COLOR CHANGE KINETICS OF CELERY LEAVES UNDERGOING MICROWAVE HEATING

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COLOR CHANGE KINETICS OF CELERYLEAVES UNDERGOING MICROWAVEHEATINGElçin Demırhan a & Belma Özbek a

a Department of Chemical Engineering , Yıldız Technical University ,Davutpaşa Campus, Esenler/Istanbul, TurkeyPublished online: 09 Jun 2011.

To cite this article: Elçin Demırhan & Belma Özbek (2011) COLOR CHANGE KINETICS OF CELERYLEAVES UNDERGOING MICROWAVE HEATING, Chemical Engineering Communications, 198:10,1189-1205, DOI: 10.1080/00986445.2010.525106

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Color Change Kinetics of Celery Leaves UndergoingMicrowave Heating

ELCIN DEMIRHAN AND BELMA OZBEK

Department of Chemical Engineering, Yıldız Technical University,Davutpasa Campus, Esenler=Istanbul, Turkey

The aim of this study was to investigate the effect of microwave output power andsample amount on color change kinetics of celery leaves (Apium graveolens L.)during microwave heating. The color parameters of the materials were quantifiedby the Hunter Lab system. These values were also used for calculation of the totalcolor change, chroma, hue angle, and browning index. The microwave heating pro-cess changed color parameters of L, a, and b, causing a color shift towards darkerregion. The mathematical modeling study of color change kinetic showed that L, a,b, and chroma fitted to a first-order kinetic model, while total color change (DE),hue angle, and browning index (BI) followed a zero-order kinetic model. For calcu-lation of the activation energy for color change kinetic parameters, the exponentialexpression based on the Arrhenius equation was used.

Keywords Activation energy; Celery leaves; Color kinetic; Microwave heating

Introduction

The removal of moisture from fresh fruits and vegetables prevents the growthand reproduction of microorganisms that cause decay, minimizes many of themoisture-mediated deteriorative reactions (Mujumdar, 1995; Zhao et al., 2005),inactivates enzymes, and provides for preservation of seasonal plants for whole year(Krokida et al., 2001; Maskan, 2000a; Avila and Silva, 1999; Lund, 1975). However,during the drying process, the food material may be exposed to temperatures thathave an adverse effect on quality and organoleptic properties of foods. Kinetic mod-els of thermal degradation are essential to design new processes resulting in a safefood product and maximum retention of quality factors (Lund, 1975; Teixeiraet al., 1969).

The color of a food is one of the most important quality factors and plays a sig-nificant role in its appearance, processing, and acceptability. This is especially impor-tant at the point of sale, where the first impact made on a consumer by a food is itsvisual appearance. It is perceived as part of the total appearance, which is the visualrecognition and assessment of the surface and subsurface properties of the foodmaterial (Tijskens et al., 2001). Color change during thermal processing takes placebecause of reactions occurring inside the food material. These reactions can be

Address correspondence to Belma Ozbek, Department of Chemical Engineering, YıldızTechnical University, Davutpasa Campus, 34210, Esenler=Istanbul, Turkey. E-mail: [email protected]

Chem. Eng. Comm., 198:1189–1205, 2011Copyright # Taylor & Francis Group, LLCISSN: 0098-6445 print=1563-5201 onlineDOI: 10.1080/00986445.2010.525106

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pigment degradation, especially carotenoids and chlorophyll, and browning reactionssuch as Maillard condensation of hexoses and amino components and oxidation ofascorbic acid (Lozano and Ibarz, 1997; Lee and Coates, 1999; Barreiro et al.,1997). Therefore, the final values of color parameters can be used as quality indicatorsto evaluate deterioration due to thermal processing (Shin and Bhownik, 1995).

Color measurements of food materials can be used as an indirect way to deter-mine quality change, since they are simpler and faster than a complete physico-chemical analysis. The color brightness coordinate L measures the whiteness valueof a color and ranges from black at 0 to white at 100. The chromaticity coordinatea measures red when positive and green when negative, and the chromaticitycoordinate b measures yellow when positive and blue when negative. Hunter colorparameters have been previously demonstrated to be valuable in describing visualcolor deterioration and providing useful information for quality control in fruitsand vegetables (Tijskens et al., 2001; Garza et al., 1999; Maskan et al., 2002;Ganasekharan et al., 1992; Tian et al., 1995, 1996; Barrett et al., 2000; Gunawanand Barringer, 2000). Additional parameters are derived from the Hunter L, a,and b scale; the total color change (DE); chroma value, which indicates the degreeof color saturation and is proportional to the strength of the color; hue angle, whichis frequently used to characterize color in food products; and browning index (BI),which represents the purity of brown color and is reported as an important para-meter in drying processes where enzymatic and nonenzymatic browning takes place(Zhao et al., 2005; Lozano and Ibarz, 1997).

Celery (Apium graveolens L.) is usually dried and used as a soup enhancer (becauseof its characteristic odor), improving flavor and aroma of the stew, and also possessessome medicinal value. The aromatic meat of its callous celeriac and aromatic leaf,which are rich in protein and carotene, are the edible parts of celery. Some other usesof celery are as aphrodisiacs, anthelmintics, antispasmodics, sedatives, stimulants,and tonics and also against asthma, bronchitis, and rheumatism (Madamba andFerdinand, 2001; Jezek et al., 2008; Engindeniz, 2008; Rupam and Bhatnagar, 2007).

Study of the color change behavior of foods during drying has recently been asubject of interest for various investigators, for example for garlic (Cui et al.,2003), rosehip (Koyuncu et al., 2003), mix of spinach and mustard leaves (Ahmedet al., 2002), green chilli puree (Ahmed et al., 2000), banana (Chen and Ramaswamy,2002; Maskan, 2000b), mango pulp (Cunha et al., 2006), melon (Pereira et al., 2006),okra (Dadali et al., 2007a), spinach (Dadali et al., 2007b), and basil (Demirhan andOzbek, 2009). While there are many literature studies on the kinetics of changes inother vegetables, kinetics of color change has not been reported for celery leaves.This information might be useful for people doing product development work withcelery leaves. Therefore, the purpose of this work is to study the kinetics of colordegradation during microwave heating of celery leaves and also to calculate the acti-vation energies for color change kinetic parameters using the exponential expressionbased on the Arrhenius equation.

Materials and Methods

Materials

Plants of fresh celery were purchased from a local supplier in Istanbul. They werewashed and stored at 4� 0.5�C in a refrigerator. Before the drying experiments,

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the samples were taken out of the refrigerator and leaves were separated from stemsand then weighed. To determine the initial moisture content, four 50 g samples weredried in an oven (Memmert UM-400) at 105�C for 12 h (Soysal, 2004). The initialmoisture content of celery leaves was determined as 6.65 kg=kg d.b. as an averageof the results obtained. The relative standard deviation of the measured valueswas calculated, and it was found to be in the range of 2%.

Drying Equipment and Drying Procedure

Drying treatments were performed in a domestic digital microwave oven (ArcelikMD 594, Turkey) with technical features of �230V, 50Hz, and 2650W with a fre-quency of 2450MHz. Full description of the drying equipment and drying procedurecan be found in previous work reported by Dadali et al. (2007c, d). The effect ofmicrowave output power and sample amount on color change kinetics of celeryleaves was investigated. Three replications of each experiment were performedaccording to a preset microwave output power and time schedule, and the data givenare an average of these results. The relative standard deviation of the measuredvalues was calculated, and it was found to be in the range of 2%.

The microwave power was applied until the weight of the sample was reduced toa level corresponding to moisture content of about 0.10 kg=kg d.b.

Microwave Output Power Measurement

The microwave output power for each level of the system was measured using theIMPI 2-liter test (Buffler, 1993). Two 1L beakers were filled with water and initialtemperatures of water were measured. Then the water was heated for 122 s withdifferent microwave output power levels and the final temperatures were recorded.The power was calculated using the following formula:

P ¼ 70� DT1 þ DT2

2

� �ð1Þ

where P is the power (W), DT1 is the temperature rise of the first beaker (�C), andDT2 is the temperature rise of the second beaker (�C).

The number 70 is used here for approximation for multiplication as stated in theliterature (Buffler, 1993). The temperature rise after 122 s was calculated by subtract-ing the initial water temperature from the final temperature as instructed in the IMPI2-liter method (Tripathy, 2009). The microwave output power measurement wasrepeated three times, and the final power was calculated from the average of thethree readings. The output powers of the microwave oven were determined as180� 2.07, 360� 2.18, 540� 2.59, 720� 2.60, and 900� 3.55W by using the IMPI2-liter test.

Color Measurements

During microwave heating, celery leaves were removed from the microwave oven atprespecified time intervals for color measurements (L, a, b) measured with a color-imeter (Konica Minolta CR-400) in a room with controlled light. The instrument

Color Change Kinetics of Celery Leaves 1191

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was calibrated before the experiments with a white ceramic plate (X¼ 93.50,Y¼ 0.3114, Z¼ 0.3190). Since the fresh and dried celery samples did not cover theentire surface area, they were scanned at five different locations to determine theaverage L, a, and b values during the measurements. The color values were expressedas L, a, and b for fresh samples and at any time. Three replications of each experi-ment were performed, and the data given are an average of these results. The relativestandard deviation of the measured values was calculated, and it was found to be inthe range of 2%.

The total color change (DE) (Equation (2)), chroma (Equation (3)), hue angle(Equation (4)), and browning index (BI) (Equation (5)) were calculated from theHunter L, a, and b values and used to describe the color change during drying:

DE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðL0 � LtÞ2 þ ða0 � atÞ2 þ ðb0 � btÞ2

qð2Þ

where L0, a0, and b0 are the initial color measurements of raw celery samples and Lt,at, and bt are the color measurements at prespecified time.

chroma ¼ða2t þ b2t Þ0:5 ð3Þ

hue angle ¼ tan�1 btat

� �ð4Þ

BI ¼ ½100ðx� 0:31Þ�0:17

ð5Þ

where

x ¼ ðat þ 1:75LtÞð5:645Lt þ at � 3:012btÞ

Statistical Analysis

The software package MATLAB 5.0 was used in the numerical calculations. Theparameters were evaluated by the nonlinear least squares method of Marquardt-Levenberg until minimal error was achieved between experimental and calculatedvalues. The residual sum of squares (SSR) is defined as the sum of the squares ofthe differences between experimental and calculated data and is given by:

SSR ¼XNd

m¼1

ðCobsm � Ccal

m Þ2 ð6Þ

where m is the observation number and Nd is total number of observations. The esti-mated variance of the error (population variance) is calculated by the SSR at itsminimum divided by its degrees of freedom:

d2 � s2 ¼ ðSSRÞmin=ðm� pÞ ð7Þ


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where p is the number of parameters and s2 is the variance. The standard error, d(the estimated standard deviation), is calculated by taking the square root of theestimated variance of the error.

Kinetic Considerations

In order to determine the color change of food materials as a function of drying time,several equations for the application of color change kinetics have been published inthe literature (Maskan, 2000a; Avila and Silva, 1999; Ahmed et al., 2002; Dadaliet al., 2007a, b). Generally, the rate of change of a quality factor C can berepresented by

dC

dt¼ �kCn ð8Þ

where k is the kinetic rate constant, C is the concentration of a quality factor at timet, and n is the order of reaction. For the majority of foods, the time-dependencerelationships appear to be described by zero-order or first-order kinetic models.By integrating Equation (8), the zero-order Equation (9) and first-order kineticmodel Equation (10) can be derived as

C ¼ C0 � kt ð9Þ

C ¼ C0 expð�ktÞ ð10Þ

where C0 is the initial value of color and C is the color value at a prespecified time. Inthe equations, (�) indicates formation and degradation of any quality parameter(Maskan et al., 2002; Prachayawarakom et al., 2004). The order of reaction forthe color parameters during microwave heating of celery leaves was determined bythe adjustment of the experimental data to the integrated Equations (9) and (10)by using linear regression analysis. In each case, the best fit was selected, and thekinetic rate constant of each process was determined.

Results and Discussion

Effect of Microwave Output Power on Color Kinetics of Celery Leaves

To investigate the effect of microwave output power on color change kinetics ofcelery leaves, five microwave output powers, 180, 360, 540, 720, and 900W, wereused for drying 25 g celery samples. The values of L, a, b, and DE obtained fromthe experimental data during microwave heating are presented in Figure 1.

The L value is illustrated in Figure 1(a). As can be seen from this figure, the Lvalue decreased with drying time. It reduced from 36.14 to 32.43 for 180W, and from35.87 to 28.22 for 900W. It has been stated that the change in the brightness of driedsamples can be taken as an indicator of browning (Avila and Silva, 1999; Teixeiraet al., 1969; Tijskens et al., 2001) during microwave heating of celery leaves. For red-ness=greenness scale, the initial color of samples showed a negative a value (about�13.2) indicating greenness (Figure 1(b)). The final a values varied from �9.61 to�9.08 as the microwave output power increased. Therefore, all celery samples


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maintained their greenness when dried by microwave. A decrease of the b value(Figure 1(c)) was also observed during microwave heating. The final b values variedfrom 12.28 to 11.28 as the microwave output power increased. This may be due topartial decomposition of chlorophyll and carotenoid pigments (Lee and Coates,1999; Palou et al., 1999; Weemaes et al., 1999), nonenzymatic Maillard browningand formation of brown pigments (Maskan, 2000a; Rhim et al., 1989; Lopez et al.,1997). As a whole, DE of celery leaves increased significantly during microwave heat-ing with drying time; it ranged from 5.99 to 9.64 as the microwave output powerincreased from 180 to 900W (Figure 1(d)).

For the mathematical modeling of color change of celery leaves, zero-order andfirst-order kinetic models were used. It was observed that L, a, and b values weresufficiently fitted to a first-order model; the DE values followed a zero-order kineticmodel. The estimated kinetic parameters of these models and the statistical values ofcoefficients of determination R2 and standard error (d) are presented in Table I.

The kinetic rate constant of L increased from 0.0032 to 0.0295min�1, for a from0.0101 to 0.0459min�1, for b from 0.0069 to 0.0406min�1, and for DE from 0.1846to 1.2707min�1 as the microwave output power increased. This implies that with anincrease in microwave output power, the degradation rate of color increases as aresult of high energy transferred to the inside of the food material, which causes

Figure 1. Kinetics of change of (a) L value, (b) a value, (c) b value, and (d) DE as a function ofdrying time at various microwave output powers for sample amount of 25 g celery leaves; &

180W, . 360W, ~ 540W, ^ 720W, D 900W, — predicted model.


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a rapid increase in temperature of the product. The results obtained were in agree-ment with studies published in the literature, and several authors have stated thata first-order kinetic model better fitted b values of kiwi fruits (Maskan, 2000a), okra(Dadali et al., 2007a), peach puree (Avila and Silva, 1999; Garza et al., 1999), andspinach (Dadali et al., 2007b) and a zero-order kinetic model was better for DEvalues of kiwi fruits (Maskan, 2000a), okra (Dadali et al., 2007a), and spinach(Dadali et al., 2007b).

Chroma, Hue Angle, and Browning Index

Chroma, hue angle, and browning index were calculated using Equations (3)–(5),and the results are illustrated in Figure 2. Chroma decreased during drying(Figure 2(a)) and the final chroma varied from 15.6 to 14.5 with increasing themicrowave output power. The final values of chroma indicate the stability ofyellowness in celery leaves. Hue angle (Figure 2(b)) decreased only slightly duringmicrowave heating, which indicates that the celery samples did not lose their greencolor (hue angle >90�) and did not turn to orange-red (hue angle <90�). The brown-ing index slightly increased depending on drying time, and hence this value wasproportional to the microwave output power applied (Figure 2(c)).

The modeling studies showed that the chroma data calculated were accuratelyfitted to a first-order model with high values for the coefficients of determination

Table I. Estimated kinetic parameters and statistical values of zero-order and first-order models for L, a, b, and DE for various microwave output powers

Power (W) Quality parameters k (min�1) C0 R2 d

180 L�� 0.0032 36.091 0.9991 0.0693a�� 0.0102 �13.318 0.9899 0.2508b�� 0.0069 15.4066 0.9963 0.1254DE� 0.1846 — 0.9958 0.2194

360 L�� 0.0056 35.927 0.9921 0.2664a�� 0.0141 �13.131 0.9913 0.2334b�� 0.0097 15.196 0.9953 0.1436DE� 0.3016 — 0.9875 0.4389

540 L�� 0.0097 35.609 0.9866 0.3894a�� 0.0238 �13.257 0.9964 0.1637b�� 0.0177 15.518 0.9870 0.2829DE� 0.5216 — 0.9845 0.5374

720 L�� 0.0189 35.921 0.9891 0.4382a�� 0.0367 �13.159 0.9981 0.1189b�� 0.0278 15.453 0.9843 0.3192DE� 0.8751 — 0.9866 0.5484

900 L�� 0.0295 35.582 0.9959 0.3086a�� 0.0459 �13.075 0.9819 0.3417b�� 0.0406 15.348 0.9911 0.2599DE� 1.2707 — 0.9894 0.5461

�Zero-order kinetic model.��First-order kinetic model.


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R2 and low values of standard error (d). On the other hand, the data of hue angleand browning index followed a zero-order kinetic model (Table II). The kinetic rateconstant of chroma increased from 0.0082 to 0.0428min�1, for hue angle from0.1338 to 0.951min�1, and for browning index from 0.0969 to 0.5284min�1 as themicrowave output power increased. The kinetic rate constants of all three para-meters were proportional to the microwave output powers applied. The resultsobtained were in agreement with the study published by Dadali et al. (2007a, b)for okra and spinach.

Effect of Sample Amount on Color Kinetics of Celery Leaves

To investigate the effect of sample amount on the color change kinetics of celeryleaves, various sample amounts of celery leaves ranging from 25 to 100 g were stud-ied with a single microwave output power of 360W. The values of L, a, b, and DEwere obtained from the experimental data during microwave heating, and the resultsare presented in Figure 3.

The same trends were obtained as in the previous section for the values of L, a,b, and DE. The L value decreased with drying time from 36.14 to 32.25 and from36.17 to 31.53 as the sample amount decreased from 100 to 25 g, respectively

Figure 2. Kinetics of change of (a) chroma, (b) hue angle, and (c) browning index as a func-tion of drying time at various microwave output powers for sample amount of 25 g celeryleaves; & 180W, . 360W, ~ 540W, ^ 720W, D 900W, — predicted model.


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(Figure 3(a)). For the a value, initial color of samples showed a negative a value(about �13.3) indicating greenness. The a value is illustrated in Figure 3(b). It wasobserved that the value of final a changed from �10 to �9.5 as the sample amountdried decreased from 100 to 25 g. A decrease of the b value was also observed duringmicrowave heating at various sample amounts (Figure 3(c)). The value of final b var-ied between 12.93 and 11.86 as the sample amount dried decreased. This may be dueto decomposition of some pigments, as explained in the previous section (Lee andCoates, 1999; Palou et al., 1999; Weemaes et al., 1999). As a whole, the DE of celeryleaves increased significantly at various sample amounts during microwave heatingwith drying time; it ranged from 5.69 to 7.01 as the sample amount decreased from100 to 25 g, respectively (Figure 3(d)).

For the mathematical modeling of color change of celery leaves dried at varioussample amounts, zero-order and first-order kinetic models were used. The estimatedkinetic parameters of these models and the statistical values of coefficients of deter-mination R2 and standard error (d) are represented in Table III. L, a, and b valueswere adequately fitted to a first-order model, while the values of DE followed azero-order kinetic model. The kinetic rate constant of L decreased from 0.0056 to0.0023min�1, for a from 0.0141 to 0.0063min�1, for b from 0.0097 to 0.0037min�1,and for DE from 0.3016 to 0.1189min�1 as the sample amount increased.

Chroma, Hue Angle, and Browning Index

Chroma, hue angle, and browning index were calculated using Equations (3)–(5),and the results are illustrated in Figure 4. The values of chroma and hue angledecreased as a function of drying time (Figures 4(a) and (b)). Chroma closely

Table II. Estimated kinetic parameters and statistical values of zero-order and first-order models for chroma, hue angle, and browning index for various microwaveoutput powers

Power (W) Quality parameters k (min�1) C0 R2 d

180 Chroma�� 0.0082 20.361 0.9963 0.1926Hue angle� 0.1338 130.584 0.9943 0.2204BI� 0.0969 20.229 0.9934 0.1722







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followed the b values. During microwave heating, the final values of chroma and hueangle increased with the increase of sample amount dried. On the other hand, brown-ing index decreased with increase in sample amount dried (Figure 4(c)).

In modeling studies, chroma data calculated were accurately fitted to a first-order model with high value for coefficients of determination R2 and low value forstandard error (d) (Table IV). On the other hand, the data of hue angle and browningindex followed a zero-order kinetic model. The kinetic rate constants of chroma, hueangle, and browning index increased as the sample amount decreased. The kineticrate constant for chroma increased from 0.0047 to 0.0115min�1, for hue angle from0.0712 to 0.2477min�1, and for browning index from 0.0483 to 0.1257min�1 as thesample amount decreased from 100 to 25 g (Table IV). This behavior can be explainedby the higher degradation of color that occurred inside the product from the hightemperatures generated by the microwaves within smaller samples.

Effect of Ratio of Microwave Output Power to Sample Amount onColor Parameters

The aim of this study was to derive a relationship between the ratio of microwaveoutput power to sample amount and L, a, b, DE, chroma, hue angle, and browning

Figure 3. Kinetics of change of (a) L value, (b) a value, (c) b value, and (d) DE as a function ofdrying time at various sample amounts at constant microwave output power of 360W; & 25 g,. 50 g, ~ 75 g, ^ 100 g, — predicted model.


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index. After evaluation of the data, the dependence of these parameters on the ratioof microwave output power to sample amount are represented with a quadraticmodel (Equation (11)) (Dadali et al., 2007a, b). The fitness of the data calculatedwith the model is illustrated in Figures 5 and 6.

C �aaþ �bb�ðP=mÞ þ �cc�ðP=mÞ2 ð11Þ

where C is the quality factor, and a (no units), b (W=g)�1, and c (W=g)�2 are thecoefficients of the model. For L, a, b, dE, chroma, hue angle, and browning index,the estimated parameters of the model coefficients of determination R2 and standarderror (d) of Equation (11) are presented in Table V.

For celery leaf samples, the final values of L and b decreased as power tosample amount ratio increased (Figures 5(a) and (c)). On the other hand, becauseof the high microwave energy exposure to the material dried at high power=sample amount, the final a values and the final dE values increased (Figures 5(b) and (d)). As can be seen from Figure 6(a), chroma values decreased as power=sample amount ratio increased. On the other hand, hue angle value decreasedwith the increase in power=sample amount ratio applied (Figure 6(b)). The brown-ing index (Figure 6(c)) increased as the power=sample amount increased. Thisresult indicates that high microwave output powers caused greater browning inthe product, probably leading to an unacceptable form of dried celery leaves. Thisresult is also in agreement with the work studied by Chutintrasri and Noomhorm(2005), where the browning index increased with a temperature increase (Dadaliet al., 2007a, b).

Table III. Estimated kinetic parameters and statistical values of zero-order and first-order models for L, a, b, and DE for various sample amounts

Sample (g) Quality parameters k (min�1) C0 R2 d

25 L�� 0.0056 35.927 0.9921 0.2664a�� 0.0141 �13.131 0.9913 0.2334b�� 0.0097 15.196 0.9953 0.1436DE� 0.3016 — 0.9875 0.4389

50 L�� 0.0039 35.868 0.9958 0.1769a�� 0.0105 �13.404 0.9923 0.2189b�� 0.0073 15.321 0.9963 0.1265DE� 0.2102 — 0.9977 0.1759

75 L�� 0.0029 36.024 0.9973 0.1355a�� 0.0082 �13.476 0.9897 0.2486b�� 0.0043 15.156 0.9971 0.0844DE� 0.1526 — 0.9986 0.1247

100 L�� 0.0023 36.089 0.9994 0.0616a�� 0.0063 �13.549 0.9823 0.3133b�� 0.0037 15.423 0.9964 0.0977DE� 0.1189 — 0.9986 0.1205



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Figure 4. Kinetics of change of (a) chroma, (b) hue angle, and (c) browning index as a func-tion of drying time at various sample amounts at constant microwave output power of 360W;& 25 g, . 50 g, ~ 75 g, ^ 100 g, — predicted model.

Table IV. Estimated kinetic parameters and statistical values of zero-order andfirst-order models for chroma, hue angle, and browning index for varioussample amounts

Sample (g) Quality parameters k (min�1) C0 R2 d







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Estimation of Activation Energy

For calculation of the activation energy for color change kinetic parameters, theexponential expression based on the Arrhenius equation was used (Dadali et al.,2007a, b):

k ¼ k0 exp�Ea m

P

� �ð12Þ

where k is the kinetic rate constant of the quality parameter (min�1), k0 is thepre-exponential constant (min�1), Ea is the activation energy (minimum energyrequired for color change during microwave heating) (W g�1), P is microwave out-put power (W), and m is the mass of raw sample (g). The kinetic rate constants forDE, hue angle, and browning index obtained from a zero-order kinetic model and thekinetic rate constants for L, a, b, and chroma obtained from a first-order kineticmodel accurately fitted to Equation (11). The calculated activation energies for eachcolor parameter, coefficients of determination R2, and standard error (d) are pre-sented in Table VI. As can be seen from this table, the Arrhenius model describedwell the power=sample amount dependence of the estimated kinetic rate constants

Figure 5. Final values of (a) L, (b) a, (c) b, and (d) DE curves as a function of power=sampleamount; — predicted model.


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for all the color parameters considered. The estimated activation energy values chan-ged within the range of 27.39–39.55W g�1, except for the estimated activationenergy value of the L value (Ea¼ 52.77W g�1). The results obtained in this studyfor activation energy values are different than the results obtained in the study of

Figure 6. Final values of (a) chroma, (b) hue angle, and (c) browning index curves as a func-tion of power=sample amount; — predicted model.

Table V. Estimated kinetic parameters and statistical values of the quadratic modelfor final L, a, b, and DE, chroma, hue angle, and browning index as a function ofpower=sample amount

Quality parameters �aa �bb (W=g)�1 �cc (W=g)�2 R2 d

Final L 32.114 0.0018 �0.0030 0.9967 0.1520Final a �10.183 0.0662 �0.0009 0.9952 0.0451Final b 13.205 �0.1045 0.0015 0.9906 0.1081Final DE 5.378 0.1111 0.0002 0.9984 0.1011Final chroma 16.596 �0.1168 0.0017 0.9890 0.1289Final hue angle 127.796 �0.2132 0.0022 0.9930 0.2414Final browning index 22.507 0.1019 �0.0010 0.9807 0.1975


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okra (Dadali et al., 2007a) (ranging between 7.45 and 11.59) and in the study of basil(Demirhan and Ozbek, 2009) (ranging between 39.81 and 43.40), since the textureand chemical composition of those food materials are different.

Conclusions

The color change of celery leaves using the L, a, and b system explained the actualbehavior of celery samples undergoing microwave heating. The final values of L, a,b, DE, chroma, and hue angle were influenced by microwave heating. The values ofbrowning index showed that microwave heating caused more brown compound(s).This result was supported by the increase in a value. Zero-order and first-order kin-etic models were used to explain the color change kinetics, and it was observed thatL, a, b, and chroma were fitted to a first-order kinetic model. On the other hand, DE,hue angle, and browning index followed a zero-order kinetic model. As a function ofpower=sample amount, the data of L, a, b, DE, chroma, hue angle, and browningindex were fitted to a quadratic model. The values of a, DE, and browning indexincreased, while L, b, chroma, and hue angle decreased when the power=sampleamount value was increased. For calculation of the activation energy for colorchange kinetic parameters of celery leaves, the exponential expression based onthe Arrhenius equation was used; and it was observed that the Arrhenius model welldescribed the power=sample amount dependence of the estimated kinetic parametersfor all the color parameters considered.

Acknowledgment

Elcin Demirhan gratefully acknowledges TUBITAK (The Scientific and Technologi-cal Research Council of Turkey) for scholarship support.

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Documents

COLOR CHANGE KINETICS OF CELERY LEAVES UNDERGOING MICROWAVE HEATING