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Kinetics of high pressure facilitated starch gelatinisation in Thai glutinous rice
Journal of Food Engineering 79 (2007) 834–841 www.elsevier.com/locate/jfoodeng
Kinetics of high pressure facilitated starch gelatinisation in Thai glutinous rice A. Ahromrit, D.A. Ledward, K. Niranjan
*
School of Food Biosciences, University of Reading, Whiteknights, P.O. Box 226, Reading RG6 6AP, UK Received 1 September 2005; accepted 1 March 2006 Available online 8 May 2006
Abstract The combined effect of pressure and temperature on the rate of gelatinisation of starch present in Thai glutinous rice was investigated. Pressure was found to initiate gelatinisation when its value exceeded 200 MPa at ambient temperature. On the other hand, complete gelatinisation was observed at 500 and 600 MPa at 70 C, when the rice was soaked in water under these conditions for 120 min. A first-order kinetic model describing the rate of gelatinisation was developed to estimate the values of the rate constants as a function of pressure and temperature in the range: 0.1–600 MPa and 20–70 C. The model, based on the well-known Arrhenius and Eyring equations, assumed the form Ea DV ln k ¼ ln k 0 P RT RT
The constants k0, Ea and DV were found to take values: 31.19 s1, 37.89 kJ mol1 and 9.98 cm3 mol1, respectively. It was further noted that the extent of gelatinisation occurring at any time, temperature and pressure, could be exclusively correlated with the grain moisture content. 2006 Elsevier Ltd. All rights reserved. Keywords: Glutinous rice; Waxy rice; High pressure; Starch gelatinisation; Kinetics; Rate constant
1. Introduction Starch gelatinisation normally occurs under the effect of water and heat during the cooking or steaming of rice. Gelatinisation occurs over a range of temperatures and can commence anywhere between 55 and 80 C depending on the rice variety (Bhattacharya, 1979). Further, it occurs in those parts of the grain where the water content is sufficiently high (water to starch ratio P0.75) (Hoseney, 1994). Application of high pressure during soaking increases the quantity of water absorbed by the Thai glutinous rice (Ahromrit, Ledward, & Niranjan, 2006). High pressures are also known to induce starch gelatinisation in rice, just
*
Corresponding author. Tel.: +44 118 9318388; fax: +44 118 9310080. E-mail address: [email protected] (K. Niranjan).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.03.003
as in other materials such as waxy maize, potato, barley and wheat starch (Douzals, Perrier-Cornet, Coquille, & Gervais, 2001; Snauwaert & Heremans, 1999; Stolt & Autio, 1999; Stolt, Oinonen, & Autio, 2001). Recent work of Bauer and Knorr (2005) also demonstrated the combined effect of pressure and heat on the gelatinisation of a variety of starches (potato, wheat and tapioca). The extent of pressure facilitated gelatinisation depended on the pressure applied, moisture content, treatment time and temperature, starch concentration, and the type of starch (Bauer & Knorr, 2004; Stute et al., 1996). In literature, the progress of gelatinisation is generally followed by plotting the water uptake and the extent of starch gelatinisation occurring in a grain as a function of time. A study of the kinetics of starch gelatinisation not only enables modelling the gelatinisation reaction rates, but also provides an insight into reaction mechanisms.
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
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Nomenclature A Ea DH k k0 M P
un-gelatinised starch in rice (%) activation energy (kJ mol1) enthalpy (J g1) rate constant (s1) constant parameter in Eq. (9) (s1) moisture content on a dry weight basis [kg moisture (kg dry matter)1] pressure (MPa)
universal gas constant (8.314 J mol1 K1 or cm3 MPa mol1 K1) temperature (K) activation volume (cm3 mol1)
R T DV
Subscripts i initial t any time
Table 1 Activation energy (Ea) values for rice starch gelatinisation at atmospheric pressure obtained by fitting first-order rate constants at various temperatures to the Arrhenius equation (Eq. (3)) Materials
Temperature (C)
Ea (kJ mol1)
References
White rice
Below 75 Above 75 Below 63 Above 63 60–75 Below 85 Above 85 Below 70 Above 70 Below 85 Above 85 73–92 Below 76 Above 76 59–72
83 34 63 11 38 77 44 287 30 187 99 42 306 43 385
Suzuki et al. (1976)
White rice (Taiwan rice) White rice (Basmati) Brown rice (short grain: S6) Rice flour (long grain: Irga) Rice starch Rice starch Rice starch Waxy rice starch (TCW 70)
Most earlier studies on the kinetics of gelatinisation assumed that the reaction followed first-order kinetics (Juliano & Perez, 1986; Okechukwu & Rao, 1996; Zanoni, Schiraldi, & Simonetta, 1995). On the other hand, Lund and Wirakartakusumah (1984) reported that gelatinisation followed the first-order kinetics only beyond a certain degree of gelatinisation. The effect of temperature on gelatinisation kinetics has often been accounted for, by applying the Arrhenius model. In this context, a number of authors have reported widely differing activation energy values depending on the rice variety and the range of temperatures studied (Table 1). There is insufficient information on gelatinisation kinetics, when the reaction is
Lin (1993) Ramesh (2001) Bakshi and Singh (1980) Ojeda et al. (2000) Birch and Priestley (1973) Kubota et al. (1979) Yeh and Li (1996) Lai and Lii (1999)
brought about by the combined effect of pressure and temperature. There are however a number of studies relating to the combined effect of pressure and temperature on the inactivation kinetics of enzymes and microorganisms (Borda, Indrawati, Smout, Loey, & Hendrickx, 2004; Ly Nguyen, Van Loey, Fachin, Verlent, & Hendrickx, 2002). In such cases, the activation volume has to be considered in addition to the activation energy. Typical activation volume values for enzyme inactivation are given in Table 2. The principal objective of this paper is to discuss the kinetics of starch gelatinisation in Thai glutinous rice occurring under the combined effects of pressure and temperature.
Table 2 Reported values of activation energy (Ea) and activation volume (DV) for enzyme inactivation under the combined application of pressure and temperature Inactivation
Pressure range (MPa)
Ea (kJ mol1)
Temperature range (C)
DV (cm3 mol1)
References
Polyphenoloxidase in avocado Pectinesterase in orange juice Pectinmethylesterase in orange juice
0.1–900 0.1–850 400–600
319–58 327–64 30–14
25–72 20–57 25–50
36 to 14 36 to 10 35 to 31
Weemaes et al. (1998) Van den Broeck et al. (2000) Nienaber and Shellhammer (2001)
Ea values are obtained by fitting first-order rate constants obtained at different temperatures at a given pressure (Eq. (3)). Likewise, the activation volume, DV is obtained by fitting rate constant data at a given temperature to Eq. (4).
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A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
2. Material and methods 2.1. Materials Commercially available milled Thai glutinous rice, Oryza sativa L. Indica was purchased from a UK based rice supplier, and mixed in a ribbon blade mixer for 5 min. The mixed product was divided into 1 kg portions and vacuum packaged. The bags were then stored at 4 C. Before use, the stored samples were allowed to equilibrate at ambient temperature. Prior to the experiments, foreign matter and damaged grains were removed by hand. The average moisture content of the raw grain was found to be 14% ± 0.1 on a dry basis. All experiments were carried out with the grains in their original dry state. Deionised water was used in all experiments. 2.2. High pressure treatment For each experiment, twenty grams of rice grains were packed in a cryovacTM polyethylene pouch (2.5 cm · 15 cm) together with 100 ml of deionised water, and stirred for 10 s. The pack was sealed such that the headspace in the pouch was kept to a minimum. The packaged sample was then transferred to a temperature-controlled pressure vessel system (vessel dimensions: 37 mm diameter and 246 mm length, Food LAB 900, Stansted Fluid Power Ltd., Stansted, UK). A mixture of castor oil and ethanol (80:20 v/ v) served as the pressure-transmitting medium. After subjecting the pouch to the specified conditions of pressure and temperature for the stipulated time, it was removed from the pressure vessel and drained. The treated grains were lightly blotted using filter paper and freeze-dried (Stokes Freeze Dryer, Model 800-001-5, F.J. Stokes Corp., Philadelphia) to a moisture content less than 5% (dry basis). The experimental design constituted six levels of pressure (100–600 MPa), four levels of temperature (20– 70 C) and four levels of time (45–120 min). Experiments at atmospheric pressure (0.1 MPa), set under the same conditions of holding time and temperature, constituted controls. All experiments were duplicated. 2.3. Determination of moisture content The moisture content of treated grains was determined on a wet weight basis, by using a Sartorious MA 40 moisture analyzer (Sartorious, Goettingen, Germany). Approximately 5 g of the treated sample was dried in the analyzer to constant weight at 102 C, and the moisture content was expressed on a dry weight basis (M). 2.4. Differential scanning calorimetry (DSC) The freeze-dried treated grains as well as the raw grains were milled in a Laboratory miller (PERTEN 3100, Huddinge, Sweden). A suspension of the resulting rice flour and deionised water (1:4 w/w) was equilibrated for 18–20 h at
ambient temperature before measuring the percentage of un-gelatinised starch using a Perkin–Elmer DSC-7 differential scanning calorimeter, equipped with a TAC/7 DX thermal analysis data station (Perkin–Elmer, St Quentin en Yvelines, France). A sample of the suspension (10–15 mg) was taken in an aluminium sample pan and hermetically sealed. The scanning temperature range and the heating rate were set to: 25–90 C and 10 C per min, respectively, to yield the energy for gelatinising and the portion of ungelatinised starch in the sample. An empty pan was used as a reference. The gelatinisation energy (DH) measured was converted into J g1 of dry weight. The data were averaged over a minimum of four replicates for each sample, and mean values were deduced. The maximum deviation from mean values were less than 1.5%. The percentage of un-gelatinised starch was determined by comparing the enthalpy change of the soaked rice (DHsoaked) with that of the raw glutinous rice (DHraw) (Riva, Fessas, & Schiraldi, 2000): At ¼ Un-gelatinised starch ð%Þ ¼ ðDH soaked =DH raw Þ 100
ð1Þ
2.5. Analysis of gelatinisation kinetics Following Bakshi and Singh (1980) and Lund and Wirakartakusumah (1984), the extent of starch gelatinisation was described by a first-order kinetic model where the percentage of un-gelatinised starch was assumed to decrease log-linearly as a function of time. In other words: ln
At ¼ kt Ai
ð2Þ
where Ai is the initial percentage of un-gelatinised starch taken to be 100%; At is the percentage after soaking for time t; and k is the first-order rate constant (s1). The value of k was determined by plotting the left hand side of Eq. (2) against the treatment time, for a given combination of pressure and temperature, covering the range: 0.1–600 MPa and 20–70 C. The temperature dependence of the first-order rate constant (k) at a given pressure, expressed with the help of activation energy (Ea), was estimated by using the Arrhenius equation (Bakshi & Singh, 1980; Turhan & Gunasekaran, 2002): ln k ¼ ln k T
Ea RT
ð3Þ
The measure of the pressure dependence of k at a given temperature, activation volume (DV), was estimated by using the Eyring equation (Van den Broeck, Ludikhuyze, Van Loey, & Hendrickx, 2000; Weemaes, Ludikhuyze, Broeck, & Hendrickx, 1998): ln k ¼ ln k P
ðDV ÞP RT
ð4Þ
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
The activation energy and the activation volume were estimated by linear-regression of the experimental data with the above equations. It may be noted that kT and kP are constants (s1), T is the absolute temperature (K), P is pressure (MPa), R is the universal gas constant (8.314 J mol1 K1 or cm3 MPa mol1 K1), Ea is the activation energy (kJ mol1), and DV is the activation volume (cm3 mol1).
3. Results and discussion 3.1. Extent of gelatinisation of pressure-soaked Thai glutinous rice grains As mentioned earlier, the glutinous rice grains together with deionised water were pressurised at 20, 50, 60 and 70 C for 45, 60, 90 and 120 min. The percentage of gelatinised starch at various pressures, temperatures and times is shown in Figs. 1(a)–(d). When gelatinisation was observed to occur, its degree was found to increase with soaking time at constant temperature and pressure until the gelatinisation
837
was complete. No gelatinisation was observed at pressures below 300 MPa and temperatures of 20 and 50 C, even after soaking for 120 min. The degree of gelatinisation also increased linearly with pressure at a fixed temperature, and the rate of the gelatinisation increased with increasing temperature as expected. Figs. 2(a) and (b) illustrate these trends for soaking times of 45 and 90 min, respectively. This is consistent with earlier reports by Bakshi and Singh (1980) and Lund and Wirakartakusumah (1984). Fig. 3 shows effect of moisture content (M) on the degree of gelatinisation for all soaking times, pressures and temperatures. It is clear that gelatinisation commences at a moisture content around 0.6 g water per g dry solids, and it is nearly complete at moisture content around 3.3 g water per g dry solids. Fig. 3 also suggests that the degree of gelatinisation can be exclusively correlated with moisture content, for all pressures, temperatures and soaking times. These parameters only influence gelatinisation to the extent that they influence the moisture uptake. The following correlation can be deduced based on 112 data points shown in the figure (r2 = 0.95):
50
Degree of starch gelatinisation (%)
Degree of starch gelatinisation (%)
100
40
30
20
10
0
80
60
40
20
0
45
60
75
90
105
120
45
135
60
75
0.1 MPa 400 MPa
(a)
100 MPa 500 MPa
90
105
120
135
Time (min)
Time (min) 200 MPa 600 MPa
300 MPa
0.1 MPa 400 MPa
(c)
100 MPa 500 MPa
200 MPa 600 MPa
300 MPa
80 Degree of starch gelatinisation (%)
Degree of starch gelatinisation (%)
100
60
40
20
0
80
60
40
20
0
45
60
75
90
105
120
45
135
60
75
Time (min)
(b)
0.1 MPa
100 MPa
200 MPa
400 MPa
500 MPa
600 MPa
90
105
120
135
Time (min) 300 MPa
(d)
0.1 MPa 400 MPa
100 MPa 500 MPa
200 MPa 600 MPa
300 MPa
Fig. 1. Effect of soaking time on the degree of gelatinisation at different pressures: (a) temperature = 20 C; (b) temperature = 50 C; (c) temperature = 60 C; (d) temperature = 70 C.
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Degree of starch gelatinisation ð%Þ ¼ 30:1ðM t Þ 16:0
Degree of starch gelatinisation (%)
100
ð5Þ
90 80 70
3.2. A kinetic model for pressure induced gelatinisation of glutinous rice
60 50 40 30 20 10 0 0
100
200
300
400
500
600
700
The extent of starch gelatinisation can be described by a first-order kinetic model. The first-order rate constant (k) is determined from the slope of the plot of ln At/Ai versus t (Eq. (2)). The temperature dependence of the rate constant is generally correlated by the Arrhenius equation (Eq. (3)), whereas the pressure dependence is correlated by the Eyring equation (Eq. (4)).
Pressure (MPa) 20°C
(a)
50°C
60°C
70°C
Degree of starch gelatinisation (%)
100 90 80 70 60 50 40 30 20 10 0 0
100
200
20°C
(b)
300 400 Pressure (MPa)
50°C
60°C
500
600
700
70°C
3.2.1. Temperature dependence of the rate constant at atmospheric pressure Fig. 4 shows that the rate of decrease of un-gelatinised starch concentration follows first-order kinetics at atmospheric pressure (Temperature = 60 C and 70 C). It is also clear that no gelatinisation occurs at 20 and 50 C. At higher temperatures, the rate of decrease is greater, which indicates higher values of k (data shown on row 1, Table 3). This set of data is consistent with observations made on the gelatinisation of whole grains at atmospheric pressure (Sayar, Turhan, & Gunasekaran, 2001; Turhan & Gunasekaran, 2002). The k values for brown rice reported by Bakshi and Singh (1980) are higher than those reported here, indicating that it is more difficult to gelatinise starch in glutinous rice. Further, Bakshi and Singh (1980) were able to report a k value at 50 C for brown rice, whereas glutinous rice showed no significant gelatinisation occurring at this temperature.
Fig. 2. Effect of pressure and temperature on the degree of gelatinisation: (a) after 45 min soaking, (b) after 90 min soaking. 0
-0.1
100
ln (At /Ai )
Degree of starch gelatinisation (%)
120
80
-0.2
-0.3
60
-0.4 40
20
% gelatinisation = 30.1 ( M t ) - 16.0 r 2 = 0.95
-0.5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (second)
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
20°C
50°C
60°C
70°C
Moisture content (g water/ g dry solids)
Fig. 3. Effect of moisture content at any time, pressure and temperature, on the degree of gelatinisation.
Fig. 4. Transient variation of the ratio of the percentage of un-gelatinised starch present at any given time (At) to that present initially (Ai), at various temperatures (P = 0.1 MPa).
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841 Table 3 First-order rate constants (k · 104 s1, Eq. (2)) at various temperatures and pressures
-7 -7.5
Temperature (C)
0.1 100 200 300 400 500 600
20
50
60
70
n/a n/a n/a 0.255 0.398 0.513 0.625
n/a n/a n/a 0.438 0.630 1.258 1.529
0.282 0.425 0.511 0.702 1.107 1.780 2.361
0.599 1.025 1.444 2.136 3.425 5.714 6.039
-8 -8.5 ln k
Pressure (MPa)
839
-9 -9.5 -10
n/a = no gelatinisation was observed under the experimental conditions.
-10.5 -11
3.2.2. Effect of pressure on the rate constant Fig. 5 shows how un-gelatinised starch concentration decreases as a function of time at various pressures, when the temperature is held constant at 20 C. The starch, at this temperature, appears to be stable and resists gelatinisation until a pressure of 200 MPa. At pressures of 300 MPa and greater, the change in un-gelatinised starch concentration follows the first-order kinetics with k values increasing progressively at elevated pressures (Table 3, column 2). Further, at this temperature, gelatinisation was incomplete even at 600 MPa. This is unlike the case of wheat starch where complete gelatinisation was reported at 600 MPa (Douzals, Cornet, Coquille, & Gervais, 1996). Potato starch, on the other hand, was reported to undergo complete gelatinisation at 800 MPa at 20 C (Kudla & Tomasik, 1992). From Table 3, it is clear that k values increase with increasing pressure and temperature. Fig. 6 shows a plot of ln k against 1/T (K1) at various pressures (P P 300 MPa). By fitting Eq. (3) to the plot, the constants kT and Ea were determined. These values are reported in
0
ln (At /Ai )
-0.1
-0.2
-0.3
-0.4
-0.5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (second) 0.1 MPa
100 MPa 400 MPa
200 MPa 500 MPa
300 MPa 600 MPa
Fig. 5. Transient variation of the ratio of the percentage of un-gelatinised starch present at any given time (At) to that present initially (Ai), at various pressures (temperature = 20 C).
0.0028
0.0029
0.0030
0.0031
0.0032
0.0033
0.0034
0.0035
1/T (K -1 ) 300 MPa
400 MPa
500 MPa
600 MPa
Fig. 6. Temperature dependence of k, represented by an Arrhenius type plot, at different pressures.
Table 4 Values of constants kT, and Ea at various pressures, obtained by fitting relevant data to Eq. (3) Pressure (MPa)
kT (s1)
Ea (kJ mol1)
r2
300 400 500 600
5.609 9.828 50.826 69.276
30.456 30.796 33.980 34.240
0.77 0.75 0.88 0.99
Table 4 together with the corresponding r2 values. The relatively low values of r2 (especially at 300 and 400 MPa) are essentially due to the inclusion of k values at the lower end of the temperature scale (i.e. 50 and 20 C) for all the pressures investigated; these values make the plot deviate from the well-established linear form. The Arrhenius plot shown in Fig. 6 appears to suggest that there are two regimes: when the temperature is greater than 50 C, the temperature has a predominant influence on the extent of gelatinisation, whereas, at lower temperatures, it is the effect of pressure that dominates. Regardless, Table 4 shows that both kT and Ea increase with pressure. However, over the range of pressures studied, Ea only increases marginally whereas the corresponding increase in kT is greater than tenfold, an observation also reported by Borda et al. (2004). In order to check the validity of Eyring’s model, ln k was plotted against pressure at various temperatures (Fig. 7). Table 5 lists the values of kP and DV. Table 5 also indicates that DV does not show any clear trend in relation to its variation with temperature, although it does not appear to vary greatly over the range studied. A similar lack of trend in the variation of DV with temperature has also been reported by Weemaes et al. (1998), Van den Broeck et al. (2000) and Tan et al. (2005).
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A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841 7
-6
6 k (experimental)
-7
ln k
-8
-9
5 4 3 2 1
-10
0 0
-11
1
2
3
4
5
k (calculated)
-12 0
100
200
300
400
500
600
700
Fig. 8. Parity plot comparing experimental values of k with those calculated using Eq. (9) (r2 = 0.86).
Pressure (MPa) 20°C
50°C
60°C
70°C
Fig. 7. Pressure dependence of ln k at different temperatures.
Table 5 Values of constants kP and DV obtained by fitting relevant data to Eq. (4) Temperature (C)
kP (·105 s1)
DV (cm3 mol1)
r2
20 50 60 70
1.131 1.156 2.770 6.560
7.064 11.816 9.690 11.122
0.97 0.96 0.99 0.99
3.2.3. The combined effect of pressure and temperature on the rate constant In order to obtain a general equation relating k with P and T, the following differential form of Eqs. (3) and (4) must be considered: o ln k Ea ¼ ð6Þ oT P RT 2 o ln k DV ð7Þ ¼ oP T RT The above equations (Eqs. (6) and (7)) can be combined to give: d ln k ¼
Ea DV dP dT RT RT 2
ð8Þ
In general, Ea can be a function of pressure, and DV can be a function of temperature. However, based on the discussion given in Section 3.2.2, Ea and DV can be assumed to be constants, and Eq. (8) can be integrated to give: Ea DV ln k ¼ ln k 0 P ð9Þ RT RT where ln k0 is the constant of integration. Instead of using mean values for Ea and DV from Tables 4 and 5, the values of k obtained under different conditions of temperature and pressure (Table 3), were fitted to Eq. (9) using multiple linear regression (MINITAB Release 14, Minitab Inc.,
Pennsylvania, USA) to deduce the following values of the constants: k0 = 31.19 s1; Ea = 37.895 kJ mol1; and DV = 9.98 cm3 mol1. It is interesting to note that the DV value for gelatinisation kinetics is an order of magnitude smaller than those reported for the kinetics of combined pressure-temperature inactivation of enzymes (see Table 2). A parity plot of the experimental k values against those given by Eq. (9), using the values of k0, Ea and DV deduced above, is shown in Fig. 8. Eq. (9) can be therefore be used to estimated the gelatinisation rate constant at any pressure and temperature within the range of variables covered in this work. 4. Conclusions Glutinous rice is not as readily gelatinised as other varieties of rice, but an appropriate combination of pressure and temperature can be used to facilitate gelatinisation. Gelatinisation was not observed at pressures below 300 MPa, and temperatures of 20 and 50 C. On the other hand, complete gelatinisation was observed at 500 and 600 MPa, temperature of 70 C and soaking time of 120 min. The degree of gelatinisation (%) occurring at any pressure, temperature and time, shows a high correlation with the moisture content (Mt) of the grain prevailing under those conditions. The rate of gelatinisation, characterised by the transient variation of the ratio of the percentage of un-gelatinised starch present at any given time to that present initially, follows the first-order kinetics at any given temperature and pressure. The rate constant k can be correlated with temperature and pressure by combining Arrhenius model with Eyring model. Acknowledgements Author A. Ahromrit gratefully acknowledges the Royal Thai Government for the financial assistance and Khon Kaen University for all the support.
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
References Ahromrit, A., Ledward, D. A., & Niranjan, K. (2006). High pressure induced water uptake characteristics of Thai glutinous rice. Journal of Food Engineering, 72(3), 225–233. Bakshi, A. S., & Singh, R. P. (1980). Kinetics of water diffusion and starch gelatinisation during rice parboiling. Journal of Food Science, 45, 1387–1392. Bauer, B. A., & Knorr, D. (2004). Electrical conductivity: a new tool for the determination of high hydrostatic pressure-induced starch gelatinisation. Innovative Food Science and Emerging Technologies, 5, 437–442. Bauer, B. A., & Knorr, D. (2005). The impact of pressure, temperature and treatment time on starches: pressure-induced starch gelatinisation as pressure time temperature indicator for high hydrostatic pressure processing. Journal of Food Engineering, 68(3), 329–334. Bhattacharya, K. R. (1979). Gelatinisation temperature of rice starch and its determination. In N. C. Brady (Ed.), Proceedings of the workshop on chemical aspects of rice grain quality (pp. 231–249). Laguna: International Rice Research Institution. Birch, G. G., & Priestley, R. J. (1973). Degree of gelatinisation of cooked rice. Starch/Starke, 25, 98–100. Borda, D., Indrawati Smout, C., Loey, A. V., & Hendrickx, M. (2004). High pressure thermal inactivation kinetics of a plasmin system. Journal of Dairy Science, 87, 2351–2358. Douzals, J. P., Cornet, J. M. P., Coquille, J. C., & Gervais, P. (1996). Microscopic study of starch gelatinisation under high hydrostatic pressure. Journal of Agricultural and Food Chemistry, 44(6), 1403–1408. Douzals, J. P., Perrier-Cornet, J. M., Coquille, J. C., & Gervais, P. (2001). Pressure-temperature phase transition diagram for wheat starch. Journal of Agricultural and Food Chemistry, 49(2), 873–876. Hoseney, R. C. (1994). Principles of cereal science and technology (pp. 29– 38). Minnesota: American Association of Cereal Chemists, Inc. Juliano, B. O., & Perez, C. M. (1986). Kinetics studies on cooking of tropical milled rice. Food Chemistry, 20, 97–105. Kubota, K., Hosokawa, Y., Suziki, K., & Hosaka, H. (1979). Studies on the gelatinisation rate of rice and potato starches. Journal of Food Science, 44, 1394–1397. Kudla, E., & Tomasik, P. (1992). The modification of starch by high pressure. Part I: air and oven dried potato starch. Starch/Starke, 44, 167–173. Lai, V. M. F., & Lii, C. Y. (1999). Effects of modulated differential scanning calorimetry (MDSC) variables on thermodynamic and kinetic characteristics during gelatinisation of waxy rice starch. Cereal Chemistry, 76(4), 519–525. Lin, S. H. (1993). Water uptake and gelatinisation of white rice. Lebensmittel-Wissenschaft und Technolologie, 26(3), 276–278. Lund, D. B., & Wirakartakusumah, M. A. (1984). A model for starch gelatinisation phenomena. In B. M. Mckenna (Ed.). Engineering and food (vol. 1, pp. 425–432). Amsterdam: Elsevier Publishers, Inc. Ly Nguyen, B., Van Loey, A., Fachin, D., Verlent, I., & Hendrickx, I. M. (2002). Purification, characterization, thermal, and high-pressure
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inactivation of pectin methyesterase from bananas (cv Cavendish). Biotechnology and Bioengineering, 78(6), 683–691. Nienaber, U., & Shellhammer, T. H. (2001). High-pressure processing of orange juice: kinetics of pectinmethylesterase inactivation. Journal of Food Science, 66(2), 328–331. Ojeda, C. A., Tolaba, M. P., & Suarez, C. (2000). Modelling starch gelatinisation kinetics of milled rice flour. Cereal Chemistry, 77(2), 145–147. Okechukwu, P. E., & Rao, M. A. (1996). Kinetics of cowpea starch gelatinisation based on granules swelling. Starch/Starke, 48, 43–47. Ramesh, M. (2001). An application of image analysis for the study of kinetics of hydration of milled rice in hot water. International Journal of Food Properties, 4(2), 271–284. Riva, M., Fessas, D., & Schiraldi, A. (2000). Starch retrogradation in cooked pasta and rice. Cereal Chemistry, 77(4), 433–438. Sayar, S., Turhan, M., & Gunasekaran, S. (2001). Analysis of chickpea soaking by simultaneous water transfer and water–starch reaction. Journal of Food Engineering, 50, 91–98. Snauwaert, J., & Heremans, K. (1999). Pressure induced swelling kinetics of starch granules. In H. Ludwig (Ed.), Advances in high pressure bioscience and biotechnology (pp. 349–352). Berlin, Heidelberg: Springer-Verlag. Stolt, M., & Autio, K. (1999). Microscopic and rheological characterization of high pressure treated starch dispersions. In H. Ludwig (Ed.), Advances in high pressure bioscience and biotechnology (pp. 357–360). Berlin, Heidelberg: Springer-Verlag. Stolt, M., Oinonen, S., & Autio, K. (2001). Effect of high pressure on the physical properties of barley starch. Innovative Food Science and Emerging Technologies, 1, 167–175. Stute, R., Heilbronn, R., Klinger, W., Boguslawski, S., Eshtiaghi, M. N., & Knorr, D. (1996). Effects of high pressure treatment on starches. Starch/Starke, 48, 399–408. Suzuki, K., Kubota, K., Omichi, M., & Hosaka, H. (1976). Kinetics studies on cooking of rice. Journal of Food Science, 41, 1180–1183. Tan, C.-Y., Xu, C.-H., Wong, J., Shen, J.-R., Sakuma, S., Yamamoto, Y., et al. (2005). Pressure equilibrium and jump study on unfolding of 23kDa protein from spinach photosystem II. Biophysical Journal, 88, 1264–1275. Turhan, M., & Gunasekaran, S. (2002). Kinetics of in situ and in vitro gelatinisation of hard and soft wheat starches during cooking in water. Journal Food Engineering, 52, 1–7. Van den Broeck, I., Ludikhuyze, L. R., Van Loey, A. M., & Hendrickx, M. E. (2000). Inactivation of orange pectinesterase by combined highpressure and-temperature treatments: a kinetic study. Journal of Agricultural and Food Chemistry, 48, 1960–1970. Weemaes, C. A., Ludikhuyze, L. R., Broeck, I. V., & Hendrickx, M. E. (1998). Kinetics of combined pressure-temperature inactivation avocado polyphenoloxidase. Biotechnology Bioengineering, 60(3), 292–300. Yeh, A., & Li, J. Y. (1996). A continuous measurement of swelling of rice starch during heating. Journal of Cereal Science, 23, 277–283. Zanoni, B., Schiraldi, A., & Simonetta, R. (1995). A naive model of starch gelatinisation kinetics. Journal of Food Engineering, 24, 25–33.
Kinetics of high pressure facilitated starch gelatinisation in Thai glutinous rice A. Ahromrit, D.A. Ledward, K. Niranjan
*
School of Food Biosciences, University of Reading, Whiteknights, P.O. Box 226, Reading RG6 6AP, UK Received 1 September 2005; accepted 1 March 2006 Available online 8 May 2006
Abstract The combined effect of pressure and temperature on the rate of gelatinisation of starch present in Thai glutinous rice was investigated. Pressure was found to initiate gelatinisation when its value exceeded 200 MPa at ambient temperature. On the other hand, complete gelatinisation was observed at 500 and 600 MPa at 70 C, when the rice was soaked in water under these conditions for 120 min. A first-order kinetic model describing the rate of gelatinisation was developed to estimate the values of the rate constants as a function of pressure and temperature in the range: 0.1–600 MPa and 20–70 C. The model, based on the well-known Arrhenius and Eyring equations, assumed the form Ea DV ln k ¼ ln k 0 P RT RT
The constants k0, Ea and DV were found to take values: 31.19 s1, 37.89 kJ mol1 and 9.98 cm3 mol1, respectively. It was further noted that the extent of gelatinisation occurring at any time, temperature and pressure, could be exclusively correlated with the grain moisture content. 2006 Elsevier Ltd. All rights reserved. Keywords: Glutinous rice; Waxy rice; High pressure; Starch gelatinisation; Kinetics; Rate constant
1. Introduction Starch gelatinisation normally occurs under the effect of water and heat during the cooking or steaming of rice. Gelatinisation occurs over a range of temperatures and can commence anywhere between 55 and 80 C depending on the rice variety (Bhattacharya, 1979). Further, it occurs in those parts of the grain where the water content is sufficiently high (water to starch ratio P0.75) (Hoseney, 1994). Application of high pressure during soaking increases the quantity of water absorbed by the Thai glutinous rice (Ahromrit, Ledward, & Niranjan, 2006). High pressures are also known to induce starch gelatinisation in rice, just
*
Corresponding author. Tel.: +44 118 9318388; fax: +44 118 9310080. E-mail address: [email protected] (K. Niranjan).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.03.003
as in other materials such as waxy maize, potato, barley and wheat starch (Douzals, Perrier-Cornet, Coquille, & Gervais, 2001; Snauwaert & Heremans, 1999; Stolt & Autio, 1999; Stolt, Oinonen, & Autio, 2001). Recent work of Bauer and Knorr (2005) also demonstrated the combined effect of pressure and heat on the gelatinisation of a variety of starches (potato, wheat and tapioca). The extent of pressure facilitated gelatinisation depended on the pressure applied, moisture content, treatment time and temperature, starch concentration, and the type of starch (Bauer & Knorr, 2004; Stute et al., 1996). In literature, the progress of gelatinisation is generally followed by plotting the water uptake and the extent of starch gelatinisation occurring in a grain as a function of time. A study of the kinetics of starch gelatinisation not only enables modelling the gelatinisation reaction rates, but also provides an insight into reaction mechanisms.
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
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Nomenclature A Ea DH k k0 M P
un-gelatinised starch in rice (%) activation energy (kJ mol1) enthalpy (J g1) rate constant (s1) constant parameter in Eq. (9) (s1) moisture content on a dry weight basis [kg moisture (kg dry matter)1] pressure (MPa)
universal gas constant (8.314 J mol1 K1 or cm3 MPa mol1 K1) temperature (K) activation volume (cm3 mol1)
R T DV
Subscripts i initial t any time
Table 1 Activation energy (Ea) values for rice starch gelatinisation at atmospheric pressure obtained by fitting first-order rate constants at various temperatures to the Arrhenius equation (Eq. (3)) Materials
Temperature (C)
Ea (kJ mol1)
References
White rice
Below 75 Above 75 Below 63 Above 63 60–75 Below 85 Above 85 Below 70 Above 70 Below 85 Above 85 73–92 Below 76 Above 76 59–72
83 34 63 11 38 77 44 287 30 187 99 42 306 43 385
Suzuki et al. (1976)
White rice (Taiwan rice) White rice (Basmati) Brown rice (short grain: S6) Rice flour (long grain: Irga) Rice starch Rice starch Rice starch Waxy rice starch (TCW 70)
Most earlier studies on the kinetics of gelatinisation assumed that the reaction followed first-order kinetics (Juliano & Perez, 1986; Okechukwu & Rao, 1996; Zanoni, Schiraldi, & Simonetta, 1995). On the other hand, Lund and Wirakartakusumah (1984) reported that gelatinisation followed the first-order kinetics only beyond a certain degree of gelatinisation. The effect of temperature on gelatinisation kinetics has often been accounted for, by applying the Arrhenius model. In this context, a number of authors have reported widely differing activation energy values depending on the rice variety and the range of temperatures studied (Table 1). There is insufficient information on gelatinisation kinetics, when the reaction is
Lin (1993) Ramesh (2001) Bakshi and Singh (1980) Ojeda et al. (2000) Birch and Priestley (1973) Kubota et al. (1979) Yeh and Li (1996) Lai and Lii (1999)
brought about by the combined effect of pressure and temperature. There are however a number of studies relating to the combined effect of pressure and temperature on the inactivation kinetics of enzymes and microorganisms (Borda, Indrawati, Smout, Loey, & Hendrickx, 2004; Ly Nguyen, Van Loey, Fachin, Verlent, & Hendrickx, 2002). In such cases, the activation volume has to be considered in addition to the activation energy. Typical activation volume values for enzyme inactivation are given in Table 2. The principal objective of this paper is to discuss the kinetics of starch gelatinisation in Thai glutinous rice occurring under the combined effects of pressure and temperature.
Table 2 Reported values of activation energy (Ea) and activation volume (DV) for enzyme inactivation under the combined application of pressure and temperature Inactivation
Pressure range (MPa)
Ea (kJ mol1)
Temperature range (C)
DV (cm3 mol1)
References
Polyphenoloxidase in avocado Pectinesterase in orange juice Pectinmethylesterase in orange juice
0.1–900 0.1–850 400–600
319–58 327–64 30–14
25–72 20–57 25–50
36 to 14 36 to 10 35 to 31
Weemaes et al. (1998) Van den Broeck et al. (2000) Nienaber and Shellhammer (2001)
Ea values are obtained by fitting first-order rate constants obtained at different temperatures at a given pressure (Eq. (3)). Likewise, the activation volume, DV is obtained by fitting rate constant data at a given temperature to Eq. (4).
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A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
2. Material and methods 2.1. Materials Commercially available milled Thai glutinous rice, Oryza sativa L. Indica was purchased from a UK based rice supplier, and mixed in a ribbon blade mixer for 5 min. The mixed product was divided into 1 kg portions and vacuum packaged. The bags were then stored at 4 C. Before use, the stored samples were allowed to equilibrate at ambient temperature. Prior to the experiments, foreign matter and damaged grains were removed by hand. The average moisture content of the raw grain was found to be 14% ± 0.1 on a dry basis. All experiments were carried out with the grains in their original dry state. Deionised water was used in all experiments. 2.2. High pressure treatment For each experiment, twenty grams of rice grains were packed in a cryovacTM polyethylene pouch (2.5 cm · 15 cm) together with 100 ml of deionised water, and stirred for 10 s. The pack was sealed such that the headspace in the pouch was kept to a minimum. The packaged sample was then transferred to a temperature-controlled pressure vessel system (vessel dimensions: 37 mm diameter and 246 mm length, Food LAB 900, Stansted Fluid Power Ltd., Stansted, UK). A mixture of castor oil and ethanol (80:20 v/ v) served as the pressure-transmitting medium. After subjecting the pouch to the specified conditions of pressure and temperature for the stipulated time, it was removed from the pressure vessel and drained. The treated grains were lightly blotted using filter paper and freeze-dried (Stokes Freeze Dryer, Model 800-001-5, F.J. Stokes Corp., Philadelphia) to a moisture content less than 5% (dry basis). The experimental design constituted six levels of pressure (100–600 MPa), four levels of temperature (20– 70 C) and four levels of time (45–120 min). Experiments at atmospheric pressure (0.1 MPa), set under the same conditions of holding time and temperature, constituted controls. All experiments were duplicated. 2.3. Determination of moisture content The moisture content of treated grains was determined on a wet weight basis, by using a Sartorious MA 40 moisture analyzer (Sartorious, Goettingen, Germany). Approximately 5 g of the treated sample was dried in the analyzer to constant weight at 102 C, and the moisture content was expressed on a dry weight basis (M). 2.4. Differential scanning calorimetry (DSC) The freeze-dried treated grains as well as the raw grains were milled in a Laboratory miller (PERTEN 3100, Huddinge, Sweden). A suspension of the resulting rice flour and deionised water (1:4 w/w) was equilibrated for 18–20 h at
ambient temperature before measuring the percentage of un-gelatinised starch using a Perkin–Elmer DSC-7 differential scanning calorimeter, equipped with a TAC/7 DX thermal analysis data station (Perkin–Elmer, St Quentin en Yvelines, France). A sample of the suspension (10–15 mg) was taken in an aluminium sample pan and hermetically sealed. The scanning temperature range and the heating rate were set to: 25–90 C and 10 C per min, respectively, to yield the energy for gelatinising and the portion of ungelatinised starch in the sample. An empty pan was used as a reference. The gelatinisation energy (DH) measured was converted into J g1 of dry weight. The data were averaged over a minimum of four replicates for each sample, and mean values were deduced. The maximum deviation from mean values were less than 1.5%. The percentage of un-gelatinised starch was determined by comparing the enthalpy change of the soaked rice (DHsoaked) with that of the raw glutinous rice (DHraw) (Riva, Fessas, & Schiraldi, 2000): At ¼ Un-gelatinised starch ð%Þ ¼ ðDH soaked =DH raw Þ 100
ð1Þ
2.5. Analysis of gelatinisation kinetics Following Bakshi and Singh (1980) and Lund and Wirakartakusumah (1984), the extent of starch gelatinisation was described by a first-order kinetic model where the percentage of un-gelatinised starch was assumed to decrease log-linearly as a function of time. In other words: ln
At ¼ kt Ai
ð2Þ
where Ai is the initial percentage of un-gelatinised starch taken to be 100%; At is the percentage after soaking for time t; and k is the first-order rate constant (s1). The value of k was determined by plotting the left hand side of Eq. (2) against the treatment time, for a given combination of pressure and temperature, covering the range: 0.1–600 MPa and 20–70 C. The temperature dependence of the first-order rate constant (k) at a given pressure, expressed with the help of activation energy (Ea), was estimated by using the Arrhenius equation (Bakshi & Singh, 1980; Turhan & Gunasekaran, 2002): ln k ¼ ln k T
Ea RT
ð3Þ
The measure of the pressure dependence of k at a given temperature, activation volume (DV), was estimated by using the Eyring equation (Van den Broeck, Ludikhuyze, Van Loey, & Hendrickx, 2000; Weemaes, Ludikhuyze, Broeck, & Hendrickx, 1998): ln k ¼ ln k P
ðDV ÞP RT
ð4Þ
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
The activation energy and the activation volume were estimated by linear-regression of the experimental data with the above equations. It may be noted that kT and kP are constants (s1), T is the absolute temperature (K), P is pressure (MPa), R is the universal gas constant (8.314 J mol1 K1 or cm3 MPa mol1 K1), Ea is the activation energy (kJ mol1), and DV is the activation volume (cm3 mol1).
3. Results and discussion 3.1. Extent of gelatinisation of pressure-soaked Thai glutinous rice grains As mentioned earlier, the glutinous rice grains together with deionised water were pressurised at 20, 50, 60 and 70 C for 45, 60, 90 and 120 min. The percentage of gelatinised starch at various pressures, temperatures and times is shown in Figs. 1(a)–(d). When gelatinisation was observed to occur, its degree was found to increase with soaking time at constant temperature and pressure until the gelatinisation
837
was complete. No gelatinisation was observed at pressures below 300 MPa and temperatures of 20 and 50 C, even after soaking for 120 min. The degree of gelatinisation also increased linearly with pressure at a fixed temperature, and the rate of the gelatinisation increased with increasing temperature as expected. Figs. 2(a) and (b) illustrate these trends for soaking times of 45 and 90 min, respectively. This is consistent with earlier reports by Bakshi and Singh (1980) and Lund and Wirakartakusumah (1984). Fig. 3 shows effect of moisture content (M) on the degree of gelatinisation for all soaking times, pressures and temperatures. It is clear that gelatinisation commences at a moisture content around 0.6 g water per g dry solids, and it is nearly complete at moisture content around 3.3 g water per g dry solids. Fig. 3 also suggests that the degree of gelatinisation can be exclusively correlated with moisture content, for all pressures, temperatures and soaking times. These parameters only influence gelatinisation to the extent that they influence the moisture uptake. The following correlation can be deduced based on 112 data points shown in the figure (r2 = 0.95):
50
Degree of starch gelatinisation (%)
Degree of starch gelatinisation (%)
100
40
30
20
10
0
80
60
40
20
0
45
60
75
90
105
120
45
135
60
75
0.1 MPa 400 MPa
(a)
100 MPa 500 MPa
90
105
120
135
Time (min)
Time (min) 200 MPa 600 MPa
300 MPa
0.1 MPa 400 MPa
(c)
100 MPa 500 MPa
200 MPa 600 MPa
300 MPa
80 Degree of starch gelatinisation (%)
Degree of starch gelatinisation (%)
100
60
40
20
0
80
60
40
20
0
45
60
75
90
105
120
45
135
60
75
Time (min)
(b)
0.1 MPa
100 MPa
200 MPa
400 MPa
500 MPa
600 MPa
90
105
120
135
Time (min) 300 MPa
(d)
0.1 MPa 400 MPa
100 MPa 500 MPa
200 MPa 600 MPa
300 MPa
Fig. 1. Effect of soaking time on the degree of gelatinisation at different pressures: (a) temperature = 20 C; (b) temperature = 50 C; (c) temperature = 60 C; (d) temperature = 70 C.
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Degree of starch gelatinisation ð%Þ ¼ 30:1ðM t Þ 16:0
Degree of starch gelatinisation (%)
100
ð5Þ
90 80 70
3.2. A kinetic model for pressure induced gelatinisation of glutinous rice
60 50 40 30 20 10 0 0
100
200
300
400
500
600
700
The extent of starch gelatinisation can be described by a first-order kinetic model. The first-order rate constant (k) is determined from the slope of the plot of ln At/Ai versus t (Eq. (2)). The temperature dependence of the rate constant is generally correlated by the Arrhenius equation (Eq. (3)), whereas the pressure dependence is correlated by the Eyring equation (Eq. (4)).
Pressure (MPa) 20°C
(a)
50°C
60°C
70°C
Degree of starch gelatinisation (%)
100 90 80 70 60 50 40 30 20 10 0 0
100
200
20°C
(b)
300 400 Pressure (MPa)
50°C
60°C
500
600
700
70°C
3.2.1. Temperature dependence of the rate constant at atmospheric pressure Fig. 4 shows that the rate of decrease of un-gelatinised starch concentration follows first-order kinetics at atmospheric pressure (Temperature = 60 C and 70 C). It is also clear that no gelatinisation occurs at 20 and 50 C. At higher temperatures, the rate of decrease is greater, which indicates higher values of k (data shown on row 1, Table 3). This set of data is consistent with observations made on the gelatinisation of whole grains at atmospheric pressure (Sayar, Turhan, & Gunasekaran, 2001; Turhan & Gunasekaran, 2002). The k values for brown rice reported by Bakshi and Singh (1980) are higher than those reported here, indicating that it is more difficult to gelatinise starch in glutinous rice. Further, Bakshi and Singh (1980) were able to report a k value at 50 C for brown rice, whereas glutinous rice showed no significant gelatinisation occurring at this temperature.
Fig. 2. Effect of pressure and temperature on the degree of gelatinisation: (a) after 45 min soaking, (b) after 90 min soaking. 0
-0.1
100
ln (At /Ai )
Degree of starch gelatinisation (%)
120
80
-0.2
-0.3
60
-0.4 40
20
% gelatinisation = 30.1 ( M t ) - 16.0 r 2 = 0.95
-0.5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (second)
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
20°C
50°C
60°C
70°C
Moisture content (g water/ g dry solids)
Fig. 3. Effect of moisture content at any time, pressure and temperature, on the degree of gelatinisation.
Fig. 4. Transient variation of the ratio of the percentage of un-gelatinised starch present at any given time (At) to that present initially (Ai), at various temperatures (P = 0.1 MPa).
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841 Table 3 First-order rate constants (k · 104 s1, Eq. (2)) at various temperatures and pressures
-7 -7.5
Temperature (C)
0.1 100 200 300 400 500 600
20
50
60
70
n/a n/a n/a 0.255 0.398 0.513 0.625
n/a n/a n/a 0.438 0.630 1.258 1.529
0.282 0.425 0.511 0.702 1.107 1.780 2.361
0.599 1.025 1.444 2.136 3.425 5.714 6.039
-8 -8.5 ln k
Pressure (MPa)
839
-9 -9.5 -10
n/a = no gelatinisation was observed under the experimental conditions.
-10.5 -11
3.2.2. Effect of pressure on the rate constant Fig. 5 shows how un-gelatinised starch concentration decreases as a function of time at various pressures, when the temperature is held constant at 20 C. The starch, at this temperature, appears to be stable and resists gelatinisation until a pressure of 200 MPa. At pressures of 300 MPa and greater, the change in un-gelatinised starch concentration follows the first-order kinetics with k values increasing progressively at elevated pressures (Table 3, column 2). Further, at this temperature, gelatinisation was incomplete even at 600 MPa. This is unlike the case of wheat starch where complete gelatinisation was reported at 600 MPa (Douzals, Cornet, Coquille, & Gervais, 1996). Potato starch, on the other hand, was reported to undergo complete gelatinisation at 800 MPa at 20 C (Kudla & Tomasik, 1992). From Table 3, it is clear that k values increase with increasing pressure and temperature. Fig. 6 shows a plot of ln k against 1/T (K1) at various pressures (P P 300 MPa). By fitting Eq. (3) to the plot, the constants kT and Ea were determined. These values are reported in
0
ln (At /Ai )
-0.1
-0.2
-0.3
-0.4
-0.5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (second) 0.1 MPa
100 MPa 400 MPa
200 MPa 500 MPa
300 MPa 600 MPa
Fig. 5. Transient variation of the ratio of the percentage of un-gelatinised starch present at any given time (At) to that present initially (Ai), at various pressures (temperature = 20 C).
0.0028
0.0029
0.0030
0.0031
0.0032
0.0033
0.0034
0.0035
1/T (K -1 ) 300 MPa
400 MPa
500 MPa
600 MPa
Fig. 6. Temperature dependence of k, represented by an Arrhenius type plot, at different pressures.
Table 4 Values of constants kT, and Ea at various pressures, obtained by fitting relevant data to Eq. (3) Pressure (MPa)
kT (s1)
Ea (kJ mol1)
r2
300 400 500 600
5.609 9.828 50.826 69.276
30.456 30.796 33.980 34.240
0.77 0.75 0.88 0.99
Table 4 together with the corresponding r2 values. The relatively low values of r2 (especially at 300 and 400 MPa) are essentially due to the inclusion of k values at the lower end of the temperature scale (i.e. 50 and 20 C) for all the pressures investigated; these values make the plot deviate from the well-established linear form. The Arrhenius plot shown in Fig. 6 appears to suggest that there are two regimes: when the temperature is greater than 50 C, the temperature has a predominant influence on the extent of gelatinisation, whereas, at lower temperatures, it is the effect of pressure that dominates. Regardless, Table 4 shows that both kT and Ea increase with pressure. However, over the range of pressures studied, Ea only increases marginally whereas the corresponding increase in kT is greater than tenfold, an observation also reported by Borda et al. (2004). In order to check the validity of Eyring’s model, ln k was plotted against pressure at various temperatures (Fig. 7). Table 5 lists the values of kP and DV. Table 5 also indicates that DV does not show any clear trend in relation to its variation with temperature, although it does not appear to vary greatly over the range studied. A similar lack of trend in the variation of DV with temperature has also been reported by Weemaes et al. (1998), Van den Broeck et al. (2000) and Tan et al. (2005).
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A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841 7
-6
6 k (experimental)
-7
ln k
-8
-9
5 4 3 2 1
-10
0 0
-11
1
2
3
4
5
k (calculated)
-12 0
100
200
300
400
500
600
700
Fig. 8. Parity plot comparing experimental values of k with those calculated using Eq. (9) (r2 = 0.86).
Pressure (MPa) 20°C
50°C
60°C
70°C
Fig. 7. Pressure dependence of ln k at different temperatures.
Table 5 Values of constants kP and DV obtained by fitting relevant data to Eq. (4) Temperature (C)
kP (·105 s1)
DV (cm3 mol1)
r2
20 50 60 70
1.131 1.156 2.770 6.560
7.064 11.816 9.690 11.122
0.97 0.96 0.99 0.99
3.2.3. The combined effect of pressure and temperature on the rate constant In order to obtain a general equation relating k with P and T, the following differential form of Eqs. (3) and (4) must be considered: o ln k Ea ¼ ð6Þ oT P RT 2 o ln k DV ð7Þ ¼ oP T RT The above equations (Eqs. (6) and (7)) can be combined to give: d ln k ¼
Ea DV dP dT RT RT 2
ð8Þ
In general, Ea can be a function of pressure, and DV can be a function of temperature. However, based on the discussion given in Section 3.2.2, Ea and DV can be assumed to be constants, and Eq. (8) can be integrated to give: Ea DV ln k ¼ ln k 0 P ð9Þ RT RT where ln k0 is the constant of integration. Instead of using mean values for Ea and DV from Tables 4 and 5, the values of k obtained under different conditions of temperature and pressure (Table 3), were fitted to Eq. (9) using multiple linear regression (MINITAB Release 14, Minitab Inc.,
Pennsylvania, USA) to deduce the following values of the constants: k0 = 31.19 s1; Ea = 37.895 kJ mol1; and DV = 9.98 cm3 mol1. It is interesting to note that the DV value for gelatinisation kinetics is an order of magnitude smaller than those reported for the kinetics of combined pressure-temperature inactivation of enzymes (see Table 2). A parity plot of the experimental k values against those given by Eq. (9), using the values of k0, Ea and DV deduced above, is shown in Fig. 8. Eq. (9) can be therefore be used to estimated the gelatinisation rate constant at any pressure and temperature within the range of variables covered in this work. 4. Conclusions Glutinous rice is not as readily gelatinised as other varieties of rice, but an appropriate combination of pressure and temperature can be used to facilitate gelatinisation. Gelatinisation was not observed at pressures below 300 MPa, and temperatures of 20 and 50 C. On the other hand, complete gelatinisation was observed at 500 and 600 MPa, temperature of 70 C and soaking time of 120 min. The degree of gelatinisation (%) occurring at any pressure, temperature and time, shows a high correlation with the moisture content (Mt) of the grain prevailing under those conditions. The rate of gelatinisation, characterised by the transient variation of the ratio of the percentage of un-gelatinised starch present at any given time to that present initially, follows the first-order kinetics at any given temperature and pressure. The rate constant k can be correlated with temperature and pressure by combining Arrhenius model with Eyring model. Acknowledgements Author A. Ahromrit gratefully acknowledges the Royal Thai Government for the financial assistance and Khon Kaen University for all the support.
A. Ahromrit et al. / Journal of Food Engineering 79 (2007) 834–841
References Ahromrit, A., Ledward, D. A., & Niranjan, K. (2006). High pressure induced water uptake characteristics of Thai glutinous rice. Journal of Food Engineering, 72(3), 225–233. Bakshi, A. S., & Singh, R. P. (1980). Kinetics of water diffusion and starch gelatinisation during rice parboiling. Journal of Food Science, 45, 1387–1392. Bauer, B. A., & Knorr, D. (2004). Electrical conductivity: a new tool for the determination of high hydrostatic pressure-induced starch gelatinisation. Innovative Food Science and Emerging Technologies, 5, 437–442. Bauer, B. A., & Knorr, D. (2005). The impact of pressure, temperature and treatment time on starches: pressure-induced starch gelatinisation as pressure time temperature indicator for high hydrostatic pressure processing. Journal of Food Engineering, 68(3), 329–334. Bhattacharya, K. R. (1979). Gelatinisation temperature of rice starch and its determination. In N. C. Brady (Ed.), Proceedings of the workshop on chemical aspects of rice grain quality (pp. 231–249). Laguna: International Rice Research Institution. Birch, G. G., & Priestley, R. J. (1973). Degree of gelatinisation of cooked rice. Starch/Starke, 25, 98–100. Borda, D., Indrawati Smout, C., Loey, A. V., & Hendrickx, M. (2004). High pressure thermal inactivation kinetics of a plasmin system. Journal of Dairy Science, 87, 2351–2358. Douzals, J. P., Cornet, J. M. P., Coquille, J. C., & Gervais, P. (1996). Microscopic study of starch gelatinisation under high hydrostatic pressure. Journal of Agricultural and Food Chemistry, 44(6), 1403–1408. Douzals, J. P., Perrier-Cornet, J. M., Coquille, J. C., & Gervais, P. (2001). Pressure-temperature phase transition diagram for wheat starch. Journal of Agricultural and Food Chemistry, 49(2), 873–876. Hoseney, R. C. (1994). Principles of cereal science and technology (pp. 29– 38). Minnesota: American Association of Cereal Chemists, Inc. Juliano, B. O., & Perez, C. M. (1986). Kinetics studies on cooking of tropical milled rice. Food Chemistry, 20, 97–105. Kubota, K., Hosokawa, Y., Suziki, K., & Hosaka, H. (1979). Studies on the gelatinisation rate of rice and potato starches. Journal of Food Science, 44, 1394–1397. Kudla, E., & Tomasik, P. (1992). The modification of starch by high pressure. Part I: air and oven dried potato starch. Starch/Starke, 44, 167–173. Lai, V. M. F., & Lii, C. Y. (1999). Effects of modulated differential scanning calorimetry (MDSC) variables on thermodynamic and kinetic characteristics during gelatinisation of waxy rice starch. Cereal Chemistry, 76(4), 519–525. Lin, S. H. (1993). Water uptake and gelatinisation of white rice. Lebensmittel-Wissenschaft und Technolologie, 26(3), 276–278. Lund, D. B., & Wirakartakusumah, M. A. (1984). A model for starch gelatinisation phenomena. In B. M. Mckenna (Ed.). Engineering and food (vol. 1, pp. 425–432). Amsterdam: Elsevier Publishers, Inc. Ly Nguyen, B., Van Loey, A., Fachin, D., Verlent, I., & Hendrickx, I. M. (2002). Purification, characterization, thermal, and high-pressure
841
inactivation of pectin methyesterase from bananas (cv Cavendish). Biotechnology and Bioengineering, 78(6), 683–691. Nienaber, U., & Shellhammer, T. H. (2001). High-pressure processing of orange juice: kinetics of pectinmethylesterase inactivation. Journal of Food Science, 66(2), 328–331. Ojeda, C. A., Tolaba, M. P., & Suarez, C. (2000). Modelling starch gelatinisation kinetics of milled rice flour. Cereal Chemistry, 77(2), 145–147. Okechukwu, P. E., & Rao, M. A. (1996). Kinetics of cowpea starch gelatinisation based on granules swelling. Starch/Starke, 48, 43–47. Ramesh, M. (2001). An application of image analysis for the study of kinetics of hydration of milled rice in hot water. International Journal of Food Properties, 4(2), 271–284. Riva, M., Fessas, D., & Schiraldi, A. (2000). Starch retrogradation in cooked pasta and rice. Cereal Chemistry, 77(4), 433–438. Sayar, S., Turhan, M., & Gunasekaran, S. (2001). Analysis of chickpea soaking by simultaneous water transfer and water–starch reaction. Journal of Food Engineering, 50, 91–98. Snauwaert, J., & Heremans, K. (1999). Pressure induced swelling kinetics of starch granules. In H. Ludwig (Ed.), Advances in high pressure bioscience and biotechnology (pp. 349–352). Berlin, Heidelberg: Springer-Verlag. Stolt, M., & Autio, K. (1999). Microscopic and rheological characterization of high pressure treated starch dispersions. In H. Ludwig (Ed.), Advances in high pressure bioscience and biotechnology (pp. 357–360). Berlin, Heidelberg: Springer-Verlag. Stolt, M., Oinonen, S., & Autio, K. (2001). Effect of high pressure on the physical properties of barley starch. Innovative Food Science and Emerging Technologies, 1, 167–175. Stute, R., Heilbronn, R., Klinger, W., Boguslawski, S., Eshtiaghi, M. N., & Knorr, D. (1996). Effects of high pressure treatment on starches. Starch/Starke, 48, 399–408. Suzuki, K., Kubota, K., Omichi, M., & Hosaka, H. (1976). Kinetics studies on cooking of rice. Journal of Food Science, 41, 1180–1183. Tan, C.-Y., Xu, C.-H., Wong, J., Shen, J.-R., Sakuma, S., Yamamoto, Y., et al. (2005). Pressure equilibrium and jump study on unfolding of 23kDa protein from spinach photosystem II. Biophysical Journal, 88, 1264–1275. Turhan, M., & Gunasekaran, S. (2002). Kinetics of in situ and in vitro gelatinisation of hard and soft wheat starches during cooking in water. Journal Food Engineering, 52, 1–7. Van den Broeck, I., Ludikhuyze, L. R., Van Loey, A. M., & Hendrickx, M. E. (2000). Inactivation of orange pectinesterase by combined highpressure and-temperature treatments: a kinetic study. Journal of Agricultural and Food Chemistry, 48, 1960–1970. Weemaes, C. A., Ludikhuyze, L. R., Broeck, I. V., & Hendrickx, M. E. (1998). Kinetics of combined pressure-temperature inactivation avocado polyphenoloxidase. Biotechnology Bioengineering, 60(3), 292–300. Yeh, A., & Li, J. Y. (1996). A continuous measurement of swelling of rice starch during heating. Journal of Cereal Science, 23, 277–283. Zanoni, B., Schiraldi, A., & Simonetta, R. (1995). A naive model of starch gelatinisation kinetics. Journal of Food Engineering, 24, 25–33.