This article was downloaded by: [York University Libraries] On: 12 August 2014, At: 05:31 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Drying Technology: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ldrt20 Color Change Kinetics of Microwave-Dried Basil Elçin Demirhan a & Belma Özbek a a Yıldız Technical University, Department of Chemical Engineering, Davutpaşa Campus , Esenler/Istanbul, Turkey Published online: 13 Jan 2009. To cite this article: Elçin Demirhan & Belma Özbek (2009) Color Change Kinetics of Microwave-Dried Basil, Drying Technology: An International Journal, 27:1, 156-166 To link to this article: http://dx.doi.org/10.1080/07373930802566101 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Transcript
This article was downloaded by: [York University Libraries]On: 12 August 2014, At: 05:31Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Drying Technology: An International JournalPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/ldrt20
Color Change Kinetics of Microwave-Dried BasilElçin Demirhan a & Belma Özbek aa Yıldız Technical University, Department of Chemical Engineering, Davutpaşa Campus ,Esenler/Istanbul, TurkeyPublished online: 13 Jan 2009.
To cite this article: Elçin Demirhan & Belma Özbek (2009) Color Change Kinetics of Microwave-Dried Basil, Drying Technology:An International Journal, 27:1, 156-166
To link to this article: http://dx.doi.org/10.1080/07373930802566101
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Elcin Demirhan and Belma OzbekYıldız Technical University, Department of Chemical Engineering, Davutpasa Campus,Esenler=Istanbul, Turkey
The aim of this study was to investigate the effect of microwaveoutput power and sample amount on color change kinetics of basil(Ocimum basilicum L.) during microwave drying. The color para-meters for the color change of the materials were quantified byHunter L (whiteness/darkness), a (redness/greenness), and b(yellowness/blueness) system. These values were also used for calcu-lation of the total color change (DE), chroma, hue angle, and brown-ing index. The microwave-drying process changed color parametersof L, a, and b, causing a color shift toward the darker region. Themathematical modeling study of color change kinetics showed that aand b fitted to a first-order kinetic model, while L and total colorchange (DE) followed a zero-order kinetic model. However, chromaand browning index (BI) followed a first-order kinetic model,whereas hue angle followed a zero-order kinetic model. For calcu-lation of the activation energy for colour change kinetic parameters,the exponential expression based on Arrhenius equation was used.
INTRODUCTION
Basil (Ocimum basilicum L.), belonging to the Lamiaceaefamily, grows in several regions with temperate and hotclimates all over the world. Fresh and dried basil arewidely used in the Mediterranean kitchen, including tom-ato products, vegetables, salads, pizza, meat, soups, andmarine foods. Basil is also well known as a plant of amedicinal value and as such is accepted officially in alot of countries. The leaves of basil are used in pharmacyfor diuretic and stimulating properties in perfumes com-positions. As a spice, dried and ground basil leaves areused in bakery products, confectionary, ice creams, vine-gars, meat, and flavor products.[1–3]
Fresh fruits and vegetables cannot be preserved fresh fora long time due their abundant moisture content and nutri-ents for microbes to grow. Drying is one of the commontechniques to restrain the microbial growth,[4] to inactivateenzymes, and to provide for preservation of seasonal plantsfor the whole year.[5–8] However, during the drying process,the food material may be exposed to temperatures that
have an adverse effect on quality and organoleptic proper-ties. The kinetic models of thermal degradation are essentialto design new processes to produce safe food product andgive a maximum retention of quality factors.[8,9]
Analysis of quality parameters of foods (color, taste,odor, and texture) are indicators of food quality through-out processing. Especially at the point of sale, the firstimpact made on a consumer of a food is its visual appear-ance. It is perceived as part of the total quality assessment,which is the visual recognition and assessment of the sur-face and subsurface properties of the food material.[10]
The color change of food materials during thermal proces-sing takes place because of the physicochemical reactions,which occur inside the food material. These reactions canbe pigment degradation, especially carotenoids and chloro-phyll; browning reactions such as Maillard condensation ofhexoses and amino components; and oxidation of ascorbicacid.[11–13] The final color measurements of the dried pro-duct can be used as quality indicators to evaluate deterio-ration caused by the thermal processing.[14]
The color measurements of food materials can be used inan indirect way to determine the quality change, since theyare simpler and faster than a complete physicochemicalanalysis. The color brightness coordinate L measures thewhiteness value of a color and ranges from black at 0 towhite at 100. The chromocity coordinate a measures redwhen positive and green when negative, and the chromocitycoordinate b measures yellow when positive and blue whennegative. Hunter color parameters have previously beendemonstrated to be valuable in describing visual colordeterioration and providing useful information for qualitycontrol in fruits and vegetables.[10,15–21] Additional para-meters are derived from the Hunter L, a, and b scale: thetotal color change (DE); chroma value, which indicatesthe degree of color saturation and is proportional to thestrength of the color; hue angle is frequently used to charac-terize color in food products; and browning index (BI),which represents the purity of brown color, is reported asan important parameter in drying processes whereenzymatic and nonenzymatic browning takes place.[4,11]
Correspondence: Belma Ozbek, Yıldız Technical University,Department of Chemical Engineering, Davutpasa Street, 34210,Esenler=Istanbul, Turkey; E-mail: [email protected]
Drying Technology, 27: 156–166, 2009
Copyright # 2009 Taylor & Francis Group, LLC
ISSN: 0737-3937 print/1532-2300 online
DOI: 10.1080/07373930802566101
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The study of the color change behavior of foods duringdrying has recently been a subject of interest for variousinvestigators such as garlic,[22] purslane,[23] rosehip,[24]
concentrated fruit pulp,[11] pestil,[16] mix of spinach andmustard leaves,[25] green chilli puree,[26] banana,[27,28]
potato,[29] mango pulp,[30] soybean,[31] okra,[32] andspinach.[33] While there are many literature studies on thekinetics of changes in vegetables, no kinetic studies relatedwith the color change during drying of basil were found inthe literature. Therefore, the purpose of this work was tostudy the kinetics of color degradation during microwavedrying of basil and to calculate the activation energies forcolor change kinetic parameters using an exponentialexpression based on Arrhenius equation.
MATERIALS AND METHODS
Materials
Plants of fresh basil were purchased from a local supplierin Istanbul. They were washed and stored at 4� 0.5�C in arefrigerator. Before the drying experiments, the samples
were taken out of the refrigerator and leaves from stemswere separated and then weighed. To determine the initialmoisture content, four 50-g samples were dried in an oven(Memmert UM-400, Buchenbach, Germany) at 105�C for24 h. The initial moisture content of basil was calculatedas 8.15 g water � g dry base�1 as an average of the resultsobtained. The reproducibility of the initial moisture contentmeasurements was within the range of �5%.
Drying Equipment and Drying Procedure
Drying treatment was performed in a domestic digitalmicrowave oven (Arcelik MD 594, Bolu, Turkey) withtechnical features of �230 V, 50 Hz, and 2650 W with a fre-quency of 2450 MHz. A full description of the dryingequipment and drying procedure can be found in previouswork reported by Dadali et al.[32,33] The effect of micro-wave output power (180, 360, 540, 720, and 900 W) andsample amount (25, 50, 75, 100 g) on color change kineticsof basil was investigated. Three replications of eachexperiment were performed according to a preset micro-wave output power and time schedule, and the data given
FIG. 1. Kinetics of change of the (a) L value, (b) a value, (c) b value, and (d) total color change (DE) as a function of drying time at various microwave
output powers for sample amount of 25 g basil; & 180 W, . 360 W, ~ 540 W, ^ 720 W, ~ 900 W,—predicted model.
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are an average of these results. The reproducibility of theexperiments was within the range of �5%. The microwavepower was applied until the weight of the sample reducedto a level corresponding to moisture content of about0.10 g water � g dry base�1.
Color Measurements
During microwave drying, basil samples were removedfrom the microwave oven at prespecified time intervalsfor color measurements (L, a, b) measured with a KonicaMinolta colorimeter (Model No: CR-400, Osaka, Japan)in a room with controlled light. The instrument wascalibrated before the experiments with a white ceramicplate (X¼ 93.50, Y¼ 0.3114, Z¼ 0.3190). Since the basilsamples did not cover the entire surface area, they werescanned at five different locations to determine the averageL, a, b values during the measurements. The color valueswere expressed as L, a, and b at any time, respectively.And the total color change (DE; Eq. (1)), chroma (Eq.(2)), hue angle (Eq. (3)), and browning index (Eq. (4)) werecalculated from the Hunter L, a, and b values and used todescribe the color change during drying.
where L0, a0, b0 are the initial color measurements of rawbasil samples and Lt, at, bt are the color measurements atprespecified time.
Chroma ¼ ða2t þ b2
t Þ0:5 ð2Þ
Hue angle ¼ tan�1 bt
at
� �ð3Þ
BI ¼ ½100ðx� 0:31Þ�0:17
ð4Þ
where x ¼ ðat þ 1:75LtÞð5:645Lt þ at � 3:012btÞ
:
Statistical Analysis
The software package MATLABTM 5.0 was used inthe numerical calculations. The parameters were evaluatedby the nonlinear least squares method of Marquardt-Levenberg until minimal error was achieved betweenexperimental and calculated values. The residual (SSR) isdefined as the sum of the squares of the differences betweenexperimental and calculated data and given by
SSR ¼XNd
m¼1
ðCobsm � Ccal
m Þ2 ð5Þ
TABLE 1The estimated kinetic parameters and the statistical values of zero-order and first-order
models for L, a, b and total color change (DE) for various microwave output powers
DEa 0.2665 — 0.9856 0.7769360 La 0.3255 35.274 0.9979 0.1525
ab 0.0334 �11.616 0.9942 0.7438bb 0.0191 14.313 0.9901 0.2455
DEa 0.5355 — 0.9853 0.8984540 La 0.5047 35.092 0.9995 0.0879
ab 0.0603 �11.356 0.9924 0.3328bb 0.0305 13.684 0.9985 0.1052
DEa 0.7960 — 0.9740 1.0991720 La 1.0335 35.166 0.9941 0.4119
ab 0.1128 �11.236 0.9969 0.2398bb 0.0540 14.151 0.9883 0.4770
DEa 1.4052 — 0.9812 1.4228900 La 1.3913 34.755 0.9834 0.7514
ab 0.1825 �10.846 0.9862 0.7374bb 0.0795 13.588 0.9824 0.6820
DEa 2.1031 — 0.9769 1.0731aZero-order kinetic model.bFirst-order kinetic model.
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where m is the observation number and Nd is total numberof observations. The estimated variance of the error (popu-lation variance) is calculated by the SSR at its minimumdivided by its degrees of freedom:
r2 � s2 ¼ ðSSRÞmin=ðm� pÞ ð6Þwhere p is the number of parameters and s2 is the variance.The standard error, d (the estimated standard deviation), iscalculated by taking the square root of the estimated vari-ance of the error.
Kinetic Considerations
In order to determine the color change of food materialsas a function of drying time, several equations for theapplication of color change kinetics have been publishedin literature.[6,7,25,32,33] Generally, the rate of change of aquality factor C can be represented by:
dC
dt¼ �kCn ð7Þ
where k is the kinetic rate constant, C is the concentrationof a quality factor C at time t, and n is the order of reac-tion. For the majority of foods, the time dependencerelationships appear to be described by zero-order orfirst-order kinetic models. By integrating Eq. (7), thezero-order Eq. (8) a first-order kinetic models Eq. (9) canbe derived as:
C ¼ C0 � kt ð8ÞC ¼ C0 expð�ktÞ ð9Þ
where C0 is the initial value of color and C is the colorvalue at a prespecified time. In the equations, (�)indicates formation and degradation of any quality para-meter.[16,31] The order of reaction for the color parametersduring microwave drying of basil was determined by theadjustment of the experimental data to the integratedEqs. (8) and (9) by using linear regression analysis. In eachcase, the best fit was selected, and the kinetic rate constantof each process was determined.
RESULTS AND DISCUSSION
Effect of Microwave Output Power on DryingKinetics of Basil
To investigate the effect of microwave output power oncolor change kinetics of basil, five microwave outputpowers, 180, 360, 540, 720, and 900 W, were used for dry-ing 25 g basil samples. The values of L, a, b and total colorchange (DE) obtained from the experimental data duringmicrowave drying are presented in Figs. 1a–1d.
The L value is illustrated in Fig. 1a. As can be seen fromthis figure, L value decreased from 35 to 26 with drying time.It has been stated that the change in the brightness of driedsamples can be taken as an indicator of browning[7,9,10] during
FIG. 2. Kinetics of change of (a) chroma, (b) hue angle, and (c) brown-
ing index as a function of drying time at various microwave output powers
for sample amount of 25 g basil; & 180 W, . 360 W, ~ 540 W, ^ 720 W,
4 900 W,—predicted model.
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TABLE 2The estimated kinetic parameters and the statistical values of zero-order and first-order
models for chroma, hue angle, and browning index for various microwave output powers
FIG. 3. Kinetics of change of the (a) L value, (b) a value, (c) b value, and (d) total color change (DE) as a function of drying time at various sample
amounts at constant microwave output power of 360 W; & 25 g, . 50 g, ~ 75 g, ^ 100 g,—predicted model.
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microwave drying of basil. For redness=greenness scale,initial color of samples showed a negative a value (about�11.4), indicating greenness (Fig. 1b). The final a values var-ied from �7.02 to �3.88 as the microwave output powerincreased. Therefore, all basil samples maintained their green-ness when dried by microwave. A decrease in the b value (Fig.1c) was also observed during microwave drying. The final bvalues varied from 11.26 to 8.75 as the microwave outputpower increased. The loss in b value indicates that the yellow-ness of sample decreased due to microwave output powerapplied and it may be due to partial decomposition of chloro-phyll and carotenoid pigments,[12,34,35] nonenzymatic Mail-lard browning, and formation of brown pigments.[6,36,37] Asa whole, the total color change (DE) of basil increased signifi-cantly during microwave drying with drying time and alsoranged from 7.11 to 13.27 as the microwave output powerincreased from 180 to 900 W, (Fig. 1d).
For the mathematical modeling of color change of basil,zero-order and first-order kinetic models were used. It wasobserved that a and b values were sufficiently fitted to afirst-order model; on the other hand, the values of L andtotal color change (DE) followed a zero-order kineticmodel. The estimated kinetic parameters of these modelsand the statistical values of coefficients of determinationR2 and standard error (d) are represented in Table 1.
The kinetic rate constant of L increased from 0.1516 to1.3913 min�1, for a value from 0.0179 to 0.1825 min�1, for bvalue from 0.0098 to 0.0795 min�1, and for total color change(DE) increased from 0.2665 to 2.1031 min�1 as the microwave
output power increased. This implies that with an increase inmicrowave output power, the degradation rate of colorincreases as a result of high energy transferred to the insideof food material, which causes a rapid increase in temperatureof the product. The results obtained were in agreement withstudies published in the literature, and several authors havestated that a first-order kinetic model better fitted b valuesof kiwifruits,[6] okra,[32] peach puree,[7,15] and spinach;[33] anda zero-order kinetic model was better for total color change(DE) values of kiwifruits,[6] okra,[32] and spinach.[33]
Chroma, Hue Angle, and Browning Index
Chroma, hue angle and browning index were calculated byusing Eqs. (2)–(4) and the results are illustrated in Figs. 2a–2c.The chroma decreased during drying (Fig. 2a) and the finalchroma varied from 13 to 9 with increasing the microwaveoutput power. This indicates the stability of yellowness inbasil sample. Hue angle (Fig. 2b) decreased only slightly dur-ing microwave drying, which indicates that the basil samplesdid not lose their greenness color (hue angle> 90�) and didnot turn to orange-red (hue angle< 90�). The browning indexincreased with an increase in microwave drying time, at lowerapplied power, and hence this value was proportional to theapplied microwave output power (Fig. 2c).
The modeling studies showed that the data of chroma andbrowning index calculated were accurately fitted to a first-order model with high values for the coefficients of determi-nation R2 and standard error (d). On the other hand, the dataof hue angle followed a zero-order kinetic model (Table 2).
TABLE 3The estimated kinetic parameters and the statistical values of zero-order and first-order
models for L, a, b, and total color change (DE) for various sample amounts
DEa 0.5355 — 0.9853 0.898450 La 0.1553 35.112 0.9985 0.0897
ab 0.0169 �10.729 0.9921 0.1955bb 0.0091 13.716 0.9972 0.0883
DEa 0.2573 — 0.9922 0.282275 La 0.0951 35.080 0.9879 0.2185
ab 0.0079 �10.686 0.9828 0.2022bb 0.0044 13.782 0.9856 0.1419
DEa 0.1491 — 0.9885 0.2762100 La 0.0651 35.322 0.9947 0.1269
ab 0.0042 �10.972 0.9896 0.1131bb 0.0024 13.782 0.9976 0.0408
DEa 0.0881 — 0.9929 0.1661aZero-order kinetic model.bFirst-order kinetic model.
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The kinetic rate constant of chroma increased from 0.0127 to0.1071 min�1, for hue angle from 0.2347 to 2.2072 min�1, andfor browning index from 0.0149 to 0.0978 min�1 as the micro-wave output power increased. The kinetic rate constants of allthese three parameters were proportional to the microwaveoutput powers applied. The results obtained were in agree-ment with the study published by Dadali et al.[32,33] for okraand spinach, where it has been stated that the first-order kin-etic model was better suited for chroma and browning indexand a zero-order kinetic model was better for hue angle.
Effect of Sample Amount on Drying Kinetics of Basil
To investigate the effect of sample amount on the colorchange kinetics of basil, various sample amounts of basilranging from 25 to 100 g were studied with a single micro-wave output power of 360 W. The values of L, a, b andtotal color change (DE) were obtained from the experi-mental data during microwave drying and the results arepresented in Figs. 3a–3d.
The same trends were obtained with previous section forthe values of L, a, b and total color change (DE). The Lvalue is illustrated in Fig. 3a. As can be seen from this fig-ure, L value decreased with drying time. It reduced from35 to 32.35 and 30.09 as the sample amount dried decreasedfrom 100 to 25 g, respectively. For a value, initial color ofsamples showed a negative a value (about �11.06), indicat-ing greenness. The a value is illustrated in Fig. 3b. It wasobserved that the value of final a changed from �9.21 to�5.9 as the sample amount dried decreased from 100 to25 g. A decrease in the b value was also observed duringmicrowave drying at various sample amounts (Fig. 3c).The value of final b was varied between 12.32 and 10.32as the sample amount dried decreased. This may be dueto decomposition of some pigments, as explained in the pre-vious section.[12,34,35] As a whole, the total color change(DE) of basil increased significantly at various sampleamounts during microwave drying with drying time andalso ranged from 3.89 to 8.29 as the sample amountdecreased from 100 to 25 g, respectively, (Fig. 3d).
For the mathematical modeling of color change ofbasil dried at various sample amounts, zero-order andfirst-order kinetic models were used. The estimated kin-etic parameters of these models and the statistical valuesof coefficients of determination R2 and standard error(d) are represented in Table 3. Values of a and b wereadequately fitted to a first-order model; on the otherhand, the values of L and total color change (DE) fol-lowed a zero-order kinetic model. The kinetic rate con-stant of L increased from 0.0651 to 0.3255 min�1, for avalue from 0.0042 to 0.0334 min�1, for b value from0.0024 to 0.0191 min�1, and for total color change (DE)increased from 0.088 to 0.5355 min�1 as the sampleamount decreased.
FIG. 4. Kinetics of change of (a) chroma, (b) hue angle, and (c) brown-
ing index as a function of drying time at various sample amounts at con-
stant microwave output power of 360 W; & 25 g, . 50 g, ~ 75 g, ^
100 g,—predicted model.
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FIG. 5. The final values of (a) L, (b) a, (c) b, and (d) total color change (DE) curves as a function of power=sample amount,—predicted model.
TABLE 4The estimated kinetic parameters and the statistical values of zero-order and first-order
models for chroma, hue angle, and browning index for various sample amounts
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Chroma, Hue Angle, and Browning Index
Chroma, hue angle, and browning index were calculatedusing Eqs. (2)–(4) and the results are illustrated in Figs. 4a–4c. The values of chroma and hue angle decreased as afunction of drying time (Figs. 4a and 4b). The chromavalues closely followed the b values. During microwavedrying, the final values of chroma and hue angle wereincreased with the increase of sample amount dried. Onthe other hand, browning index decreased with increasein sample amount dried (Fig. 4c).
In modeling studies, chroma and browning index datacalculated were accurately fitted to a first-order modelwith high value for coefficients of determination R2 andlow value for standard error (d; Table 4). On the otherhand, the data of hue angle followed a zero-order kineticmodel. The kinetic rate constants of chroma, hue angle,and browning index increased as the sample amountdecreased. The kinetic rate constant for chroma increasedfrom 0.0029 to 0.024 min�1, for hue angle from 0.0902 to0.5468 min�1, and for browning index from 0.0039 to0.0255 min�1 as the sample amount decreased from 100to 25 g (Table 4). This behavior can be explained by thehigher degradation of color that occurred inside theproduct from the high temperatures generated by themicrowaves within smaller samples.
Effect of Ratio of Microwave Output Power to SampleAmount on Color Parameters
The aim of this study was to derive a relationship betweenthe ratio of microwave output power to sample amount andL, a, b, total color change (DE), chroma, hue angle, andbrowning index. After evaluation of the data, the dependenceof these parameters on the ratio of microwave output powerto sample amount are represented with a quadratic model(Eq. (10)).[32,33] The fitness of the data calculated with themodel is illustrated in Figs. 5a–5d and 6a–6c.
C � aþ b ðP=mÞ þ c ðP=mÞ2 ð10Þwhere C is the quality factor and a (dimensionless), b(W=g)�1, and c (W=g)�2 are the coefficients of the model.For L, a, b, total color change (DE), chroma, hue angle andbrowning index, the estimated parameters of the model coef-ficients of determination R2 and standard error (d) of Eq. (10)are presented in Table 5.
For basil samples, the final values of L and b decreased aspower to sample amount ratio increased (Figs. 5a and 5c). Onthe other hand, because of the high energy generated bymicrowave at high power=sample amounts, the final a valuesand the final total color change (DE) values increased (Figs.5b and 5d). As can be seen from Fig. 6a, chroma valuesdecreased as power=sample amount increased. On the otherhand, hue angle value decreased with the increase in power-=sample amount ratio applied (Fig. 6b). The browning index
(Fig. 6c) increased as the power=sample amount increased.This result indicates that high microwave output powerscaused greater browning in the product, probably leadingto an unacceptable form of dried basil. This result is also in
FIG. 6. The final values of (a) chroma, (b) hue angle, and (c) browning
index curves as a function of power=sample amount,—predicted model.
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agreement with the work by Chutintrasri and Noomhorm[38]
where the browning index increased with a temperatureincrease.[32,33]
Estimation of Activation Energy
For calculation of the activation energy for color changekinetic parameters, the exponential expression based onArrhenius equation was used (Eq. (11)):[32,33]
k ¼ k0 exp�Ea � m
P
� �ð11Þ
where k is the kinetic rate constant of the quality parameter(min�1), k0 is the preexponential constant (min�1), Ea is theactivation energy (minimum energy required for colorchange during microwave drying; W � g�1), P is microwaveoutput power (W), and m is the weight of raw sample (g).The kinetic rate constants for L, total color change (DE),and hue angle obtained from a zero-order kinetic modeland the kinetic rate constants for a, b, chroma, and brown-ing index obtained from a first-order kinetic model accu-rately fitted to Eq. (11). The calculated activationenergies for each color parameter, coefficients of determi-nation R2, and standard error (d) are presented inTable 6. As can be seen from this table, the Arrheniusmodel described well the power=sample amount depen-dence of the estimated kinetic rate constants for all thecolor parameters considered. The estimated activationenergy values for color parameters changed within the
range of 39.81–43.40 W � g�1except for the a value (whichis about 54.27 W � g�1). The results obtained in this studyfor activation energy values are higher than the resultsobtained in the study of okra and spinach,[32,33] which ran-ged between 7.45–11.59 and 31.16–39.43 W � g�1, since thetexture and chemical composition of those food materialsare different.
CONCLUSIONS
The color change of basil using the L, a, and b systemexplained the actual behavior of basil samples undergoingmicrowave drying. The final values of L, a, b, total colorchange (DE), chroma, and hue angle were influenced bymicrowave drying. The values of browning index showedthat microwave drying caused more brown compound(s)compared to fresh samples. This result was supportedby the increase in a value. The zero-order and first-orderkinetic models were used to explain the color changekinetics and it was observed that a, b, chroma, andbrowning index were fitted to a first-order kinetic model.On the other hand, L, total color change (DE), and hueangle followed a zero-order kinetic model. As a functionof power=sample amount, the data of L, a, b, total colorchange (DE), chroma, hue angle, and browning index werefitted to a quadratic model. a, the total color change (DE),chroma, and browning index increased; on the otherhand, L, b, and hue angle decreased when the power=amount value was increased. For calculation of the
TABLE 6The activation energies calculated for the color degradation of basil for color parameters
TABLE 5The estimated kinetic parameters and the statistical values of quadratic model for final L, a, b, and total color change
(DE), chroma, hue angle, and browning index as a function of power=sample amount
Quality parameters a b (W=g)�1 c (W=g)�2 R2 d
Final L 32.806 �0.1886 8.7 10�6 0.9941 0.3178Final a �9.841 0.3366 �0.0049 0.9833 0.4492Final b 12.857 �0.2179 0.0030 0.9904 0.2310Final total color change (DE) 2.973 0.4302 �0.0039 0.9802 0.8853Final chroma 16.159 �0.3673 0.0053 0.9882 0.4136Final hue angle 126.103 �0.4868 0.00401 0.9899 0.7430Final browning index 23.622 0.6351 �0.0031 0.9847 1.4174
COLOR CHANGE KINETICS OF MICROWAVE-DRIED BASIL 165
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activation energy for color change kinetic parameters ofbasil, the exponential expression based on Arrheniusequation as demonstrated by Dadalı et al.[32,33] was used,and it was observed that Arrhenius model well describedthe power=sample amount dependence of the estimatedkinetic parameters for all the color parameters considered.
ACKNOWLEDGEMENTS
Elcin Demirhan gratefully acknowledges TUBITAK(the Scientific and Technological Research Council ofTurkey) for scholarship.
REFERENCES
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and storage of herbs and spices on the essential oil. Part I. Basil,
Ocimum basilicum L. Flavour and Fragrance Journal 1992, 7, 267–271.
2. Lawrence, B.M. A review of the world production of essential oil.
Perfumery Flavour 1985, 10, 2–16.
3. Ozcan, M.; Arslan, D.; Unver, A. Effect of drying methods on the
mineral content of basil (Ocimum Basilicum L.). Journal of Food
