constant increased with temperature and thedependence could be described using the Arrhenius equa-tion. Colour of paste was better when it was stored inHDPE pouch at 5 3C. This study clearly demonstratesthe advantage of objective measurement to study colourchanges of onion paste during both thermal processingand storage.

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