Water absorption and starch gelatinization in whole rice grain during soaking

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1 LWT 40 (2007) Water absorption and starch gelatinization in whole rice grain during soaking Marcelo O. Bello, Marcela P. Tolaba, Constantino Suarez Departamento de Industrias, Facultad de Ciencias Exactas y Naturales-UBA, Ciudad Universitaria, (1428) Buenos Aires, Argentina Received 20 May 2005; received in revised form 20 September 2005; accepted 27 September 2005 Abstract Hydration of rough rice grain in hot water as a function of time was studied at temperature range C. A simple model which considers simultaneous unsteady-state water diffusion and first-order irreversible water starch reaction phenomenon, was used to evaluate the kinetics parameters from experimental curves. The values of the diffusion coefficients and reaction rate constants were between and m 2 s 1 and and s 1, respectively. Both parameters followed a Arrheniustype equation with distinct activation energies below and above a break temperature of 60 1C. It was 25.4 and kj mol 1 for the activation energies of diffusion and reaction, respectively, below 60 1C. Above this temperature the respective values of the activation energies of diffusion and reaction were 30.0 and 16.6 kj mol 1. This break temperature was in agreement with the gelatinization temperature determined experimentally. r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Gelatinization kinetics; Water starch reaction; Simultaneous diffusion and gelatinization 1. Introduction The hydro-thermal processing of rice grain has become one of the most widespread food industries of the world (Bhattacharya, 1990). This includes the rice soaking where water diffuses slowly into the grain. When water is present at sufficient high temperatures, the starch in the endosperm undergoes a gelatinization reaction. During this, the starch granules expand, the protein bodies in the endosperm disintegrate, and the granules become closely pressed together, creating a strong cohesion between them (Tester & Morrison, 1990). Gelatinization has also been described as the bursting of granules in the presence of hot water (Kunze, 1996). There is a great need of appropriate mathematical models that describe accurately the soaking behavior of rice grain. An accurate description of bulk soaking-rates depends to a large extent on the exact description of the Corresponding author. Tel./fax: addresses: mtolaba@di.fcen.uba.ar (M.P. Tolaba), suarez@di.fcen.uba.ar (C. Suarez). water migration within the individual particle. Suzuki, Kubata, Omichi, and Osaka (1976) have studied the cooking mechanism of rice between 75 and 150 1C. They assumed a cooking model analogous to the unreacted core model for the heterogeneous catalytic reactions, i.e. cooking shell on the outer side of rice and uncooked in the middle of rice. According to these, the cooking process of rice consists of two mechanisms: below 110 1C (break temperature) it is limited by gelatinization process, above 110 1C the diffusion rate of water within the grain is the controlling step. Bakshi and Singh (1980) soaked rough rice grains in water between 50 and 120 1C, and considered simultaneously water transfer and starch gelatinization processes in spherical coordinates. In both mechanisms, water transfer and starch gelatinization, were treated as diffusion and first-order irreversible reactions, respectively. They found a break point in the activation energies of diffusion and reaction at about 85 1C, but they did not comment on a possible reason for the break point. More recently, Lin (1993) soaked white rice between 48 and 85 1C evaluating the process as a simultaneous water diffusion and starch /$30.00 r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi: /j.lwt

2 314 M.O. Bello et al. / LWT 40 (2007) gelatinization. He also found a break temperature in the activation energies of diffusion and reaction at C, attributed to structural changes in the rice grain. However, the conclusions of Lin (1993) about the controlling mechanisms above and below the break point differs from those of Bakshi and Singh (1980). While for the last one, the reaction of rice starch with water is the limiting factor during cooking at temperatures below the break point and water diffusion is the limiting factor at temperatures above the break point, Lin (1993) concluded opposite results. The present work was performed to report information on the soaking process of rice grain below and above the gelatinization temperature. The kinetics parameters, moisture diffusivity and reaction rate constant were evaluated from a mathematical model that takes into account the simultaneous process of water diffusion and water starch reaction. 2. Theoretical considerations Moisture migration in grains is usually assumed to occur by diffusion caused by the moisture gradient between the surface and the centre (Becker, 1960; Engels, Hendrickx, De Samblanx, De Gryze, & Tobback, 1986; Sayar, Turhan, & Gunasekaran, 2001, among others). Water absorption into the rice kernel can be regarded as a process of diffusion which a proportion of the absorbed water becomes immobilized during water starch reaction (gelatinization) as water diffusion proceeds (Bakshi & Singh, 1980). Such reaction will occur if the reactivity and affinity of starch with water is high enough. To simulate both diffusion and reaction rate phenomena for water on rice grains the following assumptions are made: (a) The process is considered to be a simultaneous unsteady-state water diffusion and first-order irreversible water starch reaction. (b) The diffusion coefficient and reaction rate constant are independent of moisture content. (c) There is no change of grain volume during water absorption. (d) Mass transfer coefficient is high enough to assure constant moisture content at the surface of the grain since the beginning of absorption process. (e) Thermal effects are neglected, i.e. grain temperature is assumed uniform and equal to the water temperature. (f) The shape of rice grains is assumed a sphere. The validity of the first assumption was corroborated in various studies (Bakshi & Singh, 1980; Suzuki et al., 1976). Even though there is experimental evidence of certain dependence on moisture of the diffusion coefficient, in this paper this effect was ignored in attempting to keep the model simple. The starch swells when it becomes wet, so that, in the hydration process the grain size increases; however, this effect is not considered here. Experimental work for water hydration of grains (Fortes, Okos, & Barret, 1981; Kustermann, Scherer, & Kutzbach, 1981) demonstrated that the heating process proceeds far more rapidly than hydration process; hence the two processes can be considered decoupled in any model. Although rice has not defined geometry, the grain was assumed a sphere to have an analytical solution for hydration process (Bandyopadhyay & Roy, 1978; Suzuki et al., 1976). The differential equation expressing simultaneous water diffusion and reaction process in a solid sphere is (Crank, 1975): kc, (1) qc qt ¼ D q 2 C f qr 2 þ 2 qc r qr where C is moisture content, t is the time, r the radius and D f and k are the effective diffusion coefficient and reaction rate constant, respectively. To integrate Eq. (1) the following boundary and initial conditions were used: t ¼ 0 and 0prpr e ; C ¼ C 0, (2) t40 and r ¼ 0; qc=qr ¼ 0, (3) t40 and r ¼ r e ; C ¼ C s, (4) where r e is the radius of the sphere and C s and C 0 are the saturation and initial moisture content, respectively. The integration of Eqs. (1) and (2) (4) allows to obtain the total amount of water absorbed as a function of time which, expressed in terms of the instantaneous moisture content is (Crank, 1975): m ¼ m 0 þðm 0 m s Þ8pr e D f C 0 ( ) X1 A n kr 2 e t B nr 2 e expð ða nt=r 2 e ÞÞ. ð5þ n¼1 A 2 n In this equation, m, m 0 and m s are the instantaneous, initial and saturation moisture contents, in dry basis: A n ¼ kr 2 e þ D fn 2 p 2 and B n ¼ n 2 p 2 D f. The parameters k and D f in Eq. (5) were determined using a nonlinear regression model. 3. Materials and methods Whole rice grain, long grain variety, with an initial moisture content of 0.11 kg water/kg dry solid was used in this study. The grains were cleaned and screened to obtain samples of uniform size. After that they were stored at 18 1C before use Soaking procedure Samples of whole grains (ffi25 g) were placed in flasks containing 75 ml of distilled water and the flasks were soaked in a temperature controlled water bath with mechanical agitation. The temperature of the bath was controlled to be constant (70.5 1C); the soaking temperatures were 25, 35, 45, 55, 60, 65, 75, 80 and 90 1C.

3 M.O. Bello et al. / LWT 40 (2007) Experimental water absorption curves at the temperatures mentioned above were done in duplicate. Throughout soaking the flasks were removed from the bath at regular intervals of time and the grains drained for 1 min, blotted with tissue 2 3 times to remove the surface water. This blotting procedure was established based on the preliminary test results and other reported studies (Becker, 1960; Engels et al., 1986). After blotting, the grains were placed in a metal cup and the moisture content was determinated by AOAC method (1996). In order to have an estimation of the saturation moisture content of rice grain for the soaking temperatures below gelatinization, the following procedure was performed. It can be demonstrated that when time becomes large, the limiting form of Fick s diffusion equation for sphere, without chemical reaction becomes: m m s ¼ 6 m 0 m s p 2 exp Def p2 t R 2. (6) From this equation and for any set of three moisture contents taken at equally spaced time intervals of duration j, it can be obtained for m s by the following equation: m s ¼ m im iþ2j m 2 iþj, (7) m i þ m iþ2j 2m iþj where m i and m j are the moisture contents at time i and j. Even though Eq. (7) is not necessarily valid when gelatinization process takes place, it was also used in this work, with the purpose to have an estimate of the moisture content of the gelatinized grain. As whole rice is irregularly shaped, the equivalent spherical radius was employed to represent rice grain for the mathematical model used in this work. The total volume of 50 grains was measured with volume displacement of cyclohexane and replicated thrice. The average grain volume, V e, was calculated by dividing the total sample volume by the total number of grains. The equivalent spherical radius, r e, was then computed as r e ¼ð3V e =4pÞ 1=3. The measurements corresponded to the grain with initial moisture contents (ffi0.11 kg water/kg dry solid). The equivalent spherical radius was 1.76 mm. The grains were assumed to be ellipsoid, having three characteristics diameters (apbpc), where a ¼ shortest axis, b ¼ largest axis normal to a, and c ¼ longest axis normal to a and b. The average values of the axes obtained from 50 kernels using a dial micrometer were: a ¼ 1.98, b ¼ 2.45 and c ¼ mm Rice flour Samples of whole grain rice were dehulled and milled in a bench model grain-testing mill (MT-95, Suzuki Co., Sa o Paulo, Brazil) to produce a 10% degree of milling. To prepare the flour, samples of milled rice were ground to fine particles in a laboratory mill and were passed through a sieve with mesh sizes from 74 mm. This last fraction was used for analysis by DSC Gelatinization temperature A PL-DSC calorimeter (Polymer Laboratories Ltd., Surrey, UK) was used to estimate the gelatinization temperature. Heat flow and temperature calibration were performed using pure indium with a heat of fusion of J/g and a melting temperature of C. Experimental values agreed to within 73% of the literature value. Rice flour (E4 mg) of known moisture content, weighed to an accuracy of mg, was added to the aluminum pan. Distilled water was added to the pan by a micropipette, and the pan was then hermetically sealed. A water-toflour ratio of 3:1 was used to prevent moisture evaporation loss. The water flour mixture was maintained at 24 h, in the pan before measuring in the calorimeter. Thermograms were obtained by scanning the sample from 25 to 110 1C at101c/min. An empty aluminum pan was used as reference. During the runs, the space surrounding the sample cell was flushed with dry nitrogen to avoid condensation on the outside of the cells. Onset, peak and conclusion temperatures were determined from the rice flours thermograms, by triplicate. 4. Results and discussion The water content of rice at several hydration temperatures is shown in Fig. 1 as a function of soaking time. The curves present two clear patterns of absorption. For soaking temperatures around 60 1C and below, the curves exhibited the characteristic moisture absorption behavior whereby an initial high rate of water absorption is followed by slower absorption in later stages. As soaking proceeds the amount of water absorbed becomes minimal until it reaches the saturation moisture which correspond to the maximum sorption capacity. The saturation moisture contents obtained from soaking data through Eq. (7) are given in Table 1, whose values show a general tendency to increase with temperature. This trend of m s with temperature is very much similar to that found in the literature for cereal grains (Bakshi & Singh, 1980; Cabrera, Pineda, Duran de Bazua, Segurajauregui, & Vernon, 1984; Lin, 1993). For soaking temperatures around 65 1C and above the absorption curves in Fig. 1 show a rapid increase in the hydration rate. Such increase is mainly due to the irreversible changes that occur in the starch granules as a result of the gelatinization process. During this process the rice kernels swell considerably, provoking also the breakage of hull structure which facilitates the absorption rate. These observations also indicate that the soaking temperatures of C are in the vicinity of gelatinization temperature of rice starch. It is pertinent to point out that during soaking at 65 1C and

4 316 M.O. Bello et al. / LWT 40 (2007) moisture content (kg/kg d.b.) (a) moisture content (kg/kg d.b.) (b) time (min) time (min) Fig. 1. Water absorption of whole rice grain during soaking: (a) soaking temperatures below 60 1C: (}) 251C; (+) 35 1C; (W) 451C; (J) 551C; and (x) 60 1C. (b) Soaking temperatures above 60 1C: (})651C; (J)751C; (x) 80 1C; and (+) 90 1C. Table 1 Saturation moisture content, m s, effective diffusion coefficient, D f, reaction rate constant, k, and mean relative percentage error, E(%) Temperature (1C) E(%) m s (kg water/kg dry solid) above an increase was observed in the turbidity of soaking water due to the release of part of the endosperm components Estimation of kinetics parameters D f (m 2 s 1 ) k 10 5 (s 1 ) ( ) The diffusion coefficients and reaction rate constants were found by minimizing the error between the actual and theoretical data using Eq. (5). The goodness of the fit was evaluated through the porcentual mean relative error, E(%), defined as Eð%Þ ¼ Xn jm i m pi j m pi, (8) where m i and m pi are the actual and predicted moisture contents, respectively. The E(%) values obtained from the regression analysis are given in Table 1; these were less than 3% which indicates that the goodness of fit is acceptable. The values of the effective diffusion coefficient and reaction rate constants for the different soaking temperatures are also given in Table 1. It must be mentioned that although the differences between actual and predicted moisture contents are in average quite acceptable, the differences were much higher at the very beginning of absorption process, as much as 34% during the first 45 min of soaking (about 25 g/100 g in dry basis). This was particularly true for the lower soaking temperatures; as soaking temperatures increased the discrepancies between predicted and experimental data did not exceed the value of 28% after the first 20 min of soaking, which corresponded to a water content of about 33 g/100 g, dry basis. On the other hand, for most of the tested temperatures the moisture contents predicted by Eq. (5) were higher than the measured values. This reduction in the rate of water absorption at the beginning of soaking process might be due to the siliceous hull, which does not wet easily and acts as a barrier to water penetration (Houston, 1972). As air is often trapped between hull and endosperm, this also prevents access of water to the kernel. In fact, the dehulling of rice was proposed as a method for reducing the time of soaking during rice parboiling (Kar, Jain, & Srivastav, 1999). As one of the assumptions to apply Eq. (5) is to neglect thermal effects, the Lewis number (Le ¼ a/d f ), where a is the thermal diffusivity, was calculated. According to Young (1969) the effect of heat transfer on the process is negligible when Le is greater than 60. The thermal diffusivity of rice grain was estimated following the empirical correlation proposed by Sreenarayanan and Chattopadhyay (1986), as a function of moisture content. For the lowest and highest tested temperatures, the average values of a were, respectively, and m 2 s 1, which gives a Lewis number varying between 161 and 94. This not only indicates that thermal diffusivity is less affected by temperature than mass diffusivity, but also that heat transfer effects can be neglected. From Table 1 it can be seen that the values of D f varied between and m 2 s 1 for the temperature range between 25 and 90 1C, while the corresponding reaction rate constant the range of variation was and s 1. For whole rice grain Bakshi and Singh (1980) found a range of values of D f and k between m 2 s 1 and s 1, respectively, for processing temperatures between 50 and 120 1C. For the temperature range C, Lin (1993) reported

5 M.O. Bello et al. / LWT 40 (2007) values of D f and k for white rice between m 2 s 1 and s 1, respectively, for white rice Effect of temperature The variation of D f and k with temperature were plotted in Figs. 2 and 3, respectively, according to Arrhenius equation: D f ¼ D 0 exp ð E D =RTÞ, (9) and k ¼ k 0 expð E k =RTÞ. (10) In Eq. (9) D 0 is a constant, E D is the activation energy for diffusion and R and T are the universal gas constant and absolute temperature, respectively; in Eq. (10) k 0 is a constant and E k the activation energy for water starch reaction. The values of E D and E k were calculated from the slopes of the respective curves shown in Figs. 2 and 3. As can be seen in Fig. 3, there is a marked change in the slope of activation energy around 60 1C (break point). This change, that affected both water diffusivity and reactivity of starchy endosperm, involves a significant change in the structural properties of the grain. Turhan and Sagol (2003) suggests this phenomenon associated to starch gelatinization. In this study, DSC measurements of onset, peak and D -10, effective diffusion coefficient (m 2 /s) /T (1/K) conclusion temperatures of milled rice gave the following values: 45.98, and C, respectively. It can be observed that the break temperature is included within the gelatinization range, being closed to the peak temperature of DSC thermograms. Similar changes in the Arrhenius plots of rate constants D f and k were also reported by other investigators, even though few of them commented on the possible reasons for the break point. Different values of break temperatures were reported by Bakshi and Singh (1980), Yeh, Hsin, and Shen (1992) and Lin (1993) for soaking of different varieties of rice. In all cases they explained the break point as structural changes in the rice grain. Pravisani, Califano, and Calvelo (1985) also found a break point in the Arrhenius plot of the reaction rate constant during their investigations of starch gelatinization in potato. According to these, the presence of break point is due to the alteration in the reaction mechanism due to melting of starch crystallites. The soaking process of chickpea was investigated by Sayar et al. (2001) as a simultaneous diffusion and gelatinization process occurring in a solid of spherical shape. The authors also reported a break point in the Arrhenius plots of diffusion coefficient and reaction rate constant, when temperature values were in close agreement with the gelatinization temperature determined experimentally. The values of the activation energy for diffusion and reaction are given in Table 2 for the temperature ranges above and below the break point. In this table, were also included for comparison E D and E k values given in the literature for rice grain, whole and polished, as well as for rice starch. However, the observed behavior for both magnitudes versus temperature differ from one author to another. In this work E D increased above the break point; Bakshi and Singh (1980) and Lin (1993) found a similar effect but more marked. The value of E k found here Table 2 Activation energies estimated for the diffusion coefficient (E D ) and reaction rate constant (E k ) of rice kernels and comparison with literature data Fig. 2. Arrhenius plot for the effective diffusion coefficients. Material Temperature range (1C) E D (kj mol 1 ) E k (kj mol 1 ) Reference k, reaction rate constant (1/s) 1.0E E E E E E E /T (1/K) Fig. 3. Arrhenius plot for the rate constants. Rough rice Bakshi and Singh (1980) White rice Lin (1993) White rice Suzuki et al. (1976) Rice starch o Birch and Priestley (1973) Rough rice This work

6 318 M.O. Bello et al. / LWT 40 (2007) decreased above the break point, in agreement with the results reported by Bakshi and Singh (1980). 5. Conclusions The analytical solution of the diffusion equation with irreversible chemical reaction of first-order describes satisfactorily the soaking process of rice kernels for temperatures above and below gelatinization temperature. The Arrhenius plot of the diffusion coefficient and reaction rate constant present a break point when temperature value of 60 1C is close to the gelatinization temperature of rice grain. The changes in the activation energy for diffusion and reaction at 60 1C indicates that below that temperature the reaction of water with starch was the limiting factor. Above 60 1C the diffusion of water became the limiting factor for rice cooking. Acknowledgments The authors acknowledge the financial support from Universidad de Buenos Aires, UBA, y Consejo Nacional de Investigaciones Cientı ficas y Tecnolo gicas, CONICET. References AOAC. (1996). Official methods of analysis. Washington, DC: Association of Official Analytical Chemists. Bakshi, A. S., & Singh, R. P. (1980). Kinetics of water diffusion and starch gelatinization during rice parboiling. Journal of Food Science, 45, Bandyopadhyay, S., & Roy, N. C. (1978). A semi-empirical correlation for prediction of hydration characteristics of paddy during parboiling. Journal of Food Technology, 13, Becker, H. A. (1960). On the absorption of liquid water by the wheat kernel. Cereal Chemistry, 37, Bhattacharya, K. R. (1990). Improved parboiling technologies for better product quality. Indian Food Industry, 9(5), Birch, G. G., & Priestley, R. G. (1973). Degree of gelatinization of cooked rice. Staerke, 25, Cabrera, E., Pineda, J. C., Duran de Bazua, C., Segurajauregui, J. S., & Vernon, E. J. (1984). Kinetics of water diffusion and starch gelatinization during corn nixtamalization. In B. M. McKenna (Ed.), Engineering and food, engineering sciences in the food industry, Vol. 1 (pp ). London, UK: Elsevier. Crank, J. (1975). The mathematics of diffusion (2nd ed). London: Oxford University Press. Engels, C., Hendrickx, M., De Samblanx, S., De Gryze, I., & Tobback, P. (1986). Modeling water diffusion during long-grain rice soaking. Journal of Food Engineering, 5, Fortes, M., Okos, M. R., & Barret, J. R. (1981). Heat and mass transfer analysis of intrakernel wheat drying and rewetting. Drying Technology, 9, Houston, D. F. (1972). In D. F. Houston (Ed.), Rice hulls in rice chemistry and technology (pp ). St. Paul, MN: AACC, Incorporated. Kar, N., Jain, R. K., & Srivastav, P. P. (1999). Parboiling of dehusked rice. Journal of Food Engineering, 39, Kunze, W. (1996). Technology brewing and malting (7th ed). Berlin: VLB. Kustermann, M., Scherer, R., & Kutzbach, H. O. (1981). Thermal conductivity and diffusivity of shelled corn and grain. Journal of Food Process Engineering, 4, Lin, S. H. (1993). Water uptake and gelatinization of white rice. Lebensmittel-wissenschaft und- Technologie, 26, Pravisani, C. I., Califano, A. N., & Calvelo, A. (1985). Kinetics of starch gelatinization in potato. Journal of Food Science, 50, Sayar, S., Turhan, M., & Gunasekaran, S. (2001). Analysis of chickpea soaking by simultaneous water transfer and water starch reaction. Journal of Food Engineering, 50, Sreenarayanan, V. V., & Chattopadhyay, P. K. (1986). Thermal conductivity and diffusivity of rice bran. Journal of Agricultural Engineering Research, 34, Suzuki, K., Kubata, K., Omichi, M., & Hosaka, H. (1976). Kinetic studies on cooking of rice. Journal of Food Science, 41, Tester, R. F., & Morrison, W. R. (1990). Swelling and gelatinization of cereal starches. I. Effects of amylopectin, amylose, and lipids. Cereal Chemistry, 67, Turhan, M., & Sagol, S. (2003). Abrupt changes in the rates processes occurring during hydrothermal treatment of whole starchy foods around the gelatinization temperature A review of the literature. Journal of Food Engineering, 62, Yeh, A. I., Hsin, W. H., & Shen, J. S. (1992). Moisture diffusion and gelatinization in extruded rice noodles. In J. L. Kokini, C. T. Ho, & M. V. Karwe (Eds.), Food extrusion science and technology (pp ). New York, NY, USA: Marcel Dekker. Young, J. H. (1969). Simultaneous heat and mass transfer in porous hygroscopic solid. Transactions of American Society of Agricultural Engineers, 12(2),