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A study on the degradation kinetics of visual green colour in spinach (Spinacea oleracea L.) and the effect of salt therein
Journal of Food Engineering 64 (2004) 135–142 www.elsevier.com/locate/jfoodeng
Research note
A study on the degradation kinetics of visual green colour in spinach (Spinacea oleracea L.) and the effect of salt therein P. Nisha a, R.S. Singhal
a,*
, A.B. Pandit
b
a
b
Department of Food and Fermentation Technology, Institute of Chemical Technology, University of Mumbai, Matunga 400 019, Mumbai, India Department of Chemical Engineering, Institute of Chemical Technology, University of Mumbai, Matunga 400 019, Mumbai, India Received 26 May 2003; accepted 13 September 2003
Abstract The effect of salt on the degradation of visual green colour Ôa’ in spinach puree (Spinacea oleracea L.) over a temperature range of 50–120 C (steady state temperature process) as well as under conditions of normal open pan cooking, pressure-cooking and a newly developed and patented fuel-efficient EcoCooker has been studied (unsteady state heating process). The degradation of visual green colour as measured by Ôa’ value followed a first order kinetics, where the rate constant increased with an increase in the temperature. The temperature dependence of degradation was adequately modeled by Arrhenius equation. A mathematical model has been developed using the steady state kinetic parameters obtained to predict the losses of green colour from the time–temperature data of the unsteady state heating/processing method. The results obtained indicate a protective effect of salt on the degradation of visual green colour. 2003 Elsevier Ltd. All rights reserved. Keywords: Colour degradation; Kinetics; Spinach; Effect of salt; Cookers
1. Introduction Colour plays an important role in visual recognition and assessment of the surface and the subsurface properties of the object. It has a great influence on the appearance, processing and acceptance of food materials. The degree of greenness, attributed to chlorophyll pigments, is important in determining the final quality of thermally processed green vegetables. Thermally processed green vegetables exhibit poor colour quality as compared to the fresh ones. The colour change from bright green to olive brown occurring during the processing is mainly due to the degradation of chlorophyll to pheophytin by the replacement of magnesium in the chlorophyll by hydrogen and further formation of the degradation products such as pheophorbides and chlorins (Canjura, Schwartz, & Nunes, 1991; Schwartz & Lorenzo, 1991; Schwartz & Von Elbe, 1983; White, Jones, & Gibbes, 1963). More over the conversion of chlorophyll to pheophytin is regarded as an index of the severity of the processing (Woolfe, 1979). *
Corresponding author. Fax: +91-22-2414-5614. E-mail address: [email protected] (R.S. Singhal).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.09.021
Factors such as pH, temperature, presence of salts, enzymes and surface-active ions influence the stability of chlorophyll. Chlorophyll has better stability at higher pH than at lower pH (Ryan-Stoneham & Tong, 2000; Sweeney & Martin, 1961). Salts of magnesium, calcium, sodium, ammonium and some surface-active agents are known to have some stabilizing effect on chlorophyll degradation (Eheart & Odland, 1973; Haisman & Clarke, 1975; Woolfe, 1979). Accurate knowledge of the kinetic parameters, rate constant and activation energy is essential to predict the quality changes that occur during thermal processing. Numerous researchers have evaluated the kinetics of pigments and colour degradation in fruits and vegetables such as broccoli (Weemas, Ooms, Van Loey, & Hendrickx, 1999), peas (Ryan-Stoneham & Tong, 2000; Steet & Tong, 1996), spinach (Canjura et al., 1991; Gupte, El-Bisi, & Francis, 1963; Schwartz & Von Elbe, 1983) during thermal processing and found it to follow first order reaction kinetics. It is believed that colour vision is a complex phenomenon and its measurement is much more complex than the absorption by the stimuli pigments at specific wavelengths (Govindarajan, Rajalakshmi, & Chand,
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1998). Furthermore, the spectrophotometric methods have some limitations for the colour measurement of thermally processed foods. Thermally decomposed pigments like pheophytins in the case of chlorophyll, cause interference during pigment concentration measurements (Eheart & Gott, 1965). There are several reports on the measurement of colour using instruments. Such instruments are usually based on the principle that a colour can be mathematically described as a combination of the three primary colour intensities (Clydesdale, 1978). Tristimulus colourimeter has been widely accepted as a rapid and simple instrumental method for measuring visual perception of the food products (Clydesdale, 1991; Gnanasekharan, Shewflet, & Chinnan, 1992; Hayakawa, 1977; Ozkan, Yemenicioglu, Asefi, & Cemeroglu, 2002). The colour of any food product can be represented in terms of L, a, and b values or combination of these three depending upon the nature of the pigment present in the food material. A study conducted by Weemas et al. (1999) on the kinetics of chlorophyll degradation and colour loss in heated broccoli juice shows the advantage of objective colour measurements for studying the chlorophyll degradation as compared to conventional chemical analysis and from the point of view of the consumers the green colour is more important than the residual chlorophyll content. Spinach is one of the important leafy vegetables consumed all over the world. It is consumed as fresh, pureed or processed. During thermal processing, colour changes from bright green to olive green due to the conversion of chlorophyll to pheophytin and pyropheophytin. It is a common belief that common salt (sodium chloride) has a stabilizing effect on green colour (Haisman & Clarke, 1975; Hudson, Sharples, & Gregory, 1974). However, the stabilization effect of salt is not well established and no information is available on the effect of salt on colour degradation of these products, especially in terms of visual colour. Therefore, present study was undertaken: (1) To find out the effect of salt (1% and 2% by weight) on colour degradation in spinach (Spinacea oleracea L.), a model food system, over a temperature range of 50–120 C (steady state temperature) and to determine kinetic parameters for colour degradation in spinach (S. oleracea L.). (2) To study the degradation kinetics of colour in different cooking methods (unsteady state process). (3) To develop a mathematical model relating the calculated kinetic data from the steady state temperature and the time–temperature profiles of different cooking methods (unsteady state process). (4) To apply this model to predict the green colour degradation for unsteady state heating process, from the time–temperature data of the unsteady state heating process and comparing it with the actual degradation values, which could then be used to assess the changes in the colour quality as a function of method of cooking.
2. Materials and methods Fresh green spinach leaves were obtained locally, washed thoroughly and drained. 2.1. Sample preparation Cleaned spinach leaves were trimmed and pureed with water in the ratio of 1:0.5 using a mixer grinder. It was then portioned and salt was added to get puree containing 1% and 2% by weight of NaCl since the maximum amount of salt used in cooking is 2%. 2.2. Heat treatment Heat treatments were carried out at different temperatures (50, 60, 70, 80, 90, 100 and 120 C) for 0–60 min. The temperatures were measured with ±0.1 C accuracy and the come-up time was less than 1 min. A water bath was used as a heating device for temperatures up to 100 C, while for 120 C an autoclave was used. The come-up time for all the cooking method was 5 min. Fifty grams each of the pureed spinach samples with 0%, 1% and 2% salts by weight were taken in a 100-ml beaker and heated at pre-determined temperature/time, with frequent stirring. Samples were withdrawn periodically and the Hunter L, a and b values were measured using Hunterlab DP-9000 D25A colourimeter (Hunter Associates Laboratory, Reston, VA, USA). 2.3. Cooking methods For cooking studies normal open pan cooking (15 min at a gas flow rate of 15 ml/s), pressure-cooking (10 min at a gas flow rate of 15 ml/s) and one newly developed slow cooker (Joshi & Patel, 2002) named ÔEcoCooker’ (30 min at a gas flow rate of 6 ml/s and 30 min holding period) were selected as different cooking methods. The principle of EcoCooker is based on multiple effect evaporation, heating rate matching with the pick up rate of the vessel using suitable burner, and insulation; and on the logic of combining these principles in one unit, which saves up to 70% fuel requirement. The times of cooking for different cooking methods have been selected on the basis of independent cooking studies. The samples were taken out at different time intervals and colour was measured using HunterLab DP-9000 D25A colourimeter (Hunter Associates Laboratory, Reston, VA, USA). 2.4. Time–temperature data Time–temperature data for each cooking methods was monitored using a digital thermocouple.
P. Nisha et al. / Journal of Food Engineering 64 (2004) 135–142
2.5. Evaluation of colour Visual colour was evaluated using a HunterLab colourimeter model DP-9000 D25A (Hunter Associates Laboratory, Reston, VA, USA) in terms of Hunter L (lightness), a (redness and greenness) and b (yellowness and blueness). The instrument was calibrated with standard white and black tiles. A glass cuvette containing the heat-treated puree was place above the light source and covered with the black cover provided with the instrument and Hunter L, a and b values were recorded. All the experiments were done in triplicates. 2.6. Kinetic calculations A general reaction rate expression for the degradation kinetics can be written as follows (Libuza & Riboh, 1982; Ramaswami, Van De Voort, & Ghasal, 1989; Van Boekel, 1996): d½C=dt ¼ k½Cm ;
ð1Þ
where C is the quantitative value of the component under consideration, k is the reaction rate constant, and m is the order of the reaction. Degradation of visual colour has been found to follow first order kinetics (Ahmed, Kaur, & Shivhare, 2002; Canjura et al., 1991). Following these evidences, the equation for first order kinetics after integration of Eq. (1) can be written as lnðCt =C0 Þ ¼ kt:
ð2Þ
The dependence of the degradation rate constant (kT ) on temperature was quantified by the Arrhenius equation, where kT ¼ A0 expðEa =RT Þ;
ð3Þ
where C0 , measured Hunter colour value (L; a; b) at time zero (dimensionless); Ct , measured Hunter colour value (L, a, b) or a combination of these at time t; t, heating time (s); Ea , activation energy of the reaction (kJ mol1 ); R, universal gas constant (8.3145 J mol1 K1 ); T , absolute temperature (K); A0 , frequency factor (s1 ) is a pre-exponential constant. Each experiment was done in triplicate, and for each sample four Hunter L, a and b values were recorded by rotating the glass cuvette at 0–360. Thus average of 12 readings was taken for the kinetic analysis. Kinetic data were analyzed by linear regression analysis using MS Excel. Since the major colour of green leafy vegetable puree is green, Hunter Ôa’ values was considered as the visual parameter to describe the green colour degradation during thermal processing. Therefore, Eq. (2) can be written as lnða= a0 Þ ¼ kt;
ð4Þ
137
where Ôa’, Hunter Ôa’ value at time t (dimensionless); a0 , Hunter Ôa’ value at time zero (dimensionless); k, rate constant for green colour degradation (s1 ).
3. Results and discussion 3.1. Effect of salt and temperature on visual green colour of spinach puree Table 1 shows the effect of temperature and addition of salt on the Hunter L, a and b values of spinach puree. As can be seen, there is a consistent decrease in L and Ôa’ values with an increase in the treatment time and temperature. But there was no consistent change in b values. The change in L and b values may be due to pheophytin–pyripheophytin conversion or due to degradation/reaction of other components present in the spinach puree (Weemas et al., 1999). Since the greenness is indicated by Ôa’, the study was carried out only with respect to a values. During the heat processing the puree turned olive green and the a value for spinach puree changed from an initial value of )9.56 to )8.75, )8.82 and 8.89 at 50 C at 0%, 1% and 2% salt by weight, respectively after 60 min of treatment. The corresponding value after 60 min at 120 C were )2.16, )2.24 and )2.38, respectively. It is evident from Table 1 that Ôa’ values are higher for puree containing 1% and 2% of salt by weight indicating that there is some stabilization of green colour in spinach puree containing salt. This effect is also visible from the lower k values in samples containing 1% and 2% salt up to 100 C. Chlorophyll is reported to be stable at alkaline conditions. The pHs of the puree containing 0%, 1% and 2% salt (sodium chloride) was 6.18, 6.26 and 6.43, respectively. Thus the addition of salt increases the alkalinity of the spinach puree slightly, which decreases the rate of degradation of chlorophyll. Replacement of magnesium by sodium during heat treatment may also help in the stabilization of colour (Haisman & Clarke, 1975). 3.2. Degradation kinetics of visual colour of spinach puree Using linear regression, the degradation data were analyzed using Eq. (4) to determine the overall order and rate constant for the degradation reaction. Accordingly, lnðat =a0 Þ was plotted vs t, from which rate constant, k was calculated as the slope. Figs. 1–3 show the representative values for the first order plots for the degradation of greenness (a) for spinach puree with 0%, 1% and 2% salt by weight at 50, 100 and 120 C, respectively. A correlation coefficient >0.9 in all cases confirmed that the degradation of visual green colour in spinach puree indeed follows a first order reaction at all temperatures. Table 2 documents the rate constants for the visual green colour degradation of spinach puree
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Table 1 Effect of heat treatment on L, a and b values of spinach puree containing 0%, 1% and 2% salt Temperature (C)
Time (min)
L 0%
1%
2%
50
20 40 60
22.42 22.11 22.03
22.5 22.22 19.35
22.53 22.25 19.44
)9.52 )9.20 )8.85
)9.30 )9.25 )9.18
60
20 40 60
22.23 21.14 20.49
22.04 21.8 21.73
22.37 22.06 20.97
)9.85 )9.44 )8.40
70
20 40 60
20.20 21.01 20.82
20.50 21.10 21.00
21.02 21.30 21.01
80
20 40 60
20.80 20.37 19.95
21.70 22.21 21.40
90
20 40 60
20.84 20.12 19.76
100
20 40 60
120
20 40 60
a
b
0%
1%
2%
0%
1%
2%
)9.42 )9.29 )9.25
10.26 10.22 10.10
9.88 10.65 10.94
9.81 10.57 10.95
)10.15 )9.60 )8.74
)10.25 )9.80 )9.89
11.94 11.85 11.47
11.47 12.44 11.45
12.78 12.55 12.24
)10.2 )9.49 )8.23
)10.36 )9.76 )8.42
)10.96 )10.12 )8.78
11.20 10.59 10.50
11.43 10.60 10.56
11.52 10.80 10.65
22.28 22.74 21.90
)9.22 )8.09 )5.93
)9.28 )8.14 )6.12
)9.40 )8.49 )6.70
11.52 11.52 11.35
12.01 12.13 11.68
12.44 12.51 11.70
21.69 22.28 22.55
22.25 22.46 22.04
)7.42 )5.24 )4.38
)7.09 )5.31 )4.42
)7.87 )6.00 )4.97
11.09 11.19 11.80
11.80 12.07 12.19
12.10 12.32 11.82
20.30 20.09 19.13
20.47 20.92 21.37
20.21 21.08 19.96
)5.42 )3.82 )2.80
)5.61 )3.75 )2.85
)5.52 )3.98 )3.15
10.82 10.80 11.04
10.86 11.11 10.98
10.86 11.17 10.64
21.37 20.77 18.92
19.66 21.35 19.17
19.22 19.60 19.60
)7.15 )3.60 )2.16
)7.56 )3.72 )2.24
)7.53 )3.89 )2.38
9.96 10.61 11.29
10.38 10.70 10.80
10.29 10.32 10.41
Initial L, a and b values are 22.58, )9.56 and 11.20, respectively. The standard error values were less than 0.05.
Time (s) 0.04 y = -3.17E-05x + 0.0385 2
R = 0.98(1%)
0 -0.02 0 -0.04 -0.06 -0.08 -0.1
600
1200
1800
2400
3000
3600
4200
y = -3.5E-05x + 0.0406 2
R = 0.99(0%) y = -2.67E-05x + 0.0297 2
R = 0.97(2%) 0%
1%
2%
Fig. 1. First order plot of colour (a) degradation in spinach puree leaves at 50 C.
Time(s)
0
ln(-a/-a0)
-0.2 0
600
1200
1800
Time(s) 2400
3000
3600
-0.4
y = -2.82E4x - 0.2159
-0.6
R = 0.9848(1%)
-0.8 -1 -1.2 -1.4
0.1
4200
2
y = -2.33E4x - 0.2843 2
R = 0.9932(2%)
ln(-a/-a0)
ln(-a/a0)
0.02
with 0%, 1% and 2% salt by weight. From the lower values for the rate constants in samples containing 2% and 1% salt, it is evident that green colour is more stable in puree containing 2% salt followed by 1% and 0%. However, at 120 C, the lower k values are seen only with 2% salt. A study conducted by Hudson et al. (1974) on the effect of various solutions on the conversion of chlorophyll during blanching revealed that 1.2% of NaCl treatment had greater effect on preserving colour. Haisman and Clarke (1975) reported that, at higher concentrations, sodium chloride reduced the rate of conversion of chlorophyll to pheophytin. This again confirms the protective effect of sodium chloride on green colour of vegetables.
-0.4
0
600
1200
1800
-0.9
y = -4.85E4x + 0.309
-1.4
y = -4.98E4x + 0.2788
2400
3000 3600 4200 y = -5.06E4x + 0.3398 2
R = 0.9958(1%)
2
R = 0.9967(2%)
y = -3.12E4x - 0.1261
2
R = 0.9953(0%)
2
R = 0.9915(0%)
-1.9 0%
1%
2%
Fig. 2. First order plot of colour (a) degradation in spinach puree leaves at 100 C.
0%
1%
2%
Fig. 3. First order plot of colour (a) degradation in spinach puree leaves at 120 C.
P. Nisha et al. / Journal of Food Engineering 64 (2004) 135–142
139
Table 2 Rate constant k a;b and correlation coefficient (R2 ) for green colour degradation in spinach puree with 0%, 1% and 2% of salt (NaCl) Temperature (C)
Salt added 0%
1% 1
50 60 70 80 90 100 120 a b
2% 1
k (s )
R
k (s )
R
k (s1 )
R2
3.5 · 105 6.67 · 105 9.00 · 105 1.83 · 104 2.2 · 104 3.12 · 104 4.98 · 104
0.98 0.94 0.97 0.95 0.97 0.99 0.99
3.17 · 105 6.33 · 105 8.67 · 105 1.73 · 104 1.97 · 104 2.82 · 104 5.06 · 104
0.98 0.98 0.94 0.96 0.98 0.99 0.99
2.67 · 105 6.00 · 105 9.17 · 105 1.42 · 104 1.92 · 104 2.33 · 104 4.85 · 104
0.97 0.96 0.97 0.95 0.98 0.99 0.99
2
2
Calculated from semi-log plot of lnðat = a0 Þ vs time. The standard error in k values were less than 2 · 107 .
1/T
140
-7.6
0.0031 0.0032
y = -4519.2x + 3.9294 2 R = 0.96(0%)
-8.1
lnk
0.003
-8.6 -9.1
y = -4611.4x + 4.1268 2 R = 0.97(1%)
-10.6 -11.1
100 80 60 40 20
-9.6 -10.1
120
Temperature (˚C)
-6.6 0.0024 0.0025 0.0026 0.0027 0.0028 0.0029 -7.1
0 0
y = -4634x + 4.0994 2 R = 0.97(2%)
10
Time (min)
0%
1%
2%
20 Open pan
30
40 Pressure
50
60
Eco'
Fig. 5. Time–temperature profiles of the different cooking method used.
Fig. 4. Arrhenius plot for colour degradation (a) in spinach puree.
The activation energy for green colour in spinach puree with 0%, 1% and 2% of salt was calculated to be 37.57 (R2 ¼ 0:96), 38.34 (R2 ¼ 0:97) and 38.53 (R2 ¼ 0:97) kJ mol1 , respectively (Fig. 4). Previous studies have reported wide variation in the activation energies for colour degradation of green vegetable purees. Activation energies of 28.55, 41.15 and 34.01 kJ mol1 for spinach puree, mustard leaves and a mixed puree, respectively are reported by Ahmed et al. (2002). Activation energies for colour degradation in green chilli puree (Ahmed, Shivhare, & Raghavan, 2000) and that for heated broccoli juice (Weemas et al., 1999) are reported to be 11.34–15.98 and 72.01 kJ mol1 , respectively. These variations may be attributed to the differences in the raw material and the temperature ranges used in these studies. 3.3. Time–temperature data of the three modes of cooking To extend the results obtained from steady state experiments to the unsteady state encountered the three modes of cooking, viz. open pan cooking, pressure cooking and cooking in eco cooker, time–temperature data during the processing of each was recorded (Fig. 5).
3.4. Degradation profile and kinetics of visual colour of spinach puree under the three modes of cooking and the effect of salt on the stability of colour Colour degradation was followed in each of these modes of cooking as for spinach puree under steady state conditions. The results documented in Table 3 indicate that there is a loss in colour as can be seen from the decrease in Hunter L and a values in all the methods of cooking. Spinach puree containing 2% of salt shows maximum retention of green colour as compared to the puree with 0% and 1% salt. The maximum retention of green colour is observed in open pan cooking with a value of )7.77 for spinach puree with 2% salt followed by pressure cooker, a ¼ 6:73 and EcoCooker, a ¼ 5:69. Table 4 documents the rate constant (k), correlation coefficient (R2 ) and half-life (t1=2 ) for green colour degradation (a) in spinach puree with 0%, 1% and 2% of salt (NaCl) during different cooking methods. 3.5. Prediction of colour loss during unsteady state heating processing To predict the degradation of green colour occurring during a given unsteady state heating process, the Arrhenius equation, kT ¼ A0 expðEa =RT Þ (Eq. (3)) was used, where kT is the rate constant at any absolute
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Table 3 Degradation profile of colour (L, a and b) in spinach puree with 0%, 1% and 2% of salt (NaCl) at different cooking methods Method of cooking
Time (min)
L 0%
1%
2%
0%
1%
2%
0%
1%
2%
Open pan
5 10 15
19.32 18.64 18.24
19.73 18.69 18.41
20.02 18.53 18.08
)9.23 )7.71 )7.22
)9.31 )7.97 )7.33
)9.48 )8.66 )7.77
10.65 10.68 10.82
11.00 10.92 11.04
11.10 10.99 11.07
Pressure
5 10 15
19.88 19.24 19.00
19.91 19.35 19.18
19.96 19.40 19.29
)9.40 )8.60 )6.33
)9.47 )8.67 )6.40
)9.51 )9.17 )6.73
10.63 10.68 10.88
10.65 10.77 10.86
10.65 10.69 10.75
EcoCookinga
10 20 30
21.30 19.95 18.60
21.31 20.04 19.25
21.36 20.09 19.88
)9.44 )8.70 )5.30
)9.50 )8.79 )5.37
)9.53 )8.99 )5.69
10.24 10.6 10.78
9.89 10.66 11.11
9.70 10.80 11.15
a
b
Initial L, a and b values are 22.58, )9.56 and 11.20, respectively. The standard error values were less than 0.05. a Held for 30 min as per the protocol recommended for cooking with EcoCooker.
Table 4 Rate constant k a;b and correlation coefficient (R2 ) for green colour degradation (a) in spinach puree with 0%, 1% and 2% of salt (NaCl) at different cooking methods Method of cooking
Salt (%)
Rate constant k a (s1 )
(R2 )
Open pan cooking
0 1 2
4.17 · 104 3.33 · 104 4.00 · 104
0.93 0.97 0.99
Pressure cooking
0 1 2
6.67 · 104 6.50 · 104 5.83 · 104
0.91 0.91 0.91
EcoCookingc
0 1 2
4.83 · 104 4.83 · 104 4.33 · 104
0.91 0.90 0.91
a
Calculated from semi-log plot of (at = a0 ) vs time. The standard error in k values were less than 9 · 107 . c Held for 30 min as per the protocol recommended for cooking with EcoCooker. b
temperature T and time t. Ea is the activation energy of the reaction, R is the gas constant, and A0 is a pre-exponential constant, which are already calculated for steady state heating process. The rate constant kT at each temperature was calculated using Eq. (3) substituting for T from the time–temperature data of unsteady state heating process. Knowing the rate constant kT , the rate (dC=dt), amount degraded (DC) during the short time interval zero to t (¼ DtT ) and the final concentration CtþDt can be calculated as follows: Rate ¼ rate constant (kT ) · initial concentration (Ct ). Amount degraded during DtT ðDCÞ ¼ Rate DtT (kT C DtT ). Concentration after time DtT ¼ Ct DC. These calculations were continued for the entire time period (heating and constant temperatures) at which each cooking process was done. An MS Excel based computer program was used to calculate the above parameters: The P total amount degraded after complete cooking was DC. The final concentration thus will be
P C0 DC, where C0 is the initial value of green colour (a0 ). The resulting predictions and the actual degradation, obtained experimentally, are given in Table 5. As seen, a good agreement between the actual and the predicted degradation/retention of green colour was obtained. The predicted retention for open pan and pressure cooking are higher than the actual values, while the predicted retention for EcoCooking is lower than the predicted value. This may be due to the difference in the way of cooking. In the case of open pan and pressure cooking, the cooking is vigorous due to higher gas flow rates and direct heating. In the case of EcoCooker, the heating rate is very slow and heating is indirect. Using this prediction method, the degradation of visual colour can be predicted for any processing method, if the time–temperature profile of that processing operation is known.
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141
Table 5 The actual and predicted retention of green colour in the cooking methods Method of cooking
Salt (%)
Actual retention
Predicted retention
Open pan cooking
0 1 2
7.22 7.33 7.77
7.86 7.94 8.06
Pressure cooking
0 1 2
6.33 6.40 6.73
6.9 7.09 7.31
EcoCookinga
0 1 2
5.3 5.37 5.69
4.8 5.00 5.27
a
Held for 30 min as per the protocol recommended for cooking with EcoCooker.
4. Conclusions A stabilization effect of 2% sodium chloride on green colour degradation resulting from heat treatment is confirmed from the kinetic studies, although, the exact mechanism of stabilization is not known. Slow cookers, as exemplified by EcoCooker, show no significant difference in the magnitude of retention as compared to normal open pan and pressure-cooking. Based on the colour retention and fuel savings, an overall judgment in favor of the slow EcoCooker is suggested.
Acknowledgements The authors gratefully acknowledge the financial support provided by Land Research Institute, Mumbai in carrying out this work.
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Gnanasekharan, V., Shewflet, R. L., & Chinnan, M. S. (1992). Detection of colour changes in green vegetables. Journal of Food Science, 57, 149–154. Govindarajan, V. S., Rajalakshmi, D., & Chand, N. (1998). Capsicum: Production, technology, chemistry and quality. Part VI. Evaluation of quality. CRC Critical Reviews in Food Science and Nutrition, 25, 185–282. Gupte, S. M., El-Bisi, H. M., & Francis, F. J. (1963). Kinetics of thermal degradation of chlorophyll in spinach puree. Journal of Food Science, 29, 379–382. Haisman, D. R., & Clarke, W. (1975). The interfacial factor in the heat-induced conversion of chlorophyll to pheophytin in green leaves. Journal of the Science of Food and Agriculture, 26, 1111– 1116. Hayakawa, K.-I. (1977). Influence of heat treatment on the quality of vegetables: Changes in visual green colour. Journal of Food Science, 42, 778–781. Hudson, M. A., Sharples, V. J., & Gregory, M. E. (1974). Quality of home frozen vegetables. II. Effect of blanching and/or cooling in various solutions on conversion of chlorophyll. Journal of Food Technology, 9, 105–108. Joshi, J. B., & Patel, S. B. (2002). Fuel efficient steam cooking device. US National Phase Patent Application No. 10/149, 475. Libuza, T. P., & Riboh, D. (1982). Theory and application of Arrhenius kinetics to the prediction of nutrient losses in foods. Journal of Food Science, 36, 66–74. Ozkan, M., Yemenicioglu, A., Asefi, N., & Cemeroglu, B. (2002). Degradation kinetics of anthocyanins from sour cherry, pomegranate, and strawberry juices by hydrogen peroxide. Journal of Food Science, 67, 525–529. Ramaswami, H. S., Van De Voort, F. R., & Ghasal, S. (1989). An analysis of TDT and Arrhenius methods for handling process and kinetic data. Journal of Food Science, 54, 1322–1326. Ryan-Stoneham, T., & Tong, C. H. (2000). Degradation kinetics of chlorophyll in peas as a function of pH. Journal of Food Science, 65, 1296–1302. Schwartz, S. J., & Lorenzo, T. V. (1991). Chlorophyll stability during continuous aseptic processing and storage. Journal of Food Science, 56, 1059–1062. Schwartz, S. J., & Von Elbe, J. H. (1983). Kinetics of chlorophyll degradation to pyropheophytin in vegetables. Journal of Food Science, 48, 1303–1306. Steet, J. A., & Tong, C. H. (1996). Degradation kinetics of green colour and chlorophylls in peas by colourimetry and HPLC. Journal of Food Science, 61, 924–927, 931. Sweeney, J. P., & Martin, M. E. (1961). Stability of chlorophyll in vegetables as affected by pH. Food Technology, 15, 263– 266.
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Research note
A study on the degradation kinetics of visual green colour in spinach (Spinacea oleracea L.) and the effect of salt therein P. Nisha a, R.S. Singhal
a,*
, A.B. Pandit
b
a
b
Department of Food and Fermentation Technology, Institute of Chemical Technology, University of Mumbai, Matunga 400 019, Mumbai, India Department of Chemical Engineering, Institute of Chemical Technology, University of Mumbai, Matunga 400 019, Mumbai, India Received 26 May 2003; accepted 13 September 2003
Abstract The effect of salt on the degradation of visual green colour Ôa’ in spinach puree (Spinacea oleracea L.) over a temperature range of 50–120 C (steady state temperature process) as well as under conditions of normal open pan cooking, pressure-cooking and a newly developed and patented fuel-efficient EcoCooker has been studied (unsteady state heating process). The degradation of visual green colour as measured by Ôa’ value followed a first order kinetics, where the rate constant increased with an increase in the temperature. The temperature dependence of degradation was adequately modeled by Arrhenius equation. A mathematical model has been developed using the steady state kinetic parameters obtained to predict the losses of green colour from the time–temperature data of the unsteady state heating/processing method. The results obtained indicate a protective effect of salt on the degradation of visual green colour. 2003 Elsevier Ltd. All rights reserved. Keywords: Colour degradation; Kinetics; Spinach; Effect of salt; Cookers
1. Introduction Colour plays an important role in visual recognition and assessment of the surface and the subsurface properties of the object. It has a great influence on the appearance, processing and acceptance of food materials. The degree of greenness, attributed to chlorophyll pigments, is important in determining the final quality of thermally processed green vegetables. Thermally processed green vegetables exhibit poor colour quality as compared to the fresh ones. The colour change from bright green to olive brown occurring during the processing is mainly due to the degradation of chlorophyll to pheophytin by the replacement of magnesium in the chlorophyll by hydrogen and further formation of the degradation products such as pheophorbides and chlorins (Canjura, Schwartz, & Nunes, 1991; Schwartz & Lorenzo, 1991; Schwartz & Von Elbe, 1983; White, Jones, & Gibbes, 1963). More over the conversion of chlorophyll to pheophytin is regarded as an index of the severity of the processing (Woolfe, 1979). *
Corresponding author. Fax: +91-22-2414-5614. E-mail address: [email protected] (R.S. Singhal).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.09.021
Factors such as pH, temperature, presence of salts, enzymes and surface-active ions influence the stability of chlorophyll. Chlorophyll has better stability at higher pH than at lower pH (Ryan-Stoneham & Tong, 2000; Sweeney & Martin, 1961). Salts of magnesium, calcium, sodium, ammonium and some surface-active agents are known to have some stabilizing effect on chlorophyll degradation (Eheart & Odland, 1973; Haisman & Clarke, 1975; Woolfe, 1979). Accurate knowledge of the kinetic parameters, rate constant and activation energy is essential to predict the quality changes that occur during thermal processing. Numerous researchers have evaluated the kinetics of pigments and colour degradation in fruits and vegetables such as broccoli (Weemas, Ooms, Van Loey, & Hendrickx, 1999), peas (Ryan-Stoneham & Tong, 2000; Steet & Tong, 1996), spinach (Canjura et al., 1991; Gupte, El-Bisi, & Francis, 1963; Schwartz & Von Elbe, 1983) during thermal processing and found it to follow first order reaction kinetics. It is believed that colour vision is a complex phenomenon and its measurement is much more complex than the absorption by the stimuli pigments at specific wavelengths (Govindarajan, Rajalakshmi, & Chand,
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1998). Furthermore, the spectrophotometric methods have some limitations for the colour measurement of thermally processed foods. Thermally decomposed pigments like pheophytins in the case of chlorophyll, cause interference during pigment concentration measurements (Eheart & Gott, 1965). There are several reports on the measurement of colour using instruments. Such instruments are usually based on the principle that a colour can be mathematically described as a combination of the three primary colour intensities (Clydesdale, 1978). Tristimulus colourimeter has been widely accepted as a rapid and simple instrumental method for measuring visual perception of the food products (Clydesdale, 1991; Gnanasekharan, Shewflet, & Chinnan, 1992; Hayakawa, 1977; Ozkan, Yemenicioglu, Asefi, & Cemeroglu, 2002). The colour of any food product can be represented in terms of L, a, and b values or combination of these three depending upon the nature of the pigment present in the food material. A study conducted by Weemas et al. (1999) on the kinetics of chlorophyll degradation and colour loss in heated broccoli juice shows the advantage of objective colour measurements for studying the chlorophyll degradation as compared to conventional chemical analysis and from the point of view of the consumers the green colour is more important than the residual chlorophyll content. Spinach is one of the important leafy vegetables consumed all over the world. It is consumed as fresh, pureed or processed. During thermal processing, colour changes from bright green to olive green due to the conversion of chlorophyll to pheophytin and pyropheophytin. It is a common belief that common salt (sodium chloride) has a stabilizing effect on green colour (Haisman & Clarke, 1975; Hudson, Sharples, & Gregory, 1974). However, the stabilization effect of salt is not well established and no information is available on the effect of salt on colour degradation of these products, especially in terms of visual colour. Therefore, present study was undertaken: (1) To find out the effect of salt (1% and 2% by weight) on colour degradation in spinach (Spinacea oleracea L.), a model food system, over a temperature range of 50–120 C (steady state temperature) and to determine kinetic parameters for colour degradation in spinach (S. oleracea L.). (2) To study the degradation kinetics of colour in different cooking methods (unsteady state process). (3) To develop a mathematical model relating the calculated kinetic data from the steady state temperature and the time–temperature profiles of different cooking methods (unsteady state process). (4) To apply this model to predict the green colour degradation for unsteady state heating process, from the time–temperature data of the unsteady state heating process and comparing it with the actual degradation values, which could then be used to assess the changes in the colour quality as a function of method of cooking.
2. Materials and methods Fresh green spinach leaves were obtained locally, washed thoroughly and drained. 2.1. Sample preparation Cleaned spinach leaves were trimmed and pureed with water in the ratio of 1:0.5 using a mixer grinder. It was then portioned and salt was added to get puree containing 1% and 2% by weight of NaCl since the maximum amount of salt used in cooking is 2%. 2.2. Heat treatment Heat treatments were carried out at different temperatures (50, 60, 70, 80, 90, 100 and 120 C) for 0–60 min. The temperatures were measured with ±0.1 C accuracy and the come-up time was less than 1 min. A water bath was used as a heating device for temperatures up to 100 C, while for 120 C an autoclave was used. The come-up time for all the cooking method was 5 min. Fifty grams each of the pureed spinach samples with 0%, 1% and 2% salts by weight were taken in a 100-ml beaker and heated at pre-determined temperature/time, with frequent stirring. Samples were withdrawn periodically and the Hunter L, a and b values were measured using Hunterlab DP-9000 D25A colourimeter (Hunter Associates Laboratory, Reston, VA, USA). 2.3. Cooking methods For cooking studies normal open pan cooking (15 min at a gas flow rate of 15 ml/s), pressure-cooking (10 min at a gas flow rate of 15 ml/s) and one newly developed slow cooker (Joshi & Patel, 2002) named ÔEcoCooker’ (30 min at a gas flow rate of 6 ml/s and 30 min holding period) were selected as different cooking methods. The principle of EcoCooker is based on multiple effect evaporation, heating rate matching with the pick up rate of the vessel using suitable burner, and insulation; and on the logic of combining these principles in one unit, which saves up to 70% fuel requirement. The times of cooking for different cooking methods have been selected on the basis of independent cooking studies. The samples were taken out at different time intervals and colour was measured using HunterLab DP-9000 D25A colourimeter (Hunter Associates Laboratory, Reston, VA, USA). 2.4. Time–temperature data Time–temperature data for each cooking methods was monitored using a digital thermocouple.
P. Nisha et al. / Journal of Food Engineering 64 (2004) 135–142
2.5. Evaluation of colour Visual colour was evaluated using a HunterLab colourimeter model DP-9000 D25A (Hunter Associates Laboratory, Reston, VA, USA) in terms of Hunter L (lightness), a (redness and greenness) and b (yellowness and blueness). The instrument was calibrated with standard white and black tiles. A glass cuvette containing the heat-treated puree was place above the light source and covered with the black cover provided with the instrument and Hunter L, a and b values were recorded. All the experiments were done in triplicates. 2.6. Kinetic calculations A general reaction rate expression for the degradation kinetics can be written as follows (Libuza & Riboh, 1982; Ramaswami, Van De Voort, & Ghasal, 1989; Van Boekel, 1996): d½C=dt ¼ k½Cm ;
ð1Þ
where C is the quantitative value of the component under consideration, k is the reaction rate constant, and m is the order of the reaction. Degradation of visual colour has been found to follow first order kinetics (Ahmed, Kaur, & Shivhare, 2002; Canjura et al., 1991). Following these evidences, the equation for first order kinetics after integration of Eq. (1) can be written as lnðCt =C0 Þ ¼ kt:
ð2Þ
The dependence of the degradation rate constant (kT ) on temperature was quantified by the Arrhenius equation, where kT ¼ A0 expðEa =RT Þ;
ð3Þ
where C0 , measured Hunter colour value (L; a; b) at time zero (dimensionless); Ct , measured Hunter colour value (L, a, b) or a combination of these at time t; t, heating time (s); Ea , activation energy of the reaction (kJ mol1 ); R, universal gas constant (8.3145 J mol1 K1 ); T , absolute temperature (K); A0 , frequency factor (s1 ) is a pre-exponential constant. Each experiment was done in triplicate, and for each sample four Hunter L, a and b values were recorded by rotating the glass cuvette at 0–360. Thus average of 12 readings was taken for the kinetic analysis. Kinetic data were analyzed by linear regression analysis using MS Excel. Since the major colour of green leafy vegetable puree is green, Hunter Ôa’ values was considered as the visual parameter to describe the green colour degradation during thermal processing. Therefore, Eq. (2) can be written as lnða= a0 Þ ¼ kt;
ð4Þ
137
where Ôa’, Hunter Ôa’ value at time t (dimensionless); a0 , Hunter Ôa’ value at time zero (dimensionless); k, rate constant for green colour degradation (s1 ).
3. Results and discussion 3.1. Effect of salt and temperature on visual green colour of spinach puree Table 1 shows the effect of temperature and addition of salt on the Hunter L, a and b values of spinach puree. As can be seen, there is a consistent decrease in L and Ôa’ values with an increase in the treatment time and temperature. But there was no consistent change in b values. The change in L and b values may be due to pheophytin–pyripheophytin conversion or due to degradation/reaction of other components present in the spinach puree (Weemas et al., 1999). Since the greenness is indicated by Ôa’, the study was carried out only with respect to a values. During the heat processing the puree turned olive green and the a value for spinach puree changed from an initial value of )9.56 to )8.75, )8.82 and 8.89 at 50 C at 0%, 1% and 2% salt by weight, respectively after 60 min of treatment. The corresponding value after 60 min at 120 C were )2.16, )2.24 and )2.38, respectively. It is evident from Table 1 that Ôa’ values are higher for puree containing 1% and 2% of salt by weight indicating that there is some stabilization of green colour in spinach puree containing salt. This effect is also visible from the lower k values in samples containing 1% and 2% salt up to 100 C. Chlorophyll is reported to be stable at alkaline conditions. The pHs of the puree containing 0%, 1% and 2% salt (sodium chloride) was 6.18, 6.26 and 6.43, respectively. Thus the addition of salt increases the alkalinity of the spinach puree slightly, which decreases the rate of degradation of chlorophyll. Replacement of magnesium by sodium during heat treatment may also help in the stabilization of colour (Haisman & Clarke, 1975). 3.2. Degradation kinetics of visual colour of spinach puree Using linear regression, the degradation data were analyzed using Eq. (4) to determine the overall order and rate constant for the degradation reaction. Accordingly, lnðat =a0 Þ was plotted vs t, from which rate constant, k was calculated as the slope. Figs. 1–3 show the representative values for the first order plots for the degradation of greenness (a) for spinach puree with 0%, 1% and 2% salt by weight at 50, 100 and 120 C, respectively. A correlation coefficient >0.9 in all cases confirmed that the degradation of visual green colour in spinach puree indeed follows a first order reaction at all temperatures. Table 2 documents the rate constants for the visual green colour degradation of spinach puree
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Table 1 Effect of heat treatment on L, a and b values of spinach puree containing 0%, 1% and 2% salt Temperature (C)
Time (min)
L 0%
1%
2%
50
20 40 60
22.42 22.11 22.03
22.5 22.22 19.35
22.53 22.25 19.44
)9.52 )9.20 )8.85
)9.30 )9.25 )9.18
60
20 40 60
22.23 21.14 20.49
22.04 21.8 21.73
22.37 22.06 20.97
)9.85 )9.44 )8.40
70
20 40 60
20.20 21.01 20.82
20.50 21.10 21.00
21.02 21.30 21.01
80
20 40 60
20.80 20.37 19.95
21.70 22.21 21.40
90
20 40 60
20.84 20.12 19.76
100
20 40 60
120
20 40 60
a
b
0%
1%
2%
0%
1%
2%
)9.42 )9.29 )9.25
10.26 10.22 10.10
9.88 10.65 10.94
9.81 10.57 10.95
)10.15 )9.60 )8.74
)10.25 )9.80 )9.89
11.94 11.85 11.47
11.47 12.44 11.45
12.78 12.55 12.24
)10.2 )9.49 )8.23
)10.36 )9.76 )8.42
)10.96 )10.12 )8.78
11.20 10.59 10.50
11.43 10.60 10.56
11.52 10.80 10.65
22.28 22.74 21.90
)9.22 )8.09 )5.93
)9.28 )8.14 )6.12
)9.40 )8.49 )6.70
11.52 11.52 11.35
12.01 12.13 11.68
12.44 12.51 11.70
21.69 22.28 22.55
22.25 22.46 22.04
)7.42 )5.24 )4.38
)7.09 )5.31 )4.42
)7.87 )6.00 )4.97
11.09 11.19 11.80
11.80 12.07 12.19
12.10 12.32 11.82
20.30 20.09 19.13
20.47 20.92 21.37
20.21 21.08 19.96
)5.42 )3.82 )2.80
)5.61 )3.75 )2.85
)5.52 )3.98 )3.15
10.82 10.80 11.04
10.86 11.11 10.98
10.86 11.17 10.64
21.37 20.77 18.92
19.66 21.35 19.17
19.22 19.60 19.60
)7.15 )3.60 )2.16
)7.56 )3.72 )2.24
)7.53 )3.89 )2.38
9.96 10.61 11.29
10.38 10.70 10.80
10.29 10.32 10.41
Initial L, a and b values are 22.58, )9.56 and 11.20, respectively. The standard error values were less than 0.05.
Time (s) 0.04 y = -3.17E-05x + 0.0385 2
R = 0.98(1%)
0 -0.02 0 -0.04 -0.06 -0.08 -0.1
600
1200
1800
2400
3000
3600
4200
y = -3.5E-05x + 0.0406 2
R = 0.99(0%) y = -2.67E-05x + 0.0297 2
R = 0.97(2%) 0%
1%
2%
Fig. 1. First order plot of colour (a) degradation in spinach puree leaves at 50 C.
Time(s)
0
ln(-a/-a0)
-0.2 0
600
1200
1800
Time(s) 2400
3000
3600
-0.4
y = -2.82E4x - 0.2159
-0.6
R = 0.9848(1%)
-0.8 -1 -1.2 -1.4
0.1
4200
2
y = -2.33E4x - 0.2843 2
R = 0.9932(2%)
ln(-a/-a0)
ln(-a/a0)
0.02
with 0%, 1% and 2% salt by weight. From the lower values for the rate constants in samples containing 2% and 1% salt, it is evident that green colour is more stable in puree containing 2% salt followed by 1% and 0%. However, at 120 C, the lower k values are seen only with 2% salt. A study conducted by Hudson et al. (1974) on the effect of various solutions on the conversion of chlorophyll during blanching revealed that 1.2% of NaCl treatment had greater effect on preserving colour. Haisman and Clarke (1975) reported that, at higher concentrations, sodium chloride reduced the rate of conversion of chlorophyll to pheophytin. This again confirms the protective effect of sodium chloride on green colour of vegetables.
-0.4
0
600
1200
1800
-0.9
y = -4.85E4x + 0.309
-1.4
y = -4.98E4x + 0.2788
2400
3000 3600 4200 y = -5.06E4x + 0.3398 2
R = 0.9958(1%)
2
R = 0.9967(2%)
y = -3.12E4x - 0.1261
2
R = 0.9953(0%)
2
R = 0.9915(0%)
-1.9 0%
1%
2%
Fig. 2. First order plot of colour (a) degradation in spinach puree leaves at 100 C.
0%
1%
2%
Fig. 3. First order plot of colour (a) degradation in spinach puree leaves at 120 C.
P. Nisha et al. / Journal of Food Engineering 64 (2004) 135–142
139
Table 2 Rate constant k a;b and correlation coefficient (R2 ) for green colour degradation in spinach puree with 0%, 1% and 2% of salt (NaCl) Temperature (C)
Salt added 0%
1% 1
50 60 70 80 90 100 120 a b
2% 1
k (s )
R
k (s )
R
k (s1 )
R2
3.5 · 105 6.67 · 105 9.00 · 105 1.83 · 104 2.2 · 104 3.12 · 104 4.98 · 104
0.98 0.94 0.97 0.95 0.97 0.99 0.99
3.17 · 105 6.33 · 105 8.67 · 105 1.73 · 104 1.97 · 104 2.82 · 104 5.06 · 104
0.98 0.98 0.94 0.96 0.98 0.99 0.99
2.67 · 105 6.00 · 105 9.17 · 105 1.42 · 104 1.92 · 104 2.33 · 104 4.85 · 104
0.97 0.96 0.97 0.95 0.98 0.99 0.99
2
2
Calculated from semi-log plot of lnðat = a0 Þ vs time. The standard error in k values were less than 2 · 107 .
1/T
140
-7.6
0.0031 0.0032
y = -4519.2x + 3.9294 2 R = 0.96(0%)
-8.1
lnk
0.003
-8.6 -9.1
y = -4611.4x + 4.1268 2 R = 0.97(1%)
-10.6 -11.1
100 80 60 40 20
-9.6 -10.1
120
Temperature (˚C)
-6.6 0.0024 0.0025 0.0026 0.0027 0.0028 0.0029 -7.1
0 0
y = -4634x + 4.0994 2 R = 0.97(2%)
10
Time (min)
0%
1%
2%
20 Open pan
30
40 Pressure
50
60
Eco'
Fig. 5. Time–temperature profiles of the different cooking method used.
Fig. 4. Arrhenius plot for colour degradation (a) in spinach puree.
The activation energy for green colour in spinach puree with 0%, 1% and 2% of salt was calculated to be 37.57 (R2 ¼ 0:96), 38.34 (R2 ¼ 0:97) and 38.53 (R2 ¼ 0:97) kJ mol1 , respectively (Fig. 4). Previous studies have reported wide variation in the activation energies for colour degradation of green vegetable purees. Activation energies of 28.55, 41.15 and 34.01 kJ mol1 for spinach puree, mustard leaves and a mixed puree, respectively are reported by Ahmed et al. (2002). Activation energies for colour degradation in green chilli puree (Ahmed, Shivhare, & Raghavan, 2000) and that for heated broccoli juice (Weemas et al., 1999) are reported to be 11.34–15.98 and 72.01 kJ mol1 , respectively. These variations may be attributed to the differences in the raw material and the temperature ranges used in these studies. 3.3. Time–temperature data of the three modes of cooking To extend the results obtained from steady state experiments to the unsteady state encountered the three modes of cooking, viz. open pan cooking, pressure cooking and cooking in eco cooker, time–temperature data during the processing of each was recorded (Fig. 5).
3.4. Degradation profile and kinetics of visual colour of spinach puree under the three modes of cooking and the effect of salt on the stability of colour Colour degradation was followed in each of these modes of cooking as for spinach puree under steady state conditions. The results documented in Table 3 indicate that there is a loss in colour as can be seen from the decrease in Hunter L and a values in all the methods of cooking. Spinach puree containing 2% of salt shows maximum retention of green colour as compared to the puree with 0% and 1% salt. The maximum retention of green colour is observed in open pan cooking with a value of )7.77 for spinach puree with 2% salt followed by pressure cooker, a ¼ 6:73 and EcoCooker, a ¼ 5:69. Table 4 documents the rate constant (k), correlation coefficient (R2 ) and half-life (t1=2 ) for green colour degradation (a) in spinach puree with 0%, 1% and 2% of salt (NaCl) during different cooking methods. 3.5. Prediction of colour loss during unsteady state heating processing To predict the degradation of green colour occurring during a given unsteady state heating process, the Arrhenius equation, kT ¼ A0 expðEa =RT Þ (Eq. (3)) was used, where kT is the rate constant at any absolute
140
P. Nisha et al. / Journal of Food Engineering 64 (2004) 135–142
Table 3 Degradation profile of colour (L, a and b) in spinach puree with 0%, 1% and 2% of salt (NaCl) at different cooking methods Method of cooking
Time (min)
L 0%
1%
2%
0%
1%
2%
0%
1%
2%
Open pan
5 10 15
19.32 18.64 18.24
19.73 18.69 18.41
20.02 18.53 18.08
)9.23 )7.71 )7.22
)9.31 )7.97 )7.33
)9.48 )8.66 )7.77
10.65 10.68 10.82
11.00 10.92 11.04
11.10 10.99 11.07
Pressure
5 10 15
19.88 19.24 19.00
19.91 19.35 19.18
19.96 19.40 19.29
)9.40 )8.60 )6.33
)9.47 )8.67 )6.40
)9.51 )9.17 )6.73
10.63 10.68 10.88
10.65 10.77 10.86
10.65 10.69 10.75
EcoCookinga
10 20 30
21.30 19.95 18.60
21.31 20.04 19.25
21.36 20.09 19.88
)9.44 )8.70 )5.30
)9.50 )8.79 )5.37
)9.53 )8.99 )5.69
10.24 10.6 10.78
9.89 10.66 11.11
9.70 10.80 11.15
a
b
Initial L, a and b values are 22.58, )9.56 and 11.20, respectively. The standard error values were less than 0.05. a Held for 30 min as per the protocol recommended for cooking with EcoCooker.
Table 4 Rate constant k a;b and correlation coefficient (R2 ) for green colour degradation (a) in spinach puree with 0%, 1% and 2% of salt (NaCl) at different cooking methods Method of cooking
Salt (%)
Rate constant k a (s1 )
(R2 )
Open pan cooking
0 1 2
4.17 · 104 3.33 · 104 4.00 · 104
0.93 0.97 0.99
Pressure cooking
0 1 2
6.67 · 104 6.50 · 104 5.83 · 104
0.91 0.91 0.91
EcoCookingc
0 1 2
4.83 · 104 4.83 · 104 4.33 · 104
0.91 0.90 0.91
a
Calculated from semi-log plot of (at = a0 ) vs time. The standard error in k values were less than 9 · 107 . c Held for 30 min as per the protocol recommended for cooking with EcoCooker. b
temperature T and time t. Ea is the activation energy of the reaction, R is the gas constant, and A0 is a pre-exponential constant, which are already calculated for steady state heating process. The rate constant kT at each temperature was calculated using Eq. (3) substituting for T from the time–temperature data of unsteady state heating process. Knowing the rate constant kT , the rate (dC=dt), amount degraded (DC) during the short time interval zero to t (¼ DtT ) and the final concentration CtþDt can be calculated as follows: Rate ¼ rate constant (kT ) · initial concentration (Ct ). Amount degraded during DtT ðDCÞ ¼ Rate DtT (kT C DtT ). Concentration after time DtT ¼ Ct DC. These calculations were continued for the entire time period (heating and constant temperatures) at which each cooking process was done. An MS Excel based computer program was used to calculate the above parameters: The P total amount degraded after complete cooking was DC. The final concentration thus will be
P C0 DC, where C0 is the initial value of green colour (a0 ). The resulting predictions and the actual degradation, obtained experimentally, are given in Table 5. As seen, a good agreement between the actual and the predicted degradation/retention of green colour was obtained. The predicted retention for open pan and pressure cooking are higher than the actual values, while the predicted retention for EcoCooking is lower than the predicted value. This may be due to the difference in the way of cooking. In the case of open pan and pressure cooking, the cooking is vigorous due to higher gas flow rates and direct heating. In the case of EcoCooker, the heating rate is very slow and heating is indirect. Using this prediction method, the degradation of visual colour can be predicted for any processing method, if the time–temperature profile of that processing operation is known.
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Table 5 The actual and predicted retention of green colour in the cooking methods Method of cooking
Salt (%)
Actual retention
Predicted retention
Open pan cooking
0 1 2
7.22 7.33 7.77
7.86 7.94 8.06
Pressure cooking
0 1 2
6.33 6.40 6.73
6.9 7.09 7.31
EcoCookinga
0 1 2
5.3 5.37 5.69
4.8 5.00 5.27
a
Held for 30 min as per the protocol recommended for cooking with EcoCooker.
4. Conclusions A stabilization effect of 2% sodium chloride on green colour degradation resulting from heat treatment is confirmed from the kinetic studies, although, the exact mechanism of stabilization is not known. Slow cookers, as exemplified by EcoCooker, show no significant difference in the magnitude of retention as compared to normal open pan and pressure-cooking. Based on the colour retention and fuel savings, an overall judgment in favor of the slow EcoCooker is suggested.
Acknowledgements The authors gratefully acknowledge the financial support provided by Land Research Institute, Mumbai in carrying out this work.
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