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Rheological characteristics and thermal degradation kinetics of beta-carotene in pumpkin puree
Journal of Food Engineering 76 (2006) 538–546 www.elsevier.com/locate/jfoodeng
Rheological characteristics and thermal degradation kinetics of beta-carotene in pumpkin puree Debjani Dutta, Abhishek Dutta, Utpal Raychaudhuri, Runu Chakraborty
*
Department of Food Technology and Biochemical Engineering, Jadavpur University, Kolkata 700 032, India Received 25 August 2004; received in revised form 30 April 2005; accepted 24 May 2005 Available online 10 August 2005
Abstract The degradation kinetics of both the beta-carotene and visual color of pumpkin puree (blanched for 2 min in 1% NaCl solution) were determined at a temperature range of 60–100 °C for a time period varying between 0 and 2 h. An increase in the beta-carotene content was observed when the pumpkin puree was blanched and thermally treated at 60 °C. Using the concept of fractional conversion, it was observed that the degradation of both beta-carotene and visual color followed the first-order reaction kinetics. Dependence of the rate constants followed the Arrhenius relationship. The activation energy for beta-carotene was found to be 27.2715 kJ/mol and the activation energy for visual color using La/b and DE values was found to be 33.6831 kJ/mol and 30.3943 kJ/mol respectively. Higher activation energy signifies greater temperature sensitivity of visual color. The change in visual color was found to be a direct manifestation of the change in beta-carotene content. Rheological characteristics of the puree was also studied over the temperature range of 60–100 °C. Herschel–Bulkley model was found to fit adequately over the entire temperature range. Pumpkin puree exhibited yield stress, which decreased exponentially with temperature. With the increase in temperature, the puree was found to behave as a pseudoplastic fluid. Arrhenius model gave a satisfactory description of the temperature dependence of apparent viscosity. The activation energy for apparent viscosity and consistency index of pumpkin puree was found to be 13.3845 kJ/mol and 31.9394 kJ/mol respectively. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pumpkin puree; Beta-carotene; Fractional conversion; Thermal degradation; Activation energy; Flow index behavior
1. Introduction Thermal treatment is one of the most important methods of preservation of vegetables (Lund, 1975). The thermal processing of food is primarily intended to inactivate pathogens and other deteriorative microorganisms capable of making it unsuitable for consumption. Thermal processing also improves the bio-availability of beta-carotene, since it breaks down the cellulose structure
*
Corresponding author. Tel.: +91 33 2414 6663; fax: +91 33 24146822. E-mail address: [email protected] (R. Chakraborty). 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.05.056
of plant cell. Unfortunately, sensory properties of food including nutrients, color and texture deteriorate during the process. Kinetic models of thermal destruction are essential to design new processes assuming a safe food product and giving a maximum retention of quality factors (Lund, 1975; Teixeira, Dixon, Zahradnik, & Zinsmeister, 1969). Pumpkin puree is an intermediate product and is thermally processed for the manufacture of jam, jelly, sweets, beverages and other products. Retention of color, flavor and viscosity during thermal processing is some of the parameters that affect the success of a pureed product. Maintenance of these naturally colored pigments with desired textural and viscoelastic properties have been a major challenge in food processing
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industry. Various factors are responsible for the degradation of pigment and color during thermal processing of food products. Pumpkins contain beta-carotene which show a distinct color shift during thermal processing as heat induces cis–trans isomerization reaction (Klaui & Bauernfeind, 1981), oxidation to epoxy carotenoids and apocertenals (Rodriguez-Amaya, 1999) and even hydroxylation (Marty & Bersit, 1988). Visual color and pigment degradation kinetics of food products are complex phenomena and dependable models which can predict experimental color changes, which can be used in engineering calculations are limited (Ahmed, Shivhare, & Sandhu, 2002b). Models which can accurately predict the progress of a chemical reaction that occur in a homogeneous liquid or semi-solid state phase during thermal processing and storage are useful in many engineering applications, including process optimization. Degradation of beta-carotene by thermal treatment follow first order reaction kinetics (Ahmed, Shivhare, et al., 2002b; Lavelli & Giovanelli, 2003; Minguez-Mosquera & Jaren-Galan, 1995; Tang & Chen, 2000). The kinetic parameters like reaction order, rate constant and activation energy provide useful information on the quality changes that occur during thermal processing. Visual appearance is the foremost quality considered by consumers at the time of purchasing a product. Hence, excessive discoloration caused during thermal processing renders some foods unmarketable. Many processors utilize the psychological effect of color to market their products (Maskan, 2001). Several authors have studied color of food instrumentally (Ahmed, Shivhare, & Raghavan, 2000a; Gunawan & Barringer, 2000; Hunt, 1991; Nagle, Villalon, & Burns, 1979; Rigg, 1987; Shin & Bhowmik, 1995). Measurement of color by tristimulus colorimetry in terms of Hunter scale (L, a and b) value has been accepted as simple and accurate method (Hayakawa & Timbers, 1977) of color detection as compared to spectrophotometric systems. On the other hand, measurement of pigment could quantify the actual color degradation during processing. Hence, it is important to establish correlation between pigment concentration and visual color of food products during thermal processing. Rheological behavior of food depends on various factors like temperature, composition and total soluble solids content. Knowledge of the rheological properties of food is essential for product development, quality control (Kramer & Twigg, 1970), sensory evaluation and process engineering calculations. The flow behavior of a fluid can range from Newtonian to time-dependent non-Newtonian depending on its origin, structural behavior and history. Viscosity of fluid foods is affected by thermal processing. The effect of temperature on viscosity (or apparent viscosity) determined at a specific shear rate can be expressed by Arrhenius relationship.
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Nearly no information is available on beta-carotene and visual color degradation of pumpkin puree during thermal processing. The objective of this study was to determine the relation between the kinetics of thermal degradation of beta-carotene and visual color. It was then compared with the change in viscosity of the puree during heating at various temperatures. The aim was to correlate all these parameters for the optimization of online monitoring technique for better quality.
2. Materials and method 2.1. Raw material Fresh pumpkins were purchased from the local market of Kolkata, India. It was washed thoroughly and the skin was removed using a stainless steel knife. 2.2. Preparation of puree Skinless pumpkins were cut into small pieces and washed thoroughly after removing the seeds. The pumpkin pieces were then blanched in 1% NaCl solution at 100 °C for 2 min. They were cooled immediately and dried on a filter paper to remove the excess water. After passing through a pulper, the pumpkin pieces were sieved through a 14 mesh screen to obtain a product of uniform consistency. The puree was stored in sterile glass containers at 0 °C for further processing. 2.3. Physico-chemical properties Total soluble solids (°Brix) and pH of the puree were measured with a Bellingham Stanley refractometer (Model RFM-110, UK) and a digital pH meter (Thermo Orion, Model 420, USA) respectively. 2.4. Thermal treatment Thermal degradation kinetics was studied by isothermally heating the puree at pre-selected temperatures (60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C) for 0–2 h. The samples were sealed in glass vials (i.d. 1.5 cm, 8 ml) and immersed in a thermostatic water bath (for the temperature range 60–80 °C) and in an oil-bath for (90 °C and 100 °C) for preset time (0, 30, 60, 90 and 120 min) following the method described by Weemaes, Ooms, Loey, and Hendrickx (1999) and Ahmed, Shivhare, et al. (2002b). The oil was continuously stirred by means of a stirrer fitted with the bath. An oil-recirculating pump was used to facilitate heat transfer and uniform heating through out the bath. At the specified time intervals, the samples were withdrawn from the hot bath and cooled in an ice-water mixture.
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2.5. Measurement of beta-carotene
lnðC=C 0 Þ ¼ kt
The estimation of beta-carotene was done after extraction of the sample with diacetone alcohol and petroleum ether and further purification with diacetone alcohol, methanolic KOH and distilled water. The resulting solution was filtered with anhydrous sodium sulphate and read on a spectrophotometer (Hitachi, U-2000, Japan) at 450-nm against petroleum ether as a blank. A standard graph was plotted using synthetic crystalline beta-carotene (Fluka, Germany) dissolved in petroleum ether and its optical density measured at 450 nm. From the standard graph, the concentration in microgram per ml was determined and this was used for the calculation of beta-carotene in the sample. All the analytical work were repeated three times.
where C0 = initial concentration of beta-carotene (lg/g) or measured Hunter color value (L, a, b) or a combination of these at time zero (dimensionless); C = concentration of beta-carotene (lg/g) or measured Hunter color value (L, a, b) or a combination of these at time t (dimensionless); k = temperature dependent rate constant (min1); t = heating time (min). For a reaction following a first-order kinetic model, the plot of ln(C/C0) vs time would be a straight line and the slope would be equal to k at a constant temperature. Dependence of the degradation rate constant on temperature can be represented by the Arrhenius equation:
2.6. Measurement of color Visual color was measured using a HunterLab Color Measurement System Model Color Flex 45/0 (Hunter Associates Laboratory Inc., USA) in terms of universally accepted Hunter Lab color scale. L Value signifies ‘‘lightness’’ (100 for white and 0 for black), a represents changes from ‘‘greenness to redness’’ (80 for green and 100 for red) and b from ‘‘blueness to yellowness’’ (80 for blue and 70 for yellow). The instrument (10° observer, IlluminantD-65) was calibrated against a standard white reference tile.
where k is the rate constant at temperature T (Kelvin), k0 is the pre-exponential factor, Ea is the activation energy (kJ/mol) and R is the gas constant (8.314 J/mol K). Therefore, if the temperature dependence follows ArrheniusÕ relationship, the plot of ln k vs 1/T would be a straight line and the slope equal to Ea/R. Fractional conversion is a convenient variable often used in place of concentration (Levenspiel, 1974) and has been reported to increase the accuracy of the calculation. For an irreversible first order reaction kinetics, the rate constant at constant temperature can be determined through fractional conversion, f:
2.7. Rheological measurement
f ¼ ðC 0 CÞ=ðC 0 C / Þ
Rheological measurements (shear stress and shear rate) of the puree at various temperatures were done in a rotational viscometer (Brookfield R/S–CC25 Rheometer, Middleboro, MA, USA) equipped with a Coaxial Cylinder Measuring Systems. Approximately 17 ml of puree was placed in the concentric cylindrical cup. The sample compartment was monitored at a constant temperature using a water bath. The viscometer was operated between 10 and 100 rpm and the shear stress readings were obtained directly from the instrument.
3. Kinetic analysis 3.1. Model for pigment and color analysis Degradation kinetics of both pigment and color has been found to follow first-order reaction kinetics as have been seen in case of Ahmed, Kaur, and Shivhare (2002a); Ahmed, Shivhare, et al. (2002b); Ahmed, Shivhare, and Raghavan (2000a, 2000b); Gunawan and Barringer (2000); Hutchings (1994); Steet and Tong (1996); Weemaes et al. (1999). The first-order kinetic model can be represented as,
k ¼ k 0 expðEa =RT Þ
ð1Þ
ð2Þ
ð3Þ
where C/ = measured pigment or color values at infinite time when the reaction is expected to be complete. Since, C/ for an irreversible reaction would be 0, Eq. (1), can be expressed as ln C=C 0 ¼ lnð1 f Þ ¼ kt
ð4Þ
The infinite values of both concentrations for pigment and for color were determined following the methods of Steet and Tong (1996) and Weemaes et al. (1999). Pumpkin puree was acidified using concentrated HCl and also thermally treated at selected temperatures for a long period of time (24 h) and the corresponding values were measured. It is assumed that color/pigment degradation at a constant temperature is nearly constant with respect to time on prolonged heating. This nonzero residue should be independent of reaction temperature and reaction path (Steet & Tong, 1996). 3.2. Flow models The power law model with or without yield term has been employed to describe the flow behavior of viscous food over wide ranges of shear rates (Vitali & Rao, 1984).
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546 n
s ¼ KðcÞ and s s0 ¼ K H ðcÞnH
ð5Þ ð6Þ
where s0 is the yield stress (Pa), K, KH is the consistency index (Pa sn/) and n, nH is the flow behavior index (dimensionless). s and c represent shear stress and shear rate respectively. The rheological model that has been generally used for non-Newtonian fluids, especially purees/pastes is the Herschel–Bulkley model (Eq. (7)). Although the power law has been employed extensively for characterizing foods, including shear-thinning (pseudoplastic) foods, it fails to predict the flow behavior at very low shear rates (zero shear viscosity). The temperature dependence of the apparent viscosity at constant shear rate and consistency index can be described by the Arrhenius relationship: ga ¼ Aga expðEga =RT a Þ
ð7Þ
K ¼ AK expðEK =RT a Þ
ð8Þ
where ga is the apparent viscosity (centipoise), Aga and AK are frequency factors for apparent viscosity at constant rpm and consistency index respectively. Ega and EK are activation energy for apparent viscosity at constant rpm (kJ/mol) and consistency index (kJ/mol) respectively. Ta is the absolute temperature (K) and R is the universal gas constant (J/mol K). Experiments were replicated thrice and the average values were used in the analysis. 3.3. Statistical analysis All the tests were done in triplicate and the software, Statistical Package for Social Science Research (SPSS, release 7.5., version 1, 1996) was used for statistical assessment. Significance of difference was defined at p 6 0.05.
4. Results and discussions The pH and total soluble solids (TSS) content of the pumpkin puree were found to be 4.35 and 7.2 ± 0.3 °Brix respectively. 4.1. Kinetics of pigment degradation From Table 1, it is evident that blanched pumpkin has a higher beta-carotene content than the unblanched one. This is because, blanching results in inactivation of the enzyme lipoxygenase, which can co-oxidize beta-carotene and degrade the pigment to a colorless product (Holden, 1970). Blanching is also said to remove the intracellular air, thus establishing a continuous liquid phase. The liquid phase further protects the beta-carotene from degradation.
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Table 1 Beta-carotene content of blanched, unblanched and thermally treated pumpkin at 60 °C Conditions of treatment
Beta-carotene (lg/g)
Unblanched pumpkin puree Blanched for 2 min in 1% NaCl solution Blanched for 2 min in 1% NaCl solution and thermally treated at 60 °C (for 2 h)
10.9426 12.4569 14.0531
However, thermal treatment at 60 °C leads to a further increase in the beta-carotene content. (Table 1). Thermal processing has been reported to increase the beta-carotene concentration, perhaps because of greater chemical extractability and loss of moisture due to which soluble solids concentrate the sample (GuerraVargas, Jaramillo-Flores, Dorantes-Alvarez, & Hernandez-Sanchez, 2001). Heat treatment also inactivates some oxidative enzymes and breaks some structure leading to a higher bioavailability of beta-carotene (Howard, Wong, Perry, & Klein, 1999; Vander Berg et al., 2000). The beta-carotene content of pumpkin at temperatures 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C after 2 h of thermal treatment were experimentally determined to be 8.1499, 7.7087, 7.2488, 5.8962, 4.8653 and 4.1975 lg/g respectively whereas the value at 60 °C was 14.0531 lg/g (Table 1). Thus, it is seen that above 60 °C, beta-carotene content of the pumpkin puree decreased during the thermal treatment. However, the degradation of beta-carotene pigment at 60 °C did not follow the fractional conversion kinetics (Eq. (4)). The equilibrium beta-carotene value when the puree was acidified in concentrated hydrochloric acid and thermally processed for a prolonged time (24 h) was measured as 3.0520 lg/g. The thermal degradation of beta-carotene with increase in temperature was due to oxidative and non-oxidative changes such as cis–trans isomerization and epoxide formation. The fractional conversion kinetics (Eq. (4)) was used to model the thermal degradation of beta-carotene pigment. The rate of degradation of beta-carotene in pumpkin puree was determined by linear regression of ln(1 f) against heating time (Fig. 1). It is evident from the figure that the degradation of pumpkin puree followed the first order reaction kinetics. The regression coefficients of beta-carotene degradation values with respect to time (Fig. 1) and as represented by Eq. (4) are given in Table 2. Values of R2 was greater than 0.96 for all temperatures and the standard error (SE) was less than 0.07. 4.2. Visual color degradation kinetics During thermal processing, it was observed that all three hunter values, L, a and b decreased with time at a given temperature. The initial tristimulus L, a, b values
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1
25
L
(1-f)
20 15 60°C 70°C 75°C 80°C 85°C 90°C 100°C
70°C 75°C
10
80°C 85°C
5
90°C 100°C
0
0.1 0
30
60
90
120
0
30
60
150
90
120
150
Time (min.)
Time (min.) Fig. 1. First-order beta-carotene degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (5).
Fig. 2. First-order color (La/b values) degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (10).
16
Table 2 Regression coefficients of Eq. (4)
14
R2
SE
70 75 80 85 90 100
0.0040 0.0043 0.0048 0.0059 0.0070 0.0080
0.9812 0.9923 0.9852 0.9921 0.9866 0.9624
0.0257 0.0172 0.0260 0.0238 0.0365 0.0682
for blanched pumpkin puree (2 min in 1% NaCl solution) were found to be 27.1328, 13.8695 and 14.0340. The sets of values obtained after 2 h of thermal treatment at 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C were 22.0855, 10.2222, 10.3112; 18.0232, 9.3412, 9.7512; 15.4079, 8.8888, 9.2010; 12.2494, 7.8315, 8.0000; 13.0022, 5.9963, 7.1945; 7.6592, 5.1234, 6.1321 and 5.1535, 3.4621, 5.4634 respectively. With the increase of heating time and temperature, pumpkin puree becomes darker. This corresponds to a decrease in the L-value of the color scale (Fig. 2). This is due to the degradation of thermo-labile pigments resulting in the formation of dark compounds that reduced luminosity. A similar behavior was found by Avila and Silva (1999) in peach puree. Pumpkin puree also loses its yellowness. This change is translated by a decrease in Ôb-valueÕ and Ôa-valueÕ (Figs. 3 and 4). This degradation with heat in pumpkin puree is due to the geometric isomerization of beta-carotene. Non-enzymatic browning (Maillard reaction) could also cause the degradation of color. Various combinations of Hunter Lab parameters were tried to describe the visual color change. These combinations were subjected to linear regression with respect to time as represented by Eq. (1). The combina-
12 10
b
k
8 60°C 70°C 75°C 80°C 85°C 90°C 100°C
6 4 2 0 0
30
60
90
120
150
Time (min.) Fig. 3. Thermal degradation of the L-value color parameter as a function of time and temperature.
16 14 12 10
a
Temperature (°C)
8 6
60°C 70°C 75°C 80°C 85°C 90°C 100°C
4 2 0 0
30
60
90
120
150
Time (min.) Fig. 4. Thermal degradation of the a-value color parameter as a function of time and temperature.
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
tion (La/b) was found to fit the first-order degradation kinetics. (Fig. 5) La/b has also been used earlier to express the color change of pureed products (Avila & Silva, 1999; Shin & Bhowmik, 1995). The fractional conversion (Eq. (3)) in terms of tristimulus La/b value can be written as ð9Þ
1
(1-f)
f ¼ ½ðL0 a0 =b0 Þ ðLa=bÞ=½ðL0 a0 =b0 Þ ðL/ a/ =b/ Þ
543
60ºC
Substituting f in Eq. (4),
70ºC 75ºC
ln½ðLa=bÞ ðL/ a/ =b/ Þ=½ðL0 a0 =b0 Þ ðL/ a/ =b/ Þ ¼ kt
80ºC
ð10Þ
85ºC 90ºC
La/b values with respect to time and represented by Eq. (10) were subjected to linear regression and the coefficients were determined (Table 3). R2 values were found to be greater than 0.97 and SE was less than 0.06. Total color difference (DE) of the puree was calculated from Hunter–Scotfield equation: (Avila & Silva, 1999) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 DE ¼ ðDaÞ þ ðDbÞ þ ðDLÞ ð11Þ
100ºC
0 0
30
60
90
120
150
Time (min.) Fig. 6. First-order color (TCD values) degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 80 °C, 90 °C and 100 °C (Eq. (11)).
The fractional conversion kinetics was also used to model the DE parameters (Fig. 6). DE Values were plotted at different time–temperature intervals (Fig. 7) and
30 60ºC 70ºC
25
75ºC 80ºC
20
85ºC 90ºC
ΔE
1
100° C
15 10 5
(1-f)
60ºC 70ºC
0
75ºC
0
80ºC
30
60
90
120
150
Time (min.)
85ºC
Fig. 7. Color difference evolution (DE) for pumpkin puree with time and temperature of treatment as represented by Eq. (11).
90ºC 100ºC
0.1 0
30
60
90
120
150
time (min.)
Fig. 5. Thermal degradation of the b-value color parameter as a function of time and temperature.
were found to be influenced by temperature and heating time. The maximum color change was observed at 100 °C with processing time being the same (2 h). 4.3. Effect of temperature on rate constant
Table 3 Regression coefficient of Eq. (10) Temperature (°C)
k
R2
SE
60 70 75 80 85 90 100
0.0017 0.0033 0.0042 0.0052 0.0057 0.0070 0.0085
0.9881 0.9931 0.9933 0.9937 0.9781 0.9865 0.9788
0.0085 0.0129 0.0157 0.0187 0.0381 0.0370 0.0556
Effect of temperature on the degradation rate constants of beta-carotene pigment and visual color (for both La/b and DE values) is shown in Fig. 8. Results indicated that the dependence of rate constant of both beta-carotene pigment and visual color (La/b and DE values) followed the Arrhenius equation (R2 > 0.9) (Eq. (2)). The computed activation energy for degradation of beta-carotene pigment and visual color parameters are given in Table 4. As seen from Table 4,
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in tomato pulp (Ea 22.200 kJ/mol) and paste (Ea = 20.200 kJ/mol). Higher activation energy signifies greater heat sensitivity. Thus, visual color (La/b) degradation during thermal processing is more heat sensitive than pigment degradation. Similar results were obtained by Shin and Bhowmik (1995) and Ahmed, Shivhare, et al. (2002b) while studying the kinetics of pea puree (Ea = 67.9 kJ/mol) and papaya puree (Ea = 48.07 kJ/ mol) respectively. Visual color (La/b) may, therefore be used for on-line quality control of pumpkin puree during thermal processing.
Degradation rate constant (min.-1)
0.01
β-carotene color(La/b) Color (ΔE)
Table 5 Coefficients of Eq. (12) 0.001 0.0027
0.0027
0.0028
0.0028
0.0029
0.0029
0.003
I/T (K) Fig. 8. Dependence of degradation rate constants for pigment and visual color of pumpkin puree on temperature as represented by Eq. (2).
Temperature (°C)
k1
k2
R2
SE
70 75 80
4.5732 5.8878 10.7956
2.1954 2.6602 3.1079
0.9614 0.9929 0.9912
0.7624 0.3871 0.5286
30
Table 4 Activation energy (kJ/mol) and frequency factor (min1) values for pumpkin puree k0
R2
27.2715 33.6831 30.3943
3.9956 6.1405 5.0684
0.9739 0.9781 0.9733
25
Yield stress (Pa)
Beta-carotene La/b DE
Activation energy
degradation of beta-carotene, was little influenced by temperature, that is, it has a lower energy of activation. Similar results were obtained by Lavelli and Giovanelli (2003) when they studied the beta-carotene degradation
20 15 10 5 0 40
50
60
70
80
90
100
110
Temp (°C) Fig. 10. Effect of temperature on yield stress of pumpkin puree.
25 3
20
70°C 75°C
15
80°C
10 5
Apparent viscosity (Pa.s)
Hunter color (La/b) values
30
2.5
60°C 70°C 75°C 80°C 85°C 90°C 100°C
2 1.5 1 0.5
0 5
7 9 11 β-carotene content (microgram/gram)
13 0 0
Fig. 9. Correlation between pigment and visual color of pumpkin puree at 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (12).
20
40
60
80
100
Shear rate (sec-1)
Fig. 11. Rheogram of pumpkin puree at various temperatures.
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
545
Table 6 Herschel–Bulkley and power law model coefficients for pumpkin puree against temperature Herschel–Bulkley coefficients
Power law coefficients
Temperature (°C)
Yield stress (Pa)
Consistency index, K (Pa sn)
Flow behavior index, n (dimensionless)
Consistency index, K (Pa sn)
Flow behavior index, n (dimensionless)
60 70 75 80 85 90 100
24.1756 18.3215 15.1391 11.2615 9.7331 7.0173 4.8314
1.9531 1.6315 1.3184 1.1131 0.9816 0.8153 0.6012
0.5831 0.6919 0.7252 0.7578 0.7717 0.7933 0.8016
9.3185 8.1834 7.7543 6.1370 5.8157 4.1938 3.4099
0.1970 0.2481 0.3872 0.4525 0.4916 0.5247 0.5814
4.4. Relationship between visual color and beta-carotene concentration
the Herschel–Bulkley and Power law model are given in Table 6.
Coloration is indicative of the specific carotenoids present and its concentration (Bjerkeng, 2000). In this study, change in objective color is correlated with the change in beta-carotene concentration of the pumpkin puree. Fig. 9 depicts a typical relationship between Hunter (La/b) value and the beta-carotene concentration (C) of pumpkin puree heated at 70 °C, 75 °C, and 80 °C. From the figure, it is evident that change in visual color is a direct manifestation of beta-carotene content. Thus, visual color may be used in place of beta-carotene content measurement during thermal processing of pumpkin puree. A linear equation (Eq. (12)) was found to describe the relationship between beta-carotene concentration and visual color
4.6. Effect of temperature on consistency index and apparent viscosity
La=b ¼ k 1 þ k 2 C
ð12Þ
where k1 and k2 are the coefficients of the model equation. The values of k1 and k2 evaluated from Fig. 9 are given in Table 5. 4.5. Rheological behavior of pumpkin puree Pumpkin puree exhibited yield stress, which decreased exponentially with temperature (Fig. 10). The values decreased from 24.1756 to 4.8314 Pa when the temperature was increased from 60 °C to 100 °C. At higher temperatures, due to rupture, the food structure becomes weak resulting in the lowering of yield stress (Steffe, 1992). The flow behavior index (n) was less than unity and increased with temperature from 0.5832 to 0.8012. This indicated that the puree behaved as a shear-thinning (pseudoplastic) fluid. Typical flow curves are shown in Fig. 11. From the figure, it is evident that the apparent viscosity (ga) was found to decrease with increased shear rate, which also proves its pseudoplastic or shear thinning nature. The yield stress values at selected temperatures were incorporated into the apparent viscosity value, and apparent viscosity-shear rate data fitted the Herschel–Bulkley model (Eq. (6)) adequately over the entire temperature range. The coefficients for
It is seen from Fig. 11 and Table 6 that the apparent viscosity (ga) and consistency index (K) decreased significantly whereas the flow behavior index value (n) increased with an increase in puree temperature. K Value decreased from 1.9531 to 0.6012 Pa sn (Table 6). The Arrhenius model (Eqs. (7) and (8)) gave a satisfactory description of the temperature dependence of apparent viscosity (at 100 rpm) and is in agreement with the consistency index of the Herschel–Bulkley model. The coefficients were computed using the least-square technique. Ega Value was found to be 13.3845 kJ/mol, while EK was computed to be 31.9394 kJ/mol respectively.
5. Conclusion Degradation of both beta-carotene and visual color during the thermal processing is found to follow first-order reaction kinetics and Arrhenius relationship for temperature dependence. The La/b and DE parameters were modeled using the concept of fractional conversion. The values proved to be good indicators of the total color change of heat-treated puree. Pumpkin puree exhibited pseudoplasticity and the rheological behavior fitted the Herschel–Bulkley model adequately. The K value decreased while the n values increased with the increase in temperature. The computed values of activation energy for betacarotene degradation was 27.2715 kJ/mol while the changes in color were 33.6831 kJ/mol and 30.3943 kJ/mol for La/b and DE values respectively. The activation energies for viscosity and consistency index were 13.3845 kJ/mol and 31.9395 kJ/mol respectively. All results were found to be significant till 95% confidence level. Since higher activation energy signifies greater heat sensitivity of visual color during thermal processing, it
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has been found that changes in visual color are a direct manifestation of change in beta-carotene values in pumpkin puree. Acknowledgement This research work is financially supported by the Departmental Special Assistance (DSA) programme under University Grant Commission (UGC), India. References Ahmed, J., Kaur, A., & Shivhare, U. S. (2002a). Color degradation kinetics of spinach, mustard leaves and mixed puree. Journal of Food Science, 67, 1088–1091. Ahmed, J., Shivhare, U. S., & Raghavan, G. S. V. (2000a). Rheological characterization and kinetics of color degradation of green chilli puree. Journal of Food Engineering, 44, 239–244. Ahmed, J., Shivhare, U. S., & Singh, G. (2000b). Chlorophyll and color of green chili puree as affected by mesh size and temperature. International Journal of Food Properties, 4, 305–316. Ahmed, J., Shivhare, U. S., & Sandhu, S. (2002b). Thermal degradation kinetics of carotenoids and visual color of papaya puree. Journal of Food Science, 67, 2692–2695. Avila, I. M. L. B., & Silva, C. L. M. (1999). Modeling kinetics of thermal degradation of color in peach puree. Journal of Food Engineering, 39, 161–166. Bjerkeng, B. (2000). Carotenoid pigmentation of salmonid fishes— recent progress. In: L. E. Cruz -Sua´rez, D. Ricque-Marie, M. Tapia-Salazar, M. A. Olvera-Novoa, & R. Civera-Cerecedo (Eds.). Avances en Nutricio´n Acuı´ cola V. Memorias del V Simposium Internacional de Nutricio´n Acuı´ cola. 19–22 Noviembre, 2000, Me´rida, Yucata´n. Guerra-Vargas, M., Jaramillo-Flores, M. E., Dorantes-Alvarez, L., & Hernandez-Sanchez, H. (2001). Beta-carotene retention in canned pickled Jalapeno Peppers and carrots as affected by sodium chloride, acetic acid and pasteurization. Journal of Food Science, 66(4), 620–626. Gunawan, M. I., & Barringer, S. A. (2000). Green color degradation of blanched broccoli (Brassica oleracea) due to acid and microbial growth. Journal of Food Processing and Preservation, 24, 253–263. Hayakawa, K., & Timbers, G. E. (1977). Influence of heat treatment on the quality of vegetables; changes in visual color. Journal of Food Science, 42, 778–791. Holden, M. (1970). Lipooxidase activity of leaves. Phytochemistry, 8, 2287. Howard, L. A., Wong, A. D., Perry, A. K., & Klein, B. P. (1999). bCarotene and ascorbic acid retention in fresh and processed vegetables. Journal of Food Science, 64(5), 929–936. Hunt, R. W. G. (1991). Measuring color (2nd ed., pp. 145–147). Chicester, UK: Ellis Horwood. Hutchings, J. B. (1994). Food color and appearance (pp. 220–256). London, UK: Blackie Academic and Professional Publication.
Klaui, H., & Bauernfeind, J. C. (1981). Beta-carotene as food color. In J. C. Bauernfeind (Ed.), Beta-carotene as colorants and vitamin A precursor (pp. 71–76). New York: Academic Press. Kramer, A., & Twigg, B. A. (1970). Quality control for the food industry (Vol. 1). Westport, Conn.: AVI Publishing Co. Lavelli, V., & Giovanelli, G. (2003). Evaluation of heat and oxidative damage during storage of processed tomato products II. Study of oxidative damage indices. Journal of the Science of Food and Agriculture, 83(9), 966–971. Levenspiel, O. (1974). Chemical reaction engineering (pp. 1–75). New Delhi, India: Wiley Eastern Publication. Lund, D. B. (1975). Effects of blanching, pasteurization and sterilization on nutrient. In R. S. Harris & E. Karmas (Eds.), Nutritional evaluation of food processing (pp. 205–240). New York: AVI Publishing. Marty, C., & Bersit, C. (1988). Degradation products of trans bcarotene produced during extrusion cooking. Journal of Food Science, 53, 1880–1886. Maskan, M. (2001). Kinetics of color change of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48, 169–175. Minguez-Mosquera, M. I., & Jaren-Galan, M. (1995). Kinetics of the decoloring of carotenoid pigments. Journal of the Science of Food and Agriculture, 67(2), 153–161. Nagle, B. J., Villalon, B., & Burns, E. E. (1979). Color evaluation of selected capsicums. Journal of Food Science, 44, 416–418. Rigg, B. (1987). Colorimetry and the CIE systems. In R. McDonald (Ed.), Color physics for industry (pp. 65–67). Bradford, UK: Society of Dyers and Colorists. Rodriguez-Amaya, D. B. (1999). Changes in beta-carotenes during processing and storage of foods. Archivos Latino Americanos De Nutrition, 49, 38S–47S. Shin, S., & Bhowmik, S. R. (1995). Thermal kinetics of color changes in pea puree. Journal of Food Engineering, 27, 77–86. Steet, J. A., & Tong, C. H. (1996). Degradation kinetics of green color and chlorophyll in peas by colorimetry and HPLC. Journal of Food Science, 61, 924–927, 931. Steffe, J. F. (1992). Rheological methods in food process engineering (pp. 9–30). Michigan, USA: Freeman Press. Teixeira, A. A., Dixon, J. R., Zahradnik, J. W., & Zinsmeister, G. E. (1969). Computer optimization of nutrient retention in the thermal processing of conduction—heated food. Food Technology, 23(6), 137–142. Tang, Y. C., & Chen, B. H. (2000). Pigment change of freeze dried carotenoid powder during storage. Food Chemistry, 69(1), 11–17. Vander Berg, H., Faulks, R., Fernando-Granado, H., Hirschberg, J., Olmedilla, B., Sandmann, G., Southon, S., & Stahl, W. (2000). The potential for the improvement of beta-carotene levels in foods and the likely systemic effects. Journal of the Science of Food and Agriculture, 80(7), 880–912. Vitali, A. A., & Rao, M. A. (1984). Flow properties of low pulp concentrated orange juice–serum viscosity and effect of pulp content. Journal of Food Science, 49, 876–881. Weemaes, C. A., Ooms, V., Loey, A. M., & Hendrickx, M. E. (1999). Kinetics of cholorophyll degradation and color loss in heated broccoli juice. Journal of Agricultural and Food Chemistry, 44, 2404–2409.
Rheological characteristics and thermal degradation kinetics of beta-carotene in pumpkin puree Debjani Dutta, Abhishek Dutta, Utpal Raychaudhuri, Runu Chakraborty
*
Department of Food Technology and Biochemical Engineering, Jadavpur University, Kolkata 700 032, India Received 25 August 2004; received in revised form 30 April 2005; accepted 24 May 2005 Available online 10 August 2005
Abstract The degradation kinetics of both the beta-carotene and visual color of pumpkin puree (blanched for 2 min in 1% NaCl solution) were determined at a temperature range of 60–100 °C for a time period varying between 0 and 2 h. An increase in the beta-carotene content was observed when the pumpkin puree was blanched and thermally treated at 60 °C. Using the concept of fractional conversion, it was observed that the degradation of both beta-carotene and visual color followed the first-order reaction kinetics. Dependence of the rate constants followed the Arrhenius relationship. The activation energy for beta-carotene was found to be 27.2715 kJ/mol and the activation energy for visual color using La/b and DE values was found to be 33.6831 kJ/mol and 30.3943 kJ/mol respectively. Higher activation energy signifies greater temperature sensitivity of visual color. The change in visual color was found to be a direct manifestation of the change in beta-carotene content. Rheological characteristics of the puree was also studied over the temperature range of 60–100 °C. Herschel–Bulkley model was found to fit adequately over the entire temperature range. Pumpkin puree exhibited yield stress, which decreased exponentially with temperature. With the increase in temperature, the puree was found to behave as a pseudoplastic fluid. Arrhenius model gave a satisfactory description of the temperature dependence of apparent viscosity. The activation energy for apparent viscosity and consistency index of pumpkin puree was found to be 13.3845 kJ/mol and 31.9394 kJ/mol respectively. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pumpkin puree; Beta-carotene; Fractional conversion; Thermal degradation; Activation energy; Flow index behavior
1. Introduction Thermal treatment is one of the most important methods of preservation of vegetables (Lund, 1975). The thermal processing of food is primarily intended to inactivate pathogens and other deteriorative microorganisms capable of making it unsuitable for consumption. Thermal processing also improves the bio-availability of beta-carotene, since it breaks down the cellulose structure
*
Corresponding author. Tel.: +91 33 2414 6663; fax: +91 33 24146822. E-mail address: [email protected] (R. Chakraborty). 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.05.056
of plant cell. Unfortunately, sensory properties of food including nutrients, color and texture deteriorate during the process. Kinetic models of thermal destruction are essential to design new processes assuming a safe food product and giving a maximum retention of quality factors (Lund, 1975; Teixeira, Dixon, Zahradnik, & Zinsmeister, 1969). Pumpkin puree is an intermediate product and is thermally processed for the manufacture of jam, jelly, sweets, beverages and other products. Retention of color, flavor and viscosity during thermal processing is some of the parameters that affect the success of a pureed product. Maintenance of these naturally colored pigments with desired textural and viscoelastic properties have been a major challenge in food processing
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industry. Various factors are responsible for the degradation of pigment and color during thermal processing of food products. Pumpkins contain beta-carotene which show a distinct color shift during thermal processing as heat induces cis–trans isomerization reaction (Klaui & Bauernfeind, 1981), oxidation to epoxy carotenoids and apocertenals (Rodriguez-Amaya, 1999) and even hydroxylation (Marty & Bersit, 1988). Visual color and pigment degradation kinetics of food products are complex phenomena and dependable models which can predict experimental color changes, which can be used in engineering calculations are limited (Ahmed, Shivhare, & Sandhu, 2002b). Models which can accurately predict the progress of a chemical reaction that occur in a homogeneous liquid or semi-solid state phase during thermal processing and storage are useful in many engineering applications, including process optimization. Degradation of beta-carotene by thermal treatment follow first order reaction kinetics (Ahmed, Shivhare, et al., 2002b; Lavelli & Giovanelli, 2003; Minguez-Mosquera & Jaren-Galan, 1995; Tang & Chen, 2000). The kinetic parameters like reaction order, rate constant and activation energy provide useful information on the quality changes that occur during thermal processing. Visual appearance is the foremost quality considered by consumers at the time of purchasing a product. Hence, excessive discoloration caused during thermal processing renders some foods unmarketable. Many processors utilize the psychological effect of color to market their products (Maskan, 2001). Several authors have studied color of food instrumentally (Ahmed, Shivhare, & Raghavan, 2000a; Gunawan & Barringer, 2000; Hunt, 1991; Nagle, Villalon, & Burns, 1979; Rigg, 1987; Shin & Bhowmik, 1995). Measurement of color by tristimulus colorimetry in terms of Hunter scale (L, a and b) value has been accepted as simple and accurate method (Hayakawa & Timbers, 1977) of color detection as compared to spectrophotometric systems. On the other hand, measurement of pigment could quantify the actual color degradation during processing. Hence, it is important to establish correlation between pigment concentration and visual color of food products during thermal processing. Rheological behavior of food depends on various factors like temperature, composition and total soluble solids content. Knowledge of the rheological properties of food is essential for product development, quality control (Kramer & Twigg, 1970), sensory evaluation and process engineering calculations. The flow behavior of a fluid can range from Newtonian to time-dependent non-Newtonian depending on its origin, structural behavior and history. Viscosity of fluid foods is affected by thermal processing. The effect of temperature on viscosity (or apparent viscosity) determined at a specific shear rate can be expressed by Arrhenius relationship.
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Nearly no information is available on beta-carotene and visual color degradation of pumpkin puree during thermal processing. The objective of this study was to determine the relation between the kinetics of thermal degradation of beta-carotene and visual color. It was then compared with the change in viscosity of the puree during heating at various temperatures. The aim was to correlate all these parameters for the optimization of online monitoring technique for better quality.
2. Materials and method 2.1. Raw material Fresh pumpkins were purchased from the local market of Kolkata, India. It was washed thoroughly and the skin was removed using a stainless steel knife. 2.2. Preparation of puree Skinless pumpkins were cut into small pieces and washed thoroughly after removing the seeds. The pumpkin pieces were then blanched in 1% NaCl solution at 100 °C for 2 min. They were cooled immediately and dried on a filter paper to remove the excess water. After passing through a pulper, the pumpkin pieces were sieved through a 14 mesh screen to obtain a product of uniform consistency. The puree was stored in sterile glass containers at 0 °C for further processing. 2.3. Physico-chemical properties Total soluble solids (°Brix) and pH of the puree were measured with a Bellingham Stanley refractometer (Model RFM-110, UK) and a digital pH meter (Thermo Orion, Model 420, USA) respectively. 2.4. Thermal treatment Thermal degradation kinetics was studied by isothermally heating the puree at pre-selected temperatures (60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C) for 0–2 h. The samples were sealed in glass vials (i.d. 1.5 cm, 8 ml) and immersed in a thermostatic water bath (for the temperature range 60–80 °C) and in an oil-bath for (90 °C and 100 °C) for preset time (0, 30, 60, 90 and 120 min) following the method described by Weemaes, Ooms, Loey, and Hendrickx (1999) and Ahmed, Shivhare, et al. (2002b). The oil was continuously stirred by means of a stirrer fitted with the bath. An oil-recirculating pump was used to facilitate heat transfer and uniform heating through out the bath. At the specified time intervals, the samples were withdrawn from the hot bath and cooled in an ice-water mixture.
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2.5. Measurement of beta-carotene
lnðC=C 0 Þ ¼ kt
The estimation of beta-carotene was done after extraction of the sample with diacetone alcohol and petroleum ether and further purification with diacetone alcohol, methanolic KOH and distilled water. The resulting solution was filtered with anhydrous sodium sulphate and read on a spectrophotometer (Hitachi, U-2000, Japan) at 450-nm against petroleum ether as a blank. A standard graph was plotted using synthetic crystalline beta-carotene (Fluka, Germany) dissolved in petroleum ether and its optical density measured at 450 nm. From the standard graph, the concentration in microgram per ml was determined and this was used for the calculation of beta-carotene in the sample. All the analytical work were repeated three times.
where C0 = initial concentration of beta-carotene (lg/g) or measured Hunter color value (L, a, b) or a combination of these at time zero (dimensionless); C = concentration of beta-carotene (lg/g) or measured Hunter color value (L, a, b) or a combination of these at time t (dimensionless); k = temperature dependent rate constant (min1); t = heating time (min). For a reaction following a first-order kinetic model, the plot of ln(C/C0) vs time would be a straight line and the slope would be equal to k at a constant temperature. Dependence of the degradation rate constant on temperature can be represented by the Arrhenius equation:
2.6. Measurement of color Visual color was measured using a HunterLab Color Measurement System Model Color Flex 45/0 (Hunter Associates Laboratory Inc., USA) in terms of universally accepted Hunter Lab color scale. L Value signifies ‘‘lightness’’ (100 for white and 0 for black), a represents changes from ‘‘greenness to redness’’ (80 for green and 100 for red) and b from ‘‘blueness to yellowness’’ (80 for blue and 70 for yellow). The instrument (10° observer, IlluminantD-65) was calibrated against a standard white reference tile.
where k is the rate constant at temperature T (Kelvin), k0 is the pre-exponential factor, Ea is the activation energy (kJ/mol) and R is the gas constant (8.314 J/mol K). Therefore, if the temperature dependence follows ArrheniusÕ relationship, the plot of ln k vs 1/T would be a straight line and the slope equal to Ea/R. Fractional conversion is a convenient variable often used in place of concentration (Levenspiel, 1974) and has been reported to increase the accuracy of the calculation. For an irreversible first order reaction kinetics, the rate constant at constant temperature can be determined through fractional conversion, f:
2.7. Rheological measurement
f ¼ ðC 0 CÞ=ðC 0 C / Þ
Rheological measurements (shear stress and shear rate) of the puree at various temperatures were done in a rotational viscometer (Brookfield R/S–CC25 Rheometer, Middleboro, MA, USA) equipped with a Coaxial Cylinder Measuring Systems. Approximately 17 ml of puree was placed in the concentric cylindrical cup. The sample compartment was monitored at a constant temperature using a water bath. The viscometer was operated between 10 and 100 rpm and the shear stress readings were obtained directly from the instrument.
3. Kinetic analysis 3.1. Model for pigment and color analysis Degradation kinetics of both pigment and color has been found to follow first-order reaction kinetics as have been seen in case of Ahmed, Kaur, and Shivhare (2002a); Ahmed, Shivhare, et al. (2002b); Ahmed, Shivhare, and Raghavan (2000a, 2000b); Gunawan and Barringer (2000); Hutchings (1994); Steet and Tong (1996); Weemaes et al. (1999). The first-order kinetic model can be represented as,
k ¼ k 0 expðEa =RT Þ
ð1Þ
ð2Þ
ð3Þ
where C/ = measured pigment or color values at infinite time when the reaction is expected to be complete. Since, C/ for an irreversible reaction would be 0, Eq. (1), can be expressed as ln C=C 0 ¼ lnð1 f Þ ¼ kt
ð4Þ
The infinite values of both concentrations for pigment and for color were determined following the methods of Steet and Tong (1996) and Weemaes et al. (1999). Pumpkin puree was acidified using concentrated HCl and also thermally treated at selected temperatures for a long period of time (24 h) and the corresponding values were measured. It is assumed that color/pigment degradation at a constant temperature is nearly constant with respect to time on prolonged heating. This nonzero residue should be independent of reaction temperature and reaction path (Steet & Tong, 1996). 3.2. Flow models The power law model with or without yield term has been employed to describe the flow behavior of viscous food over wide ranges of shear rates (Vitali & Rao, 1984).
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546 n
s ¼ KðcÞ and s s0 ¼ K H ðcÞnH
ð5Þ ð6Þ
where s0 is the yield stress (Pa), K, KH is the consistency index (Pa sn/) and n, nH is the flow behavior index (dimensionless). s and c represent shear stress and shear rate respectively. The rheological model that has been generally used for non-Newtonian fluids, especially purees/pastes is the Herschel–Bulkley model (Eq. (7)). Although the power law has been employed extensively for characterizing foods, including shear-thinning (pseudoplastic) foods, it fails to predict the flow behavior at very low shear rates (zero shear viscosity). The temperature dependence of the apparent viscosity at constant shear rate and consistency index can be described by the Arrhenius relationship: ga ¼ Aga expðEga =RT a Þ
ð7Þ
K ¼ AK expðEK =RT a Þ
ð8Þ
where ga is the apparent viscosity (centipoise), Aga and AK are frequency factors for apparent viscosity at constant rpm and consistency index respectively. Ega and EK are activation energy for apparent viscosity at constant rpm (kJ/mol) and consistency index (kJ/mol) respectively. Ta is the absolute temperature (K) and R is the universal gas constant (J/mol K). Experiments were replicated thrice and the average values were used in the analysis. 3.3. Statistical analysis All the tests were done in triplicate and the software, Statistical Package for Social Science Research (SPSS, release 7.5., version 1, 1996) was used for statistical assessment. Significance of difference was defined at p 6 0.05.
4. Results and discussions The pH and total soluble solids (TSS) content of the pumpkin puree were found to be 4.35 and 7.2 ± 0.3 °Brix respectively. 4.1. Kinetics of pigment degradation From Table 1, it is evident that blanched pumpkin has a higher beta-carotene content than the unblanched one. This is because, blanching results in inactivation of the enzyme lipoxygenase, which can co-oxidize beta-carotene and degrade the pigment to a colorless product (Holden, 1970). Blanching is also said to remove the intracellular air, thus establishing a continuous liquid phase. The liquid phase further protects the beta-carotene from degradation.
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Table 1 Beta-carotene content of blanched, unblanched and thermally treated pumpkin at 60 °C Conditions of treatment
Beta-carotene (lg/g)
Unblanched pumpkin puree Blanched for 2 min in 1% NaCl solution Blanched for 2 min in 1% NaCl solution and thermally treated at 60 °C (for 2 h)
10.9426 12.4569 14.0531
However, thermal treatment at 60 °C leads to a further increase in the beta-carotene content. (Table 1). Thermal processing has been reported to increase the beta-carotene concentration, perhaps because of greater chemical extractability and loss of moisture due to which soluble solids concentrate the sample (GuerraVargas, Jaramillo-Flores, Dorantes-Alvarez, & Hernandez-Sanchez, 2001). Heat treatment also inactivates some oxidative enzymes and breaks some structure leading to a higher bioavailability of beta-carotene (Howard, Wong, Perry, & Klein, 1999; Vander Berg et al., 2000). The beta-carotene content of pumpkin at temperatures 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C after 2 h of thermal treatment were experimentally determined to be 8.1499, 7.7087, 7.2488, 5.8962, 4.8653 and 4.1975 lg/g respectively whereas the value at 60 °C was 14.0531 lg/g (Table 1). Thus, it is seen that above 60 °C, beta-carotene content of the pumpkin puree decreased during the thermal treatment. However, the degradation of beta-carotene pigment at 60 °C did not follow the fractional conversion kinetics (Eq. (4)). The equilibrium beta-carotene value when the puree was acidified in concentrated hydrochloric acid and thermally processed for a prolonged time (24 h) was measured as 3.0520 lg/g. The thermal degradation of beta-carotene with increase in temperature was due to oxidative and non-oxidative changes such as cis–trans isomerization and epoxide formation. The fractional conversion kinetics (Eq. (4)) was used to model the thermal degradation of beta-carotene pigment. The rate of degradation of beta-carotene in pumpkin puree was determined by linear regression of ln(1 f) against heating time (Fig. 1). It is evident from the figure that the degradation of pumpkin puree followed the first order reaction kinetics. The regression coefficients of beta-carotene degradation values with respect to time (Fig. 1) and as represented by Eq. (4) are given in Table 2. Values of R2 was greater than 0.96 for all temperatures and the standard error (SE) was less than 0.07. 4.2. Visual color degradation kinetics During thermal processing, it was observed that all three hunter values, L, a and b decreased with time at a given temperature. The initial tristimulus L, a, b values
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1
25
L
(1-f)
20 15 60°C 70°C 75°C 80°C 85°C 90°C 100°C
70°C 75°C
10
80°C 85°C
5
90°C 100°C
0
0.1 0
30
60
90
120
0
30
60
150
90
120
150
Time (min.)
Time (min.) Fig. 1. First-order beta-carotene degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (5).
Fig. 2. First-order color (La/b values) degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (10).
16
Table 2 Regression coefficients of Eq. (4)
14
R2
SE
70 75 80 85 90 100
0.0040 0.0043 0.0048 0.0059 0.0070 0.0080
0.9812 0.9923 0.9852 0.9921 0.9866 0.9624
0.0257 0.0172 0.0260 0.0238 0.0365 0.0682
for blanched pumpkin puree (2 min in 1% NaCl solution) were found to be 27.1328, 13.8695 and 14.0340. The sets of values obtained after 2 h of thermal treatment at 60 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C were 22.0855, 10.2222, 10.3112; 18.0232, 9.3412, 9.7512; 15.4079, 8.8888, 9.2010; 12.2494, 7.8315, 8.0000; 13.0022, 5.9963, 7.1945; 7.6592, 5.1234, 6.1321 and 5.1535, 3.4621, 5.4634 respectively. With the increase of heating time and temperature, pumpkin puree becomes darker. This corresponds to a decrease in the L-value of the color scale (Fig. 2). This is due to the degradation of thermo-labile pigments resulting in the formation of dark compounds that reduced luminosity. A similar behavior was found by Avila and Silva (1999) in peach puree. Pumpkin puree also loses its yellowness. This change is translated by a decrease in Ôb-valueÕ and Ôa-valueÕ (Figs. 3 and 4). This degradation with heat in pumpkin puree is due to the geometric isomerization of beta-carotene. Non-enzymatic browning (Maillard reaction) could also cause the degradation of color. Various combinations of Hunter Lab parameters were tried to describe the visual color change. These combinations were subjected to linear regression with respect to time as represented by Eq. (1). The combina-
12 10
b
k
8 60°C 70°C 75°C 80°C 85°C 90°C 100°C
6 4 2 0 0
30
60
90
120
150
Time (min.) Fig. 3. Thermal degradation of the L-value color parameter as a function of time and temperature.
16 14 12 10
a
Temperature (°C)
8 6
60°C 70°C 75°C 80°C 85°C 90°C 100°C
4 2 0 0
30
60
90
120
150
Time (min.) Fig. 4. Thermal degradation of the a-value color parameter as a function of time and temperature.
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tion (La/b) was found to fit the first-order degradation kinetics. (Fig. 5) La/b has also been used earlier to express the color change of pureed products (Avila & Silva, 1999; Shin & Bhowmik, 1995). The fractional conversion (Eq. (3)) in terms of tristimulus La/b value can be written as ð9Þ
1
(1-f)
f ¼ ½ðL0 a0 =b0 Þ ðLa=bÞ=½ðL0 a0 =b0 Þ ðL/ a/ =b/ Þ
543
60ºC
Substituting f in Eq. (4),
70ºC 75ºC
ln½ðLa=bÞ ðL/ a/ =b/ Þ=½ðL0 a0 =b0 Þ ðL/ a/ =b/ Þ ¼ kt
80ºC
ð10Þ
85ºC 90ºC
La/b values with respect to time and represented by Eq. (10) were subjected to linear regression and the coefficients were determined (Table 3). R2 values were found to be greater than 0.97 and SE was less than 0.06. Total color difference (DE) of the puree was calculated from Hunter–Scotfield equation: (Avila & Silva, 1999) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 DE ¼ ðDaÞ þ ðDbÞ þ ðDLÞ ð11Þ
100ºC
0 0
30
60
90
120
150
Time (min.) Fig. 6. First-order color (TCD values) degradation kinetics of pumpkin puree at selected temperatures of 60 °C, 70 °C, 80 °C, 90 °C and 100 °C (Eq. (11)).
The fractional conversion kinetics was also used to model the DE parameters (Fig. 6). DE Values were plotted at different time–temperature intervals (Fig. 7) and
30 60ºC 70ºC
25
75ºC 80ºC
20
85ºC 90ºC
ΔE
1
100° C
15 10 5
(1-f)
60ºC 70ºC
0
75ºC
0
80ºC
30
60
90
120
150
Time (min.)
85ºC
Fig. 7. Color difference evolution (DE) for pumpkin puree with time and temperature of treatment as represented by Eq. (11).
90ºC 100ºC
0.1 0
30
60
90
120
150
time (min.)
Fig. 5. Thermal degradation of the b-value color parameter as a function of time and temperature.
were found to be influenced by temperature and heating time. The maximum color change was observed at 100 °C with processing time being the same (2 h). 4.3. Effect of temperature on rate constant
Table 3 Regression coefficient of Eq. (10) Temperature (°C)
k
R2
SE
60 70 75 80 85 90 100
0.0017 0.0033 0.0042 0.0052 0.0057 0.0070 0.0085
0.9881 0.9931 0.9933 0.9937 0.9781 0.9865 0.9788
0.0085 0.0129 0.0157 0.0187 0.0381 0.0370 0.0556
Effect of temperature on the degradation rate constants of beta-carotene pigment and visual color (for both La/b and DE values) is shown in Fig. 8. Results indicated that the dependence of rate constant of both beta-carotene pigment and visual color (La/b and DE values) followed the Arrhenius equation (R2 > 0.9) (Eq. (2)). The computed activation energy for degradation of beta-carotene pigment and visual color parameters are given in Table 4. As seen from Table 4,
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in tomato pulp (Ea 22.200 kJ/mol) and paste (Ea = 20.200 kJ/mol). Higher activation energy signifies greater heat sensitivity. Thus, visual color (La/b) degradation during thermal processing is more heat sensitive than pigment degradation. Similar results were obtained by Shin and Bhowmik (1995) and Ahmed, Shivhare, et al. (2002b) while studying the kinetics of pea puree (Ea = 67.9 kJ/mol) and papaya puree (Ea = 48.07 kJ/ mol) respectively. Visual color (La/b) may, therefore be used for on-line quality control of pumpkin puree during thermal processing.
Degradation rate constant (min.-1)
0.01
β-carotene color(La/b) Color (ΔE)
Table 5 Coefficients of Eq. (12) 0.001 0.0027
0.0027
0.0028
0.0028
0.0029
0.0029
0.003
I/T (K) Fig. 8. Dependence of degradation rate constants for pigment and visual color of pumpkin puree on temperature as represented by Eq. (2).
Temperature (°C)
k1
k2
R2
SE
70 75 80
4.5732 5.8878 10.7956
2.1954 2.6602 3.1079
0.9614 0.9929 0.9912
0.7624 0.3871 0.5286
30
Table 4 Activation energy (kJ/mol) and frequency factor (min1) values for pumpkin puree k0
R2
27.2715 33.6831 30.3943
3.9956 6.1405 5.0684
0.9739 0.9781 0.9733
25
Yield stress (Pa)
Beta-carotene La/b DE
Activation energy
degradation of beta-carotene, was little influenced by temperature, that is, it has a lower energy of activation. Similar results were obtained by Lavelli and Giovanelli (2003) when they studied the beta-carotene degradation
20 15 10 5 0 40
50
60
70
80
90
100
110
Temp (°C) Fig. 10. Effect of temperature on yield stress of pumpkin puree.
25 3
20
70°C 75°C
15
80°C
10 5
Apparent viscosity (Pa.s)
Hunter color (La/b) values
30
2.5
60°C 70°C 75°C 80°C 85°C 90°C 100°C
2 1.5 1 0.5
0 5
7 9 11 β-carotene content (microgram/gram)
13 0 0
Fig. 9. Correlation between pigment and visual color of pumpkin puree at 70 °C, 75 °C, 80 °C, 85 °C, 90 °C and 100 °C as represented by Eq. (12).
20
40
60
80
100
Shear rate (sec-1)
Fig. 11. Rheogram of pumpkin puree at various temperatures.
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Table 6 Herschel–Bulkley and power law model coefficients for pumpkin puree against temperature Herschel–Bulkley coefficients
Power law coefficients
Temperature (°C)
Yield stress (Pa)
Consistency index, K (Pa sn)
Flow behavior index, n (dimensionless)
Consistency index, K (Pa sn)
Flow behavior index, n (dimensionless)
60 70 75 80 85 90 100
24.1756 18.3215 15.1391 11.2615 9.7331 7.0173 4.8314
1.9531 1.6315 1.3184 1.1131 0.9816 0.8153 0.6012
0.5831 0.6919 0.7252 0.7578 0.7717 0.7933 0.8016
9.3185 8.1834 7.7543 6.1370 5.8157 4.1938 3.4099
0.1970 0.2481 0.3872 0.4525 0.4916 0.5247 0.5814
4.4. Relationship between visual color and beta-carotene concentration
the Herschel–Bulkley and Power law model are given in Table 6.
Coloration is indicative of the specific carotenoids present and its concentration (Bjerkeng, 2000). In this study, change in objective color is correlated with the change in beta-carotene concentration of the pumpkin puree. Fig. 9 depicts a typical relationship between Hunter (La/b) value and the beta-carotene concentration (C) of pumpkin puree heated at 70 °C, 75 °C, and 80 °C. From the figure, it is evident that change in visual color is a direct manifestation of beta-carotene content. Thus, visual color may be used in place of beta-carotene content measurement during thermal processing of pumpkin puree. A linear equation (Eq. (12)) was found to describe the relationship between beta-carotene concentration and visual color
4.6. Effect of temperature on consistency index and apparent viscosity
La=b ¼ k 1 þ k 2 C
ð12Þ
where k1 and k2 are the coefficients of the model equation. The values of k1 and k2 evaluated from Fig. 9 are given in Table 5. 4.5. Rheological behavior of pumpkin puree Pumpkin puree exhibited yield stress, which decreased exponentially with temperature (Fig. 10). The values decreased from 24.1756 to 4.8314 Pa when the temperature was increased from 60 °C to 100 °C. At higher temperatures, due to rupture, the food structure becomes weak resulting in the lowering of yield stress (Steffe, 1992). The flow behavior index (n) was less than unity and increased with temperature from 0.5832 to 0.8012. This indicated that the puree behaved as a shear-thinning (pseudoplastic) fluid. Typical flow curves are shown in Fig. 11. From the figure, it is evident that the apparent viscosity (ga) was found to decrease with increased shear rate, which also proves its pseudoplastic or shear thinning nature. The yield stress values at selected temperatures were incorporated into the apparent viscosity value, and apparent viscosity-shear rate data fitted the Herschel–Bulkley model (Eq. (6)) adequately over the entire temperature range. The coefficients for
It is seen from Fig. 11 and Table 6 that the apparent viscosity (ga) and consistency index (K) decreased significantly whereas the flow behavior index value (n) increased with an increase in puree temperature. K Value decreased from 1.9531 to 0.6012 Pa sn (Table 6). The Arrhenius model (Eqs. (7) and (8)) gave a satisfactory description of the temperature dependence of apparent viscosity (at 100 rpm) and is in agreement with the consistency index of the Herschel–Bulkley model. The coefficients were computed using the least-square technique. Ega Value was found to be 13.3845 kJ/mol, while EK was computed to be 31.9394 kJ/mol respectively.
5. Conclusion Degradation of both beta-carotene and visual color during the thermal processing is found to follow first-order reaction kinetics and Arrhenius relationship for temperature dependence. The La/b and DE parameters were modeled using the concept of fractional conversion. The values proved to be good indicators of the total color change of heat-treated puree. Pumpkin puree exhibited pseudoplasticity and the rheological behavior fitted the Herschel–Bulkley model adequately. The K value decreased while the n values increased with the increase in temperature. The computed values of activation energy for betacarotene degradation was 27.2715 kJ/mol while the changes in color were 33.6831 kJ/mol and 30.3943 kJ/mol for La/b and DE values respectively. The activation energies for viscosity and consistency index were 13.3845 kJ/mol and 31.9395 kJ/mol respectively. All results were found to be significant till 95% confidence level. Since higher activation energy signifies greater heat sensitivity of visual color during thermal processing, it
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has been found that changes in visual color are a direct manifestation of change in beta-carotene values in pumpkin puree. Acknowledgement This research work is financially supported by the Departmental Special Assistance (DSA) programme under University Grant Commission (UGC), India. References Ahmed, J., Kaur, A., & Shivhare, U. S. (2002a). Color degradation kinetics of spinach, mustard leaves and mixed puree. Journal of Food Science, 67, 1088–1091. Ahmed, J., Shivhare, U. S., & Raghavan, G. S. V. (2000a). Rheological characterization and kinetics of color degradation of green chilli puree. Journal of Food Engineering, 44, 239–244. Ahmed, J., Shivhare, U. S., & Singh, G. (2000b). Chlorophyll and color of green chili puree as affected by mesh size and temperature. International Journal of Food Properties, 4, 305–316. Ahmed, J., Shivhare, U. S., & Sandhu, S. (2002b). Thermal degradation kinetics of carotenoids and visual color of papaya puree. Journal of Food Science, 67, 2692–2695. Avila, I. M. L. B., & Silva, C. L. M. (1999). Modeling kinetics of thermal degradation of color in peach puree. Journal of Food Engineering, 39, 161–166. Bjerkeng, B. (2000). Carotenoid pigmentation of salmonid fishes— recent progress. In: L. E. Cruz -Sua´rez, D. Ricque-Marie, M. Tapia-Salazar, M. A. Olvera-Novoa, & R. Civera-Cerecedo (Eds.). Avances en Nutricio´n Acuı´ cola V. Memorias del V Simposium Internacional de Nutricio´n Acuı´ cola. 19–22 Noviembre, 2000, Me´rida, Yucata´n. Guerra-Vargas, M., Jaramillo-Flores, M. E., Dorantes-Alvarez, L., & Hernandez-Sanchez, H. (2001). Beta-carotene retention in canned pickled Jalapeno Peppers and carrots as affected by sodium chloride, acetic acid and pasteurization. Journal of Food Science, 66(4), 620–626. Gunawan, M. I., & Barringer, S. A. (2000). Green color degradation of blanched broccoli (Brassica oleracea) due to acid and microbial growth. Journal of Food Processing and Preservation, 24, 253–263. Hayakawa, K., & Timbers, G. E. (1977). Influence of heat treatment on the quality of vegetables; changes in visual color. Journal of Food Science, 42, 778–791. Holden, M. (1970). Lipooxidase activity of leaves. Phytochemistry, 8, 2287. Howard, L. A., Wong, A. D., Perry, A. K., & Klein, B. P. (1999). bCarotene and ascorbic acid retention in fresh and processed vegetables. Journal of Food Science, 64(5), 929–936. Hunt, R. W. G. (1991). Measuring color (2nd ed., pp. 145–147). Chicester, UK: Ellis Horwood. Hutchings, J. B. (1994). Food color and appearance (pp. 220–256). London, UK: Blackie Academic and Professional Publication.
Klaui, H., & Bauernfeind, J. C. (1981). Beta-carotene as food color. In J. C. Bauernfeind (Ed.), Beta-carotene as colorants and vitamin A precursor (pp. 71–76). New York: Academic Press. Kramer, A., & Twigg, B. A. (1970). Quality control for the food industry (Vol. 1). Westport, Conn.: AVI Publishing Co. Lavelli, V., & Giovanelli, G. (2003). Evaluation of heat and oxidative damage during storage of processed tomato products II. Study of oxidative damage indices. Journal of the Science of Food and Agriculture, 83(9), 966–971. Levenspiel, O. (1974). Chemical reaction engineering (pp. 1–75). New Delhi, India: Wiley Eastern Publication. Lund, D. B. (1975). Effects of blanching, pasteurization and sterilization on nutrient. In R. S. Harris & E. Karmas (Eds.), Nutritional evaluation of food processing (pp. 205–240). New York: AVI Publishing. Marty, C., & Bersit, C. (1988). Degradation products of trans bcarotene produced during extrusion cooking. Journal of Food Science, 53, 1880–1886. Maskan, M. (2001). Kinetics of color change of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48, 169–175. Minguez-Mosquera, M. I., & Jaren-Galan, M. (1995). Kinetics of the decoloring of carotenoid pigments. Journal of the Science of Food and Agriculture, 67(2), 153–161. Nagle, B. J., Villalon, B., & Burns, E. E. (1979). Color evaluation of selected capsicums. Journal of Food Science, 44, 416–418. Rigg, B. (1987). Colorimetry and the CIE systems. In R. McDonald (Ed.), Color physics for industry (pp. 65–67). Bradford, UK: Society of Dyers and Colorists. Rodriguez-Amaya, D. B. (1999). Changes in beta-carotenes during processing and storage of foods. Archivos Latino Americanos De Nutrition, 49, 38S–47S. Shin, S., & Bhowmik, S. R. (1995). Thermal kinetics of color changes in pea puree. Journal of Food Engineering, 27, 77–86. Steet, J. A., & Tong, C. H. (1996). Degradation kinetics of green color and chlorophyll in peas by colorimetry and HPLC. Journal of Food Science, 61, 924–927, 931. Steffe, J. F. (1992). Rheological methods in food process engineering (pp. 9–30). Michigan, USA: Freeman Press. Teixeira, A. A., Dixon, J. R., Zahradnik, J. W., & Zinsmeister, G. E. (1969). Computer optimization of nutrient retention in the thermal processing of conduction—heated food. Food Technology, 23(6), 137–142. Tang, Y. C., & Chen, B. H. (2000). Pigment change of freeze dried carotenoid powder during storage. Food Chemistry, 69(1), 11–17. Vander Berg, H., Faulks, R., Fernando-Granado, H., Hirschberg, J., Olmedilla, B., Sandmann, G., Southon, S., & Stahl, W. (2000). The potential for the improvement of beta-carotene levels in foods and the likely systemic effects. Journal of the Science of Food and Agriculture, 80(7), 880–912. Vitali, A. A., & Rao, M. A. (1984). Flow properties of low pulp concentrated orange juice–serum viscosity and effect of pulp content. Journal of Food Science, 49, 876–881. Weemaes, C. A., Ooms, V., Loey, A. M., & Hendrickx, M. E. (1999). Kinetics of cholorophyll degradation and color loss in heated broccoli juice. Journal of Agricultural and Food Chemistry, 44, 2404–2409.