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Production of pomegranate (Punica granatum L.) juice concentrate by various heating methods: colour degradation and kinetics
Journal of Food Engineering 72 (2006) 218–224 www.elsevier.com/locate/jfoodeng
Production of pomegranate (Punica granatum L.) juice concentrate by various heating methods: colour degradation and kinetics Medeni Maskan
*
Food Engineering Department, Engineering Faculty, University of Gaziantep, Gaziantep 27310, Turkey Received 3 June 2004; accepted 23 November 2004 Available online 24 December 2004
Abstract Pomegranate juice was concentrated by various heating methods. The final juice concentration of 60.5 °Brix was achieved in 23, 108 and 190 min by using microwave, rotary vacuum and atmospheric heating processes, respectively. The colour change during concentration processes was investigated. Total colour differences, Hunter L, a and b parameters were used to estimate the extent of colour loss. All Hunter colour parameters decreased with time. It was observed that the severity of colour loss was higher in rotary vacuum heating process than the others. The zero-order, first-order and a combined kinetics model were applied to the changes in colour parameters. Results indicated that variation in TCD followed both first-order and combined kinetics models, and parameters L, a and b followed only combined model. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Pomegranate juice; Concentration; Colour; Kinetics
1. Introduction Pomegranate (Punica granatum L.) is one of the important fruits grown in Turkey, Iran, USA, Middle East, Mediterranean and Arabic countries. It is originated from south-east Asia. The edible part of the fruit contains considerable amount of acids, sugars, vitamins, polysaccharides, polyphenols and important minerals (Al-Maiman & Ahmad, 2002; Vardin & Fenerciog˘lu, 2003). It has been reported that pomegranate juice has potent anti-atherogenic effects in healthy humans and atherosclerotic effects in mice that may be attributable to its anti-oxidative properties (Negi, Jayaprakasha, & Jena, 2003). The fruit is consumed directly as fresh seeds as well as fresh juice which can also be used in beverages for jellies, and flavouring and colouring agents. The kernels are also used as a garnish for desserts and salads *
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(Al-Maiman & Ahmad, 2002). Pomegranate contains richly coloured grains which give a delicious juice. The pomegranate juice concentrate is commonly used for salads and in many dishes in Turkey. Juices are produced from various fruits that are not necessarily harvested the whole year round. Before shipping to its final destination the extracted juice is concentrated to ensure longer storage life (because of its low water activity) and easier transportation. The concentration of fruit juices requires partly removal of water without changes in solids composition, leaving all the original solid components such as fruit sugars, minerals and vitamins to the more concentrated solution (Toribo & Lozano, 1986). Concentration of fruit juices, a major unit operation in fruit processing industry, is of critical importance as it determines the quality of the final product such as flavour, colour, aroma, appearance and mouth feel (Jiao, Cassano, & Drioli, 2004; Ramteke, Singh, Rekha, & Eipeson, 1993). The production of concentrated fruit juices is of interest at industrial level since
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they can be used as ingredients in many products such as ice creams, fruit syrups, jellies and fruit juices beverages (Cassano, Jioa, & Drioli, 2004). It is known that concentration of fruit juices by conventional evaporation methods results in colour degradation and loss of most volatile compounds with a consequent remarkable qualitative decline due to thermal effects (Cassano et al., 2004; Jiao et al., 2004). The visual colour is an important attribute because it is usually the first property the consumer observes. It is an indicator of pigment concentration which can be measured instantaneously using tristimulus colorimeters for on line quality control. It has been reported (Barreiro, Milano, & Sandoval, 1997; Suh, Noh, Kang, Kim, & Lee, 2003) that many reactions can take place during thermal processing that affect the colour. Among them, the most common are pigment degradation, especially carotenoids (lycopene, xanthophylls, etc.), anthocyanin and chlorophyll, and browning reactions such as the Maillard reaction, enzymatic browning and oxidation of ascorbic acid (Barreiro et al., 1997; Ibarz, Pagan, & Garza, 1999; Lozano & Ibarz, 1997). The Hunter colour parameters L, a and b have been widely used to describe colour changes during thermal processing of fruit and vegetable products. While working on the drying of banana, kiwi fruit, grape leather and grape juice concentrate, Maskan (2000, 2001) and Maskan, Kaya, and Maskan (2002) reported that Hunter colour values changed during heat treatments. Many investigators observed similar results in their studies such as thermal treatment of concentrated tomato paste (Barreiro et al., 1997), peach puree (Avila & Silva, 1999), mulberry fruit extract (Suh et al., 2003) and concentrated fruit pulp (Lozano & Ibarz, 1997). However, studies on concentrating the pomegranate juice by microwave energy and kinetics of degradation of colour of pomegranate during concentration processes are lacking. Therefore, the present study was undertaken to investigate; (1) production of pomegranate juice concentrate by evaporation processes such as microwave heating, rotary vacuum evaporator and heating at atmospheric pressure, (2) the kinetics of degradation of visual colour using Hunter tristimulus colour scale (L, a and b values) during concentration processes.
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encloses hundreds of fleshy sacs was removed. The juice that is localised in the sacs was manually pressed and extracted. The juice obtained had a deep-red colour. It was stored at 4 °C overnight, for settling of suspended particles, then filtered and concentrated. 2.2. Pomegranate juice concentration Three different heating/evaporation processes were employed for production of pomegranate juice concentrate. The juice was concentrated to a final 60.5°Brix from an initial °Brix of 17.5 by the following processes; (1) By microwave heating: A programmable domestic microwave oven (Arc¸elik ARMD 580, TURKEY) with maximum output of 700 W at 2450 MHz was used. The oven has adjustable power (wattage) and time controllers and was fitted with a turntable. Heating above or below 350 W power level resulted in some problems such as foaming, charring of juice or lengthy of concentration time. Therefore, the study was carried out at 350 W power level. A 500 ml of the juice sample was put in a beaker and replaced on the center of turn table in the microwave cavity. Samples were taken for measurement of °Brix and colour periodically and replaced again. (2) By rotary vacuum evaporator: A 500 ml of the juice sample was concentrated in a laboratory rotary vacuum evaporator (RE 100 Model, Bibby Sterilin Ltd., England) rotating at 66 rpm and 40 °C. Samples were taken from the bulk of juice periodically for °Brix and colour measurements and replaced again after used. (3) By evaporating at atmospheric pressure: The pomegranate juice was concentrated by using an electromagnetic heater (VELP Scientifica, Italy). A 500 ml of the juice sample was put in a beaker and replaced on the heater open to atmosphere. The sample was continuously heated and stirred during this process. Samples were taken for measurement of °Brix and colour periodically and replaced again after used.
2.3. Soluble solids content determination 2. Materials and methods 2.1. Preparation of fresh pomegranate juice Fresh pomegranate fruits (Punica granatum L.) of Gaziantep cultivars were obtained from a local market (Gaziantep—Turkey) in November. Sweet-mature fruits were prefered for the production of concentrate. Fruits were washed in cold tap water and drained. They were manually cut-up and the outer leathery skin which
During concentration processes, the soluble solids content of the juice samples was measured by an OPTON model Abbe refractometer (F.G. Bode & Co., Hamburg) at 20 °C and expressed in °Brix. 2.4. Colour measurement Colour measurements of the juice samples were carried out using a HunterLab Colorflex (A-60-1010-615 Model Colorimeter, HunterLab, Reston, VA). The
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instrument was standardized each time with a black and a white (L = 91.10, a = 1.12, b = 1.26) tile. The colour values were expressed as L (whiteness or brightness/ darkness), a (redness/greenness) and b (yellowness/blueness) at any time, respectively. 2.5. Statistical analysis One-way analysis of variance (ANOVA) was conducted to determine the effect of the three concentration processes on soluble solids content and colour parameters of pomegranate juice using Statgraphics software (Statgraphics, 1991). Each measurement was replicated three times. In order to determine which means are significantly different from each other, LSD multiple range test method was used. Trends were considered significant when means of compared parameters differed at P < 0.05 significance level. The parameters of kinetics models (Eqs. (1)–(5)) were estimated by the non-linear regression iterative procedure of the SigmaPlot (SigmaPlot 8.0 Windows version, SPSS Inc.). 2.6. Kinetics of colour changes
of the coloured polymers into non-coloured compounds following a first-order kinetics. According to this combined kinetics, the colour change process can be expressed by Eq. (4) (Garza, Ibarz, Pagan, & Giner, 1999). k0 k0 C 0 expðk 1 tÞ ð4Þ C¼ k1 k1 The terms C and C0 are the concentrations of colour parameters at any time t and initial concentration, respectively; k0 is the zero-order kinetics constant and k1 is the first-order kinetics constant in Eqs. (2)–(4). Another informative model is the total colour difference (TCD) which is a combination of parameters L, a and b-values. It is a colorimetric parameter extensively used to characterize the variation of colour in foods during processing. It was calculated from Eq. (5). qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 TCD ¼ ðL0 LÞ þ ða0 aÞ þ ðb0 bÞ ð5Þ where, L0 (23.78), a0 (34.46) and b0 (8.69) refer to reference values, i.e., colour parameters of fresh juice, and L, a and b refer to colour values at various times during concentration process.
The change in concentration (°Brix) of juice samples against time was fitted to a three-parameter exponential equation (Eq. (1)).
3. Results and discussion
B ¼ B0 þ B1 expðk tÞ
3.1. Change in concentration during evaporation processes
ð1Þ
where, B and B0 are the soluble solids concentration of samples at any time t and initial concentration (°Brix), respectively; B1 is a constant and k is the evaporation rate constant (min1). The complexity of fruit juices and derivatives implies a wide range of enzymatic and non-enzymatic browning reactions caused by thermal treatments. Consequently it is difficult to establish a reaction mechanism and to obtain a kinetics model describing the global process adequately (Ibarz et al., 1999). There are numerous references on the kinetics of colour of food materials in the literature. The majority of these works report zero-order (Eq. (2)) or first-order (Eq. (3)) degradation reaction kinetics. C ¼ C0 k0 t
ð2Þ
C ¼ C 0 expðk 1 tÞ
ð3Þ
where (+) and () indicate formation and degradation of any quality parameter, respectively. On the other hand, sometimes, relatively simple models described (Eqs. (2) and (3)) do not adequately represent the colour change phenomena. Therefore, a combined kinetics has been developed in which colour change reactions are considered to consist of two stages. A first stage of coloured polymeric compound formation following a zeroorder kinetics, the second stage supposes decomposition
Evaporation is a special case of heat transfer, which deals with the evaporation of a volatile solvent such as water from a non-volatile solute material in a solution. In evaporation the vapour from a boiling liquid solution is removed and a more concentrated solution remains (Geankoplis, 1983). Many efforts have been devoted to develop improved methods such as freeze concentration, sublimation concentration and membranes (ultrafiltration and reverse osmosis) for concentrated juice processing. The most promising alternative is membrane concentration. However, main disadvantages of this method are its high operating coast and inability to reach the concentration of standard products produced by evaporation because of high pressure limitation (Jiao et al., 2004). Therefore, microwave energy was tested in production of pomegranate juice in this study. It has the advantage of heating the juice rapidly and uniformly, thus inactivating enzymes more quickly and minimizing browning (Gerard & Roberts, 2004). Fig. 1 shows soluble solids concentration (°Brix) of pomegranate juice against time of concentration process for three evaporation techniques. The time required to obtain the desired final concentration (60.5 °Brix) was 23, 108 and 190 min, respectively for microwave, rotary vacuum and atmospheric heating processes. ANOVA results showed a significant difference between the mean
M. Maskan / Journal of Food Engineering 72 (2006) 218–224
°Brix
50
40
30
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
20
10 0
25
50
75
100
125
150
175
200
Time (min) Fig. 1. Change in pomegranate juice concentration (°Brix) produced by various concentration processes.
evaporation times of the three processes (P < 0.05). The experimental data were fitted to a single, three-parameter exponential equation. Fitted parameters of Eq. (1) and corresponding correlation coefficients (r) values for the change in the concentration of pomegranate juice during concentration process were reported in Table 1. The r values were greater than 0.99 (indicating good fit) in all cases. The evaporation rate constant (k) for microwave heating process was 5.8 and 8.4 times greater than rotary vacuum and atmospheric heating concentration processes, respectively. This is because of the rapid heating and, hence, evaporating water effect of microwave process.
exert health protective effects for human (Vardin & Fenerciog˘lu, 2003). It was observed that all concentration processes decreased the colour parameters (L, a, and b values) of pomegranate juice significantly (Figs. 2–5) and the products turned reddish brown. The extent of colour degradation increased with soluble solids content. Sugar and sugar degradation products have been found to be effective on accelerating anthocyanin (pomegranate pigment) breakdown and enhance non-enzymatic browning during thermal processing (Cemerog˘lu, Veliog˘lu, & Isßık, 1994; Suh et al., 2003). Lightness (Hunter L value) decreased with treatment time (Fig. 2). Decrease in L value was 43.4%, 55.3% and 46.8 % for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. As it is seen, the extent of loss of L value is higher in the samples treated with rotary vacuum heating evaporation process. However, there was not a statistically significant
26
22
Concentration process
B0 ± SE
B1 ± SE
k ± SE
r
Microwave Rotary vacuum Atmospheric
18.87 ± 0.95 19.04 ± 0.97 19.26 ± 1.68
0.039 ± 0.031 0.162 ± 0.096 0.043 ± 0.055
0.302 ± 0.034 0.051 ± 0.005 0.036 ± 0.006
0.997 0.996 0.995
18 16
12 10 0
25
50
75
100
125
150
175
200
Time (min) Fig. 2. Variation of Hunter colour L value of pomegranate juice produced by various concentration processes.
36 Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
34
Hunter a Value
Table 1 Kinetics parameters of Eq. (1) for changes in the concentration (°Brix) of pomegranate juice during concentration process
20
14
3.2. Change in colour parameters during evaporation processes The red, blue and orange colour in, flowers, fruits and vegetables is due to anthocyanins; these have to be accounted for in processing (Ahmed, Shivhare, & Raghavan, 2004; Salunkhe, Bolin, & Reddy, 1991). The colour of pomegranate varies from light pink to violet which arises from the various anthocyanin pigments. Anthocyanin concentrations of pomegranate juice generally vary between 10 and 700 mg/l depending on the pomegranate cultivar. Nutritionists recommend to preserving these compounds during fruit juice processing, because they
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
24
Hunter L Value
60
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32 30 28 26 24
SE: Standard error of estimation, r: correlation coefficient.
0
25
50
75
100
125
150
175
200
Time (min) Fig. 3. Variation of Hunter colour a value of pomegranate juice produced by various concentration processes.
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M. Maskan / Journal of Food Engineering 72 (2006) 218–224 10.0
Hunter b Value
9.5 9.0 8.5 8.0 7.5 Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
7.0 6.5 0
25
50
75
100
125
150
175
200
Time (min) Fig. 4. Variation of Hunter colour b value of pomegranate juice produced by various concentration processes.
18 16 14
TCD
12 10 8 6 4
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
2 0 0
20
40
60
80
100
120
140
160
180
200
Time (min) Fig. 5. Change in total colour difference value of pomegranate juice concentrate produced by various concentration processes.
difference (P > 0.05) between the mean L values of different evaporation processes. Since L value is a measure of the colour in the light–dark axis, this falling value indicates that the samples were turning darker. Similar results were obtained by various investigators and it has been reported that decreases in L value correlated well with increases in the browning of food materials and pigment destruction (Ahmed et al., 2004; Ibarz et al., 1999; Maskan et al., 2002; Suh et al., 2003). Fig. 3 shows change in Hunter a value of pomegranate juice during concentration processes. The a value decreased during processing by either method with time. A similar behaviour for this parameter was found by other authors in blackcurrant syrups (Skrede, 1985), grape juice (Rhim, Nunes, Jones, & Swartzel, 1989), blood orange juice (Arena, Fallico, & Maccarone, 2000) and purple carrots (Uyan, Baysal, Yurdagel, & El, 2004). The reduction in this parameter was not very severe as
compared to L value. It was 25.8%, 27.2% and 19.4% for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. Concentrating the juice by various heating methods had no effect on Hunter a value statistically (P > 0.05). The decrease of the Hunter L and a values was due to fading of the red colour as heat destroyed anthocyanin pigments which are unstable in fruit juices (Rhim et al., 1989) and polymerisation of anthocyanins with other phenolics (GarciaViguera et al., 1999). Hunter b value showed fluctuation during all concentration processes (Fig. 4). The experimental points distributed in a disperse form with a decreasing trend. Such a trend was observed by Garza et al. (1999) in peach puree during heating at 80 °C. The b value decreased from an initial value of 8.69–7.18, 6.78 and 7.78, respectively for microwave, rotary vacuum and atmospheric heating processes. It corresponds to 17.4%, 21.9% and 10.5% reduction which demonstrates that the samples lost their yellow hues. Statistically the change in Hunter b values for these processes was not significantly different (P > 0.05). Similar results for the decrease in b value were found by many authors in blackcurrant syrups during storage (Skrede, 1985), hot air and microwave drying of banana (Maskan, 2000) and kiwifruits (Maskan, 2001), peach puree (Avila & Silva, 1999; Garza et al., 1999) and plum puree during heating (Ahmed et al., 2004). The least reduction was obtained in b value of pomegranate juice concentrate compared to L and a values, in this study. Also, Hunter L, a and b values were used to calculate total colour differences during concentration processes, which indicated the magnitude of overall colour difference between fresh and concentrated juice. TCD values increased with time during all concentration processes as shown in Fig. 5. The values at the pomegranate final soluble solids content of 60.5 °Brix were 13.72, 16.24 and 13.02 for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. Interestingly, the largest colour change was observed in rotary vacuum heating process. It indicates more colour change occurred due to this process. However, analysis of variance results showed no statistically significant difference (P > 0.05) between the mean of TCD values of the three processes employed. Comparing these results and those previously reported by other authors for heating of pear puree at high temperatures (Ibarz et al., 1999), heating peach puree at 110–135 °C (Avila & Silva, 1999), and microwave-hot air drying of banana (Maskan, 2000) and kiwifruits (Maskan, 2001), the TCD values in this study are quite similar despite of different systems and the heat treatment applied. From the colour parameters investigated (Hunter L, a, b and TCD), it was revealed that the extent of loss was higher in rotary vacuum heating process (L: 55.3%, a: 27.2%, b: 21.9% and TCD: 16.24) than the
M. Maskan / Journal of Food Engineering 72 (2006) 218–224
others. It may be due to low temperature (40 °C) employed which could not inactivated the oxidative enzymes naturally present that cause colour change during processing (Avila & Silva, 1999; Palou, Lopez-Malo, Barbosa-Canovas, Welti-Chanes, & Swanson, 1999; Rhim et al., 1989) in addition to the other factors. Table 2 Kinetics parameters of zero-order model (Eq. (2)) for L, a and TCD values Colour parameter
Concentration process
C0 ± SE
k0 ± SE
r
L
Microwave Rotary vacuum Atmospheric
25.21 ± 1.07 24.52 ± 0.77 24.72 ± 1.19
0.436 ± 0.073 0.112 ± 0.010 0.055 ± 0.009
0.947 0.977 0.959
a
Microwave Rotary vacuum Atmospheric
35.36 ± 1.26 35.64 ± 1.17 35.28 ± 1.03
0.319 ± 0.086 0.073 ± 0.016 0.032 ± 0.008
0.879 0.893 0.916
TCD
Microwave Rotary vacuum Atmospheric
1.59 ± 1.57 1.18 ± 1.24 1.21 ± 1.53
0.540 ± 0.107 0.135 ± 0.017 0.063 ± 0.012
0.929 0.959 0.950
SE: Standard error of estimation, r: correlation coefficient.
Table 3 Kinetics parameters of first-order model (Eq. (3)) for L, a and TCD values Colour parameter
Concentration process
C0 ± SE
k1 ± SE
r
L
Microwave Rotary vacuum Atmospheric
25.26 ± 1.38 24.75 ± 1.12 24.77 ± 1.55
0.021 ± 0.004 0.006 ± 0.001 0.002 ± 0.001
0.925 0.959 0.939
a
Microwave Rotary vacuum Atmospheric
35.37 ± 1.36 35.64 ± 1.28 35.28 ± 1.11
0.009 ± 0.002 0.002 ± 0.001 0.001 ± 0.000
0.867 0.880 0.906
TCD
Microwave Rotary vacuum Atmospheric
0.48 ± 0.109 0.95 ± 0.210 0.66 ± 0.140
0.144 ± 0.010 0.026 ± 0.002 0.015 ± 0.001
0.996 0.993 0.997
SE: Standard error of estimation, r: correlation coefficient.
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3.3. Kinetics consideration of colour parameters In the present study, variation of colour parameters L, a and TCD with the treatment time was fitted to a zero-, first-order and combined kinetics model by non-linear regression. Since Hunter b value showed fluctuation during all concentration processes without a constant trend, it gave very poor correlation. Therefore, this parameter was not involved in zero- and first-order kinetics calculations. Correlation coefficients and initial value of parameters estimated were used as the basis to select the model which best described the experimental data. The values of parameters estimated from the fittings were given in Tables 2–4. The kinetics constants from different models for the colour degradation were observed to have different values (P < 0.05) for the three heating processes. For example, the constants for L value obtained from zero- and first-order kinetics models were 0.436 and 0.021 min1, respectively (Tables 2 and 3) due to microwave application. Similar trends were observed in the other colour parameters of juice for different processes also. The zero-order model kinetics constant values (k0) were higher in all cases than those of first-order model (k1). This is not in agreement with the results of Garza et al. (1999) and Maskan (2001). It can be calculated from Tables 2 and 3 that when microwave heating was applied, regardless of zero or first-order models, the kinetics constant for L value was about 3.5–5.5 times greater than that of rotary vacuum heating and 8–10.5 times greater than atmospheric heating process. A similar pattern was also found for the other parameters. It implies severe destruction of parameters. But since the time required to achieving the final pomegranate juice concentration of 60.5 °Brix by microwave heating was smaller than that of the other methods, these results seem reasonable. The correlation coefficients of zero- and firstorder models were not significantly different from each other (P > 0.05) except those of TCD. However, the
Table 4 Kinetics parameters of combined model (Eq. (4)) for L, a, b and TCD values Colour parameter
Concentration process
C0 ± SE
k0 ± SE
k1 ± SE
r
L
Microwave Rotary vacuum Atmospheric
23.86 ± 0.12 23.39 ± 0.43 23.58 ± 0.36
2.730 ± 0.146 0.412 ± 0.075 0.298 ± 0.044
0.110 ± 0.007 0.015 ± 0.003 0.012 ± 0.002
0.999 0.995 0.998
a
Microwave Rotary vacuum Atmospheric
33.71 ± 0.47 34.10 ± 0.17 34.31 ± 0.17
8.032 ± 1.996 1.623 ± 0.134 0.668 ± 0.073
0.238 ± 0.061 0.047 ± 0.004 0.019 ± 0.002
0.984 0.998 0.998
b
Microwave Rotary vacuum Atmospheric
8.46 ± 0.229 9.13 ± 0.230 8.77 ± 0.169
2.811 ± 1.153 0.675 ± 0.148 0.668 ± 0.127
0.332 ± 0.140 0.073 ± 0.017 0.076 ± 0.000
0.828 0.926 0.992
TCD
Microwave Rotary vacuum Atmospheric
0.43 ± 0.430 0.65 ± 0.601 0.31 ± 0.441
0.111 ± 0.093 0.015 ± 0.027 0.009 ± 0.010
0.142 ± 0.022 0.023 ± 0.005 0.013 ± 0.024
0.996 0.994 0.997
SE: Standard error of estimation, r: correlation coefficient.
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M. Maskan / Journal of Food Engineering 72 (2006) 218–224
regression analysis revealed that combined model described the experimental data of parameters L, a, b and TCD better than the zero- and first-order models due to high correlation coefficients and reasonable C0 values obtained (Table 4). An excellent correlation existed between the colour parameters and concentration times at all heating processes. On the other hand, there was no significant difference between the r values of firstorder and combined model for TCD value analysis. Both models can describe the TCD data adequately. Therefore, only the plots of combined model were given (Figs. 2–5). Comparing both constants of combined model (Table 4), it was observed that k0 value is notably higher than k1 for parameters L, a and b which implies that the two stages (colour formation and pigment destruction) supposed by the model occur. This indicates that the rate of colour formation is higher than the colour destruction for all processes. The results are in agreement with previous reports of Garza et al. (1999). The correlation between Hunter parameters (L–a, L–b and a–b) was analysed for the three processes. Only the linear correlation between Hunter L and a values was found significant. The equations of positive linear relation and corresponding correlation coefficients were L = 18.59 + 1.22 * a (r = 0.965), L = 24.24 + 1.34 * a (r = 0.967) and L = 33.00 + 1.63 * a (r = 0.992) for microwave, rotary vacuum and atmospheric heating processes, respectively. 4. Conclusions According to the results obtained, microwave energy could be used in production of pomegranate juice concentrate successfully. The combined model used can describe the experimental colour parameters better than the zero- and first-order kinetics models. This model implied that the colour formation and pigment destruction occurred during concentration processes of pomegranate juice. References Ahmed, J., Shivhare, U. S., & Raghavan, G. S. V. (2004). Thermal degradation kinetics of anthocyanin and visual colour of plum puree. European Food Research and Technology, 218, 525–528. Al-Maiman, S. A., & Ahmad, D. (2002). Changes in physical and chemical properties during pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437–441. Arena, E., Fallico, B., & Maccarone, E. (2000). Influence of carotenoids and pulps on the color modification of blood orange juice. Journal of Food Science, 65, 458–460. Avila, I. M. L. B., & Silva, C. L. M. (1999). Modelling kinetics of thermal degradation of colour in peach puree. Journal of Food Engineering, 39, 161–166. Barreiro, J. A., Milano, M., & Sandoval, A. J. (1997). Kinetics of colour change of double concentrated tomato paste during thermal treatment. Journal of Food Engineering, 33, 359–371.
Cassano, A., Jioa, B., & Drioli, E. (2004). Production of concentrated kiwifruit juice by integrated membrane process. Food Research International, 37, 139–148. Cemerog˘lu, B., Veliog˘lu, S., & Isßık, S. (1994). Degradation kinetics of anthocyanins in sour cherry juice and concentrate. Journal of Food Science, 59, 1216–1218. Garcia-Viguera, C., Zafrilla, P., Romero, F., Abellan, P., Artes, F., & Tomas-Barberan, F. A. (1999). Color stability of strawberry jam as affected by cultivar and storage temperature. Journal of Food Science, 64, 243–247. Garza, S., Ibarz, A., Pagan, J., & Giner, J. (1999). Non-enzymatic browning in peach puree during heating. Food Research International, 32, 335–343. Geankoplis, C. J. (1983). Transport processes and unit operations (pp. 479–507) (second ed.). Newton, MA: Allyn and Bacon, Inc. Gerard, K. A., & Roberts, J. S. (2004). Microwave heating of apple mash to improve juice yield and quality. Lebensmittel-Wissenschaft & Technologie, 37(5), 551–557. Ibarz, A., Pagan, J., & Garza, S. (1999). Kinetic models for colour changes in pear puree during heating at relatively high temperatures. Journal of Food Engineering, 39, 415–422. Jiao, B., Cassano, A., & Drioli, E. (2004). Recent advances on membrane processes for the concentration of fruit juices: a review. Journal of Food Engineering, 63, 303–324. Lozano, J. E., & Ibarz, A. (1997). Colour changes in concentrated fruit pulp during heating at high temperatures. Journal of Food Engineering, 31, 365–373. Maskan, A., Kaya, S., & Maskan, M. (2002). Effect of concentration and drying processes on color change of grape juice and leather (pestil). Journal of Food Engineering, 54, 75–80. Maskan, M. (2000). Microwave/air and microwave finish drying of banana. Journal of Food Engineering, 44, 71–78. Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48, 169–175. Negi, P. S., Jayaprakasha, G. K., & Jena, B. S. (2003). Antioxidant and antimutagenic activities of pomegranate peel extracts. Food Chemistry, 80, 393–397. Palou, E., Lopez-Malo, A., Barbosa-Canovas, G. V., Welti-Chanes, J., & Swanson, B. G. (1999). Polyphenoloxidase activity and color of blanched and high hydrostatic pressure treated banana puree. Journal of Food Science, 64, 42–45. Ramteke, R. S., Singh, N. I., Rekha, M. N., & Eipeson, W. E. (1993). Methods for concentration of fruit juices—a critical evaluation. Journal of Food Science and Technology—Mysore, 30, 391–402. Rhim, J. W., Nunes, R. V., Jones, V. A., & Swartzel, K. R. (1989). Kinetics of colour change of grape juice generated using linearly increasing temperature. Journal of Food Science, 54, 776–777. Salunkhe, D. K., Bolin, H. R., & Reddy, N. R. (1991). Storage, processing, and nutritional quality of fruits and vegetables, 2. Boca Raton, FL: CRC Press. Skrede, G. (1985). Color quality of blackcurrant syrups during storage evaluated by Hunter L, a, b values. Journal of Food Science, 50, 514–517, 525. Statgraphics, A. (1991). Statistical graphics system reference manual, 1. USA: Statistical Graphics Corporation, Inc. Suh, H. J., Noh, D. O., Kang, C. S., Kim, J. M., & Lee, S. W. (2003). Thermal kinetics of color degradation of mulberry fruit extract. Nahrung, 47, 132–135. Toribo, J. L., & Lozano, J. E. (1986). Heat induced browning of clarified apple juice at high temperatures. Journal of Food Science, 51, 172–175, 179. ¨ , & El, S. N. (2004). Effects of Uyan, S. E., Baysal, T., Yurdagel, U drying on antioxidant activity of purple carrots. Nahrung, 48, 57–60. Vardin, H., & Fenerciog˘lu, H. (2003). Study on the development of pomegranate juice processing technology: clarification of pomegranate juice. Nahrung, 47, 300–303.
Production of pomegranate (Punica granatum L.) juice concentrate by various heating methods: colour degradation and kinetics Medeni Maskan
*
Food Engineering Department, Engineering Faculty, University of Gaziantep, Gaziantep 27310, Turkey Received 3 June 2004; accepted 23 November 2004 Available online 24 December 2004
Abstract Pomegranate juice was concentrated by various heating methods. The final juice concentration of 60.5 °Brix was achieved in 23, 108 and 190 min by using microwave, rotary vacuum and atmospheric heating processes, respectively. The colour change during concentration processes was investigated. Total colour differences, Hunter L, a and b parameters were used to estimate the extent of colour loss. All Hunter colour parameters decreased with time. It was observed that the severity of colour loss was higher in rotary vacuum heating process than the others. The zero-order, first-order and a combined kinetics model were applied to the changes in colour parameters. Results indicated that variation in TCD followed both first-order and combined kinetics models, and parameters L, a and b followed only combined model. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Pomegranate juice; Concentration; Colour; Kinetics
1. Introduction Pomegranate (Punica granatum L.) is one of the important fruits grown in Turkey, Iran, USA, Middle East, Mediterranean and Arabic countries. It is originated from south-east Asia. The edible part of the fruit contains considerable amount of acids, sugars, vitamins, polysaccharides, polyphenols and important minerals (Al-Maiman & Ahmad, 2002; Vardin & Fenerciog˘lu, 2003). It has been reported that pomegranate juice has potent anti-atherogenic effects in healthy humans and atherosclerotic effects in mice that may be attributable to its anti-oxidative properties (Negi, Jayaprakasha, & Jena, 2003). The fruit is consumed directly as fresh seeds as well as fresh juice which can also be used in beverages for jellies, and flavouring and colouring agents. The kernels are also used as a garnish for desserts and salads *
Tel.: 90 342 3601200/2309; fax: 90 342 3601105. E-mail address: [email protected]
0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.11.012
(Al-Maiman & Ahmad, 2002). Pomegranate contains richly coloured grains which give a delicious juice. The pomegranate juice concentrate is commonly used for salads and in many dishes in Turkey. Juices are produced from various fruits that are not necessarily harvested the whole year round. Before shipping to its final destination the extracted juice is concentrated to ensure longer storage life (because of its low water activity) and easier transportation. The concentration of fruit juices requires partly removal of water without changes in solids composition, leaving all the original solid components such as fruit sugars, minerals and vitamins to the more concentrated solution (Toribo & Lozano, 1986). Concentration of fruit juices, a major unit operation in fruit processing industry, is of critical importance as it determines the quality of the final product such as flavour, colour, aroma, appearance and mouth feel (Jiao, Cassano, & Drioli, 2004; Ramteke, Singh, Rekha, & Eipeson, 1993). The production of concentrated fruit juices is of interest at industrial level since
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they can be used as ingredients in many products such as ice creams, fruit syrups, jellies and fruit juices beverages (Cassano, Jioa, & Drioli, 2004). It is known that concentration of fruit juices by conventional evaporation methods results in colour degradation and loss of most volatile compounds with a consequent remarkable qualitative decline due to thermal effects (Cassano et al., 2004; Jiao et al., 2004). The visual colour is an important attribute because it is usually the first property the consumer observes. It is an indicator of pigment concentration which can be measured instantaneously using tristimulus colorimeters for on line quality control. It has been reported (Barreiro, Milano, & Sandoval, 1997; Suh, Noh, Kang, Kim, & Lee, 2003) that many reactions can take place during thermal processing that affect the colour. Among them, the most common are pigment degradation, especially carotenoids (lycopene, xanthophylls, etc.), anthocyanin and chlorophyll, and browning reactions such as the Maillard reaction, enzymatic browning and oxidation of ascorbic acid (Barreiro et al., 1997; Ibarz, Pagan, & Garza, 1999; Lozano & Ibarz, 1997). The Hunter colour parameters L, a and b have been widely used to describe colour changes during thermal processing of fruit and vegetable products. While working on the drying of banana, kiwi fruit, grape leather and grape juice concentrate, Maskan (2000, 2001) and Maskan, Kaya, and Maskan (2002) reported that Hunter colour values changed during heat treatments. Many investigators observed similar results in their studies such as thermal treatment of concentrated tomato paste (Barreiro et al., 1997), peach puree (Avila & Silva, 1999), mulberry fruit extract (Suh et al., 2003) and concentrated fruit pulp (Lozano & Ibarz, 1997). However, studies on concentrating the pomegranate juice by microwave energy and kinetics of degradation of colour of pomegranate during concentration processes are lacking. Therefore, the present study was undertaken to investigate; (1) production of pomegranate juice concentrate by evaporation processes such as microwave heating, rotary vacuum evaporator and heating at atmospheric pressure, (2) the kinetics of degradation of visual colour using Hunter tristimulus colour scale (L, a and b values) during concentration processes.
219
encloses hundreds of fleshy sacs was removed. The juice that is localised in the sacs was manually pressed and extracted. The juice obtained had a deep-red colour. It was stored at 4 °C overnight, for settling of suspended particles, then filtered and concentrated. 2.2. Pomegranate juice concentration Three different heating/evaporation processes were employed for production of pomegranate juice concentrate. The juice was concentrated to a final 60.5°Brix from an initial °Brix of 17.5 by the following processes; (1) By microwave heating: A programmable domestic microwave oven (Arc¸elik ARMD 580, TURKEY) with maximum output of 700 W at 2450 MHz was used. The oven has adjustable power (wattage) and time controllers and was fitted with a turntable. Heating above or below 350 W power level resulted in some problems such as foaming, charring of juice or lengthy of concentration time. Therefore, the study was carried out at 350 W power level. A 500 ml of the juice sample was put in a beaker and replaced on the center of turn table in the microwave cavity. Samples were taken for measurement of °Brix and colour periodically and replaced again. (2) By rotary vacuum evaporator: A 500 ml of the juice sample was concentrated in a laboratory rotary vacuum evaporator (RE 100 Model, Bibby Sterilin Ltd., England) rotating at 66 rpm and 40 °C. Samples were taken from the bulk of juice periodically for °Brix and colour measurements and replaced again after used. (3) By evaporating at atmospheric pressure: The pomegranate juice was concentrated by using an electromagnetic heater (VELP Scientifica, Italy). A 500 ml of the juice sample was put in a beaker and replaced on the heater open to atmosphere. The sample was continuously heated and stirred during this process. Samples were taken for measurement of °Brix and colour periodically and replaced again after used.
2.3. Soluble solids content determination 2. Materials and methods 2.1. Preparation of fresh pomegranate juice Fresh pomegranate fruits (Punica granatum L.) of Gaziantep cultivars were obtained from a local market (Gaziantep—Turkey) in November. Sweet-mature fruits were prefered for the production of concentrate. Fruits were washed in cold tap water and drained. They were manually cut-up and the outer leathery skin which
During concentration processes, the soluble solids content of the juice samples was measured by an OPTON model Abbe refractometer (F.G. Bode & Co., Hamburg) at 20 °C and expressed in °Brix. 2.4. Colour measurement Colour measurements of the juice samples were carried out using a HunterLab Colorflex (A-60-1010-615 Model Colorimeter, HunterLab, Reston, VA). The
220
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instrument was standardized each time with a black and a white (L = 91.10, a = 1.12, b = 1.26) tile. The colour values were expressed as L (whiteness or brightness/ darkness), a (redness/greenness) and b (yellowness/blueness) at any time, respectively. 2.5. Statistical analysis One-way analysis of variance (ANOVA) was conducted to determine the effect of the three concentration processes on soluble solids content and colour parameters of pomegranate juice using Statgraphics software (Statgraphics, 1991). Each measurement was replicated three times. In order to determine which means are significantly different from each other, LSD multiple range test method was used. Trends were considered significant when means of compared parameters differed at P < 0.05 significance level. The parameters of kinetics models (Eqs. (1)–(5)) were estimated by the non-linear regression iterative procedure of the SigmaPlot (SigmaPlot 8.0 Windows version, SPSS Inc.). 2.6. Kinetics of colour changes
of the coloured polymers into non-coloured compounds following a first-order kinetics. According to this combined kinetics, the colour change process can be expressed by Eq. (4) (Garza, Ibarz, Pagan, & Giner, 1999). k0 k0 C 0 expðk 1 tÞ ð4Þ C¼ k1 k1 The terms C and C0 are the concentrations of colour parameters at any time t and initial concentration, respectively; k0 is the zero-order kinetics constant and k1 is the first-order kinetics constant in Eqs. (2)–(4). Another informative model is the total colour difference (TCD) which is a combination of parameters L, a and b-values. It is a colorimetric parameter extensively used to characterize the variation of colour in foods during processing. It was calculated from Eq. (5). qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 TCD ¼ ðL0 LÞ þ ða0 aÞ þ ðb0 bÞ ð5Þ where, L0 (23.78), a0 (34.46) and b0 (8.69) refer to reference values, i.e., colour parameters of fresh juice, and L, a and b refer to colour values at various times during concentration process.
The change in concentration (°Brix) of juice samples against time was fitted to a three-parameter exponential equation (Eq. (1)).
3. Results and discussion
B ¼ B0 þ B1 expðk tÞ
3.1. Change in concentration during evaporation processes
ð1Þ
where, B and B0 are the soluble solids concentration of samples at any time t and initial concentration (°Brix), respectively; B1 is a constant and k is the evaporation rate constant (min1). The complexity of fruit juices and derivatives implies a wide range of enzymatic and non-enzymatic browning reactions caused by thermal treatments. Consequently it is difficult to establish a reaction mechanism and to obtain a kinetics model describing the global process adequately (Ibarz et al., 1999). There are numerous references on the kinetics of colour of food materials in the literature. The majority of these works report zero-order (Eq. (2)) or first-order (Eq. (3)) degradation reaction kinetics. C ¼ C0 k0 t
ð2Þ
C ¼ C 0 expðk 1 tÞ
ð3Þ
where (+) and () indicate formation and degradation of any quality parameter, respectively. On the other hand, sometimes, relatively simple models described (Eqs. (2) and (3)) do not adequately represent the colour change phenomena. Therefore, a combined kinetics has been developed in which colour change reactions are considered to consist of two stages. A first stage of coloured polymeric compound formation following a zeroorder kinetics, the second stage supposes decomposition
Evaporation is a special case of heat transfer, which deals with the evaporation of a volatile solvent such as water from a non-volatile solute material in a solution. In evaporation the vapour from a boiling liquid solution is removed and a more concentrated solution remains (Geankoplis, 1983). Many efforts have been devoted to develop improved methods such as freeze concentration, sublimation concentration and membranes (ultrafiltration and reverse osmosis) for concentrated juice processing. The most promising alternative is membrane concentration. However, main disadvantages of this method are its high operating coast and inability to reach the concentration of standard products produced by evaporation because of high pressure limitation (Jiao et al., 2004). Therefore, microwave energy was tested in production of pomegranate juice in this study. It has the advantage of heating the juice rapidly and uniformly, thus inactivating enzymes more quickly and minimizing browning (Gerard & Roberts, 2004). Fig. 1 shows soluble solids concentration (°Brix) of pomegranate juice against time of concentration process for three evaporation techniques. The time required to obtain the desired final concentration (60.5 °Brix) was 23, 108 and 190 min, respectively for microwave, rotary vacuum and atmospheric heating processes. ANOVA results showed a significant difference between the mean
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°Brix
50
40
30
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
20
10 0
25
50
75
100
125
150
175
200
Time (min) Fig. 1. Change in pomegranate juice concentration (°Brix) produced by various concentration processes.
evaporation times of the three processes (P < 0.05). The experimental data were fitted to a single, three-parameter exponential equation. Fitted parameters of Eq. (1) and corresponding correlation coefficients (r) values for the change in the concentration of pomegranate juice during concentration process were reported in Table 1. The r values were greater than 0.99 (indicating good fit) in all cases. The evaporation rate constant (k) for microwave heating process was 5.8 and 8.4 times greater than rotary vacuum and atmospheric heating concentration processes, respectively. This is because of the rapid heating and, hence, evaporating water effect of microwave process.
exert health protective effects for human (Vardin & Fenerciog˘lu, 2003). It was observed that all concentration processes decreased the colour parameters (L, a, and b values) of pomegranate juice significantly (Figs. 2–5) and the products turned reddish brown. The extent of colour degradation increased with soluble solids content. Sugar and sugar degradation products have been found to be effective on accelerating anthocyanin (pomegranate pigment) breakdown and enhance non-enzymatic browning during thermal processing (Cemerog˘lu, Veliog˘lu, & Isßık, 1994; Suh et al., 2003). Lightness (Hunter L value) decreased with treatment time (Fig. 2). Decrease in L value was 43.4%, 55.3% and 46.8 % for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. As it is seen, the extent of loss of L value is higher in the samples treated with rotary vacuum heating evaporation process. However, there was not a statistically significant
26
22
Concentration process
B0 ± SE
B1 ± SE
k ± SE
r
Microwave Rotary vacuum Atmospheric
18.87 ± 0.95 19.04 ± 0.97 19.26 ± 1.68
0.039 ± 0.031 0.162 ± 0.096 0.043 ± 0.055
0.302 ± 0.034 0.051 ± 0.005 0.036 ± 0.006
0.997 0.996 0.995
18 16
12 10 0
25
50
75
100
125
150
175
200
Time (min) Fig. 2. Variation of Hunter colour L value of pomegranate juice produced by various concentration processes.
36 Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
34
Hunter a Value
Table 1 Kinetics parameters of Eq. (1) for changes in the concentration (°Brix) of pomegranate juice during concentration process
20
14
3.2. Change in colour parameters during evaporation processes The red, blue and orange colour in, flowers, fruits and vegetables is due to anthocyanins; these have to be accounted for in processing (Ahmed, Shivhare, & Raghavan, 2004; Salunkhe, Bolin, & Reddy, 1991). The colour of pomegranate varies from light pink to violet which arises from the various anthocyanin pigments. Anthocyanin concentrations of pomegranate juice generally vary between 10 and 700 mg/l depending on the pomegranate cultivar. Nutritionists recommend to preserving these compounds during fruit juice processing, because they
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
24
Hunter L Value
60
221
32 30 28 26 24
SE: Standard error of estimation, r: correlation coefficient.
0
25
50
75
100
125
150
175
200
Time (min) Fig. 3. Variation of Hunter colour a value of pomegranate juice produced by various concentration processes.
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M. Maskan / Journal of Food Engineering 72 (2006) 218–224 10.0
Hunter b Value
9.5 9.0 8.5 8.0 7.5 Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
7.0 6.5 0
25
50
75
100
125
150
175
200
Time (min) Fig. 4. Variation of Hunter colour b value of pomegranate juice produced by various concentration processes.
18 16 14
TCD
12 10 8 6 4
Microwave heating Rotary vacuum heating Atmospheric heating Predicted (Eq.(4))
2 0 0
20
40
60
80
100
120
140
160
180
200
Time (min) Fig. 5. Change in total colour difference value of pomegranate juice concentrate produced by various concentration processes.
difference (P > 0.05) between the mean L values of different evaporation processes. Since L value is a measure of the colour in the light–dark axis, this falling value indicates that the samples were turning darker. Similar results were obtained by various investigators and it has been reported that decreases in L value correlated well with increases in the browning of food materials and pigment destruction (Ahmed et al., 2004; Ibarz et al., 1999; Maskan et al., 2002; Suh et al., 2003). Fig. 3 shows change in Hunter a value of pomegranate juice during concentration processes. The a value decreased during processing by either method with time. A similar behaviour for this parameter was found by other authors in blackcurrant syrups (Skrede, 1985), grape juice (Rhim, Nunes, Jones, & Swartzel, 1989), blood orange juice (Arena, Fallico, & Maccarone, 2000) and purple carrots (Uyan, Baysal, Yurdagel, & El, 2004). The reduction in this parameter was not very severe as
compared to L value. It was 25.8%, 27.2% and 19.4% for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. Concentrating the juice by various heating methods had no effect on Hunter a value statistically (P > 0.05). The decrease of the Hunter L and a values was due to fading of the red colour as heat destroyed anthocyanin pigments which are unstable in fruit juices (Rhim et al., 1989) and polymerisation of anthocyanins with other phenolics (GarciaViguera et al., 1999). Hunter b value showed fluctuation during all concentration processes (Fig. 4). The experimental points distributed in a disperse form with a decreasing trend. Such a trend was observed by Garza et al. (1999) in peach puree during heating at 80 °C. The b value decreased from an initial value of 8.69–7.18, 6.78 and 7.78, respectively for microwave, rotary vacuum and atmospheric heating processes. It corresponds to 17.4%, 21.9% and 10.5% reduction which demonstrates that the samples lost their yellow hues. Statistically the change in Hunter b values for these processes was not significantly different (P > 0.05). Similar results for the decrease in b value were found by many authors in blackcurrant syrups during storage (Skrede, 1985), hot air and microwave drying of banana (Maskan, 2000) and kiwifruits (Maskan, 2001), peach puree (Avila & Silva, 1999; Garza et al., 1999) and plum puree during heating (Ahmed et al., 2004). The least reduction was obtained in b value of pomegranate juice concentrate compared to L and a values, in this study. Also, Hunter L, a and b values were used to calculate total colour differences during concentration processes, which indicated the magnitude of overall colour difference between fresh and concentrated juice. TCD values increased with time during all concentration processes as shown in Fig. 5. The values at the pomegranate final soluble solids content of 60.5 °Brix were 13.72, 16.24 and 13.02 for microwave, rotary vacuum and atmospheric heating concentration processes, respectively. Interestingly, the largest colour change was observed in rotary vacuum heating process. It indicates more colour change occurred due to this process. However, analysis of variance results showed no statistically significant difference (P > 0.05) between the mean of TCD values of the three processes employed. Comparing these results and those previously reported by other authors for heating of pear puree at high temperatures (Ibarz et al., 1999), heating peach puree at 110–135 °C (Avila & Silva, 1999), and microwave-hot air drying of banana (Maskan, 2000) and kiwifruits (Maskan, 2001), the TCD values in this study are quite similar despite of different systems and the heat treatment applied. From the colour parameters investigated (Hunter L, a, b and TCD), it was revealed that the extent of loss was higher in rotary vacuum heating process (L: 55.3%, a: 27.2%, b: 21.9% and TCD: 16.24) than the
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others. It may be due to low temperature (40 °C) employed which could not inactivated the oxidative enzymes naturally present that cause colour change during processing (Avila & Silva, 1999; Palou, Lopez-Malo, Barbosa-Canovas, Welti-Chanes, & Swanson, 1999; Rhim et al., 1989) in addition to the other factors. Table 2 Kinetics parameters of zero-order model (Eq. (2)) for L, a and TCD values Colour parameter
Concentration process
C0 ± SE
k0 ± SE
r
L
Microwave Rotary vacuum Atmospheric
25.21 ± 1.07 24.52 ± 0.77 24.72 ± 1.19
0.436 ± 0.073 0.112 ± 0.010 0.055 ± 0.009
0.947 0.977 0.959
a
Microwave Rotary vacuum Atmospheric
35.36 ± 1.26 35.64 ± 1.17 35.28 ± 1.03
0.319 ± 0.086 0.073 ± 0.016 0.032 ± 0.008
0.879 0.893 0.916
TCD
Microwave Rotary vacuum Atmospheric
1.59 ± 1.57 1.18 ± 1.24 1.21 ± 1.53
0.540 ± 0.107 0.135 ± 0.017 0.063 ± 0.012
0.929 0.959 0.950
SE: Standard error of estimation, r: correlation coefficient.
Table 3 Kinetics parameters of first-order model (Eq. (3)) for L, a and TCD values Colour parameter
Concentration process
C0 ± SE
k1 ± SE
r
L
Microwave Rotary vacuum Atmospheric
25.26 ± 1.38 24.75 ± 1.12 24.77 ± 1.55
0.021 ± 0.004 0.006 ± 0.001 0.002 ± 0.001
0.925 0.959 0.939
a
Microwave Rotary vacuum Atmospheric
35.37 ± 1.36 35.64 ± 1.28 35.28 ± 1.11
0.009 ± 0.002 0.002 ± 0.001 0.001 ± 0.000
0.867 0.880 0.906
TCD
Microwave Rotary vacuum Atmospheric
0.48 ± 0.109 0.95 ± 0.210 0.66 ± 0.140
0.144 ± 0.010 0.026 ± 0.002 0.015 ± 0.001
0.996 0.993 0.997
SE: Standard error of estimation, r: correlation coefficient.
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3.3. Kinetics consideration of colour parameters In the present study, variation of colour parameters L, a and TCD with the treatment time was fitted to a zero-, first-order and combined kinetics model by non-linear regression. Since Hunter b value showed fluctuation during all concentration processes without a constant trend, it gave very poor correlation. Therefore, this parameter was not involved in zero- and first-order kinetics calculations. Correlation coefficients and initial value of parameters estimated were used as the basis to select the model which best described the experimental data. The values of parameters estimated from the fittings were given in Tables 2–4. The kinetics constants from different models for the colour degradation were observed to have different values (P < 0.05) for the three heating processes. For example, the constants for L value obtained from zero- and first-order kinetics models were 0.436 and 0.021 min1, respectively (Tables 2 and 3) due to microwave application. Similar trends were observed in the other colour parameters of juice for different processes also. The zero-order model kinetics constant values (k0) were higher in all cases than those of first-order model (k1). This is not in agreement with the results of Garza et al. (1999) and Maskan (2001). It can be calculated from Tables 2 and 3 that when microwave heating was applied, regardless of zero or first-order models, the kinetics constant for L value was about 3.5–5.5 times greater than that of rotary vacuum heating and 8–10.5 times greater than atmospheric heating process. A similar pattern was also found for the other parameters. It implies severe destruction of parameters. But since the time required to achieving the final pomegranate juice concentration of 60.5 °Brix by microwave heating was smaller than that of the other methods, these results seem reasonable. The correlation coefficients of zero- and firstorder models were not significantly different from each other (P > 0.05) except those of TCD. However, the
Table 4 Kinetics parameters of combined model (Eq. (4)) for L, a, b and TCD values Colour parameter
Concentration process
C0 ± SE
k0 ± SE
k1 ± SE
r
L
Microwave Rotary vacuum Atmospheric
23.86 ± 0.12 23.39 ± 0.43 23.58 ± 0.36
2.730 ± 0.146 0.412 ± 0.075 0.298 ± 0.044
0.110 ± 0.007 0.015 ± 0.003 0.012 ± 0.002
0.999 0.995 0.998
a
Microwave Rotary vacuum Atmospheric
33.71 ± 0.47 34.10 ± 0.17 34.31 ± 0.17
8.032 ± 1.996 1.623 ± 0.134 0.668 ± 0.073
0.238 ± 0.061 0.047 ± 0.004 0.019 ± 0.002
0.984 0.998 0.998
b
Microwave Rotary vacuum Atmospheric
8.46 ± 0.229 9.13 ± 0.230 8.77 ± 0.169
2.811 ± 1.153 0.675 ± 0.148 0.668 ± 0.127
0.332 ± 0.140 0.073 ± 0.017 0.076 ± 0.000
0.828 0.926 0.992
TCD
Microwave Rotary vacuum Atmospheric
0.43 ± 0.430 0.65 ± 0.601 0.31 ± 0.441
0.111 ± 0.093 0.015 ± 0.027 0.009 ± 0.010
0.142 ± 0.022 0.023 ± 0.005 0.013 ± 0.024
0.996 0.994 0.997
SE: Standard error of estimation, r: correlation coefficient.
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M. Maskan / Journal of Food Engineering 72 (2006) 218–224
regression analysis revealed that combined model described the experimental data of parameters L, a, b and TCD better than the zero- and first-order models due to high correlation coefficients and reasonable C0 values obtained (Table 4). An excellent correlation existed between the colour parameters and concentration times at all heating processes. On the other hand, there was no significant difference between the r values of firstorder and combined model for TCD value analysis. Both models can describe the TCD data adequately. Therefore, only the plots of combined model were given (Figs. 2–5). Comparing both constants of combined model (Table 4), it was observed that k0 value is notably higher than k1 for parameters L, a and b which implies that the two stages (colour formation and pigment destruction) supposed by the model occur. This indicates that the rate of colour formation is higher than the colour destruction for all processes. The results are in agreement with previous reports of Garza et al. (1999). The correlation between Hunter parameters (L–a, L–b and a–b) was analysed for the three processes. Only the linear correlation between Hunter L and a values was found significant. The equations of positive linear relation and corresponding correlation coefficients were L = 18.59 + 1.22 * a (r = 0.965), L = 24.24 + 1.34 * a (r = 0.967) and L = 33.00 + 1.63 * a (r = 0.992) for microwave, rotary vacuum and atmospheric heating processes, respectively. 4. Conclusions According to the results obtained, microwave energy could be used in production of pomegranate juice concentrate successfully. The combined model used can describe the experimental colour parameters better than the zero- and first-order kinetics models. This model implied that the colour formation and pigment destruction occurred during concentration processes of pomegranate juice. References Ahmed, J., Shivhare, U. S., & Raghavan, G. S. V. (2004). Thermal degradation kinetics of anthocyanin and visual colour of plum puree. European Food Research and Technology, 218, 525–528. Al-Maiman, S. A., & Ahmad, D. (2002). Changes in physical and chemical properties during pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437–441. Arena, E., Fallico, B., & Maccarone, E. (2000). Influence of carotenoids and pulps on the color modification of blood orange juice. Journal of Food Science, 65, 458–460. Avila, I. M. L. B., & Silva, C. L. M. (1999). Modelling kinetics of thermal degradation of colour in peach puree. Journal of Food Engineering, 39, 161–166. Barreiro, J. A., Milano, M., & Sandoval, A. J. (1997). Kinetics of colour change of double concentrated tomato paste during thermal treatment. Journal of Food Engineering, 33, 359–371.
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