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
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
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.
539
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.
540
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
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
K ¼ AK expðEK =RT a Þ
ð7Þ
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.
541
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
542
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
30
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
ð11Þ
DE ¼ ðDaÞ þ ðDbÞ þ ðDLÞ
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,
544
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
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
546
D. Dutta et al. / Journal of Food Engineering 76 (2006) 538–546
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.
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