Water uptake rate

In general, the absorption of water by a given rice is a function of time, temperature, and initial moisture content. In practice, variety, degree of milling, and storage conditions should be considered in evaluating the hydration rate of rice. Varietal differences in hydration rate of japonica type milled rice have been reported (Kim et al., 1984b; Kim et al., 1985; Lee et al., 1983). The equilibrium moisture content (EMC) of milled rice at room temperature is 30-32% for japonica type (Cho et al, 1980; Lee et al., 1983; Suzuki et al., 1977; Takeuchi et al, 1997a), 28-29% for indica type, (Bhattacharya et al., 1972) and 30.4% (Chiang and Yeh, 2002) and 34-37% for waxy rice (Bhattacharya et al., 1982). It was reported that EMC of milled rice was very highly significantly related inversely to amylose content and directly to kernel chalkiness index (Bhattacharya et al., 1982).

The diffusion coefficients of milled rice reported in the literature are summarized in Table 19.2. Steff and Singh (1980) and Suzuki et al. (1977) assumed that the rice grain is spherical, and the others assumed it to be prolate spheroid. This gives a difference in the calculations of S and V of rice grain, hence affecting the calculated value of diffusion coefficient. Degree of milling (Yoon, 2002) and defatting (Kim et al., 1986) of milled rice also affect the hydration rate.

Zhang et al. (1984) developed a computer-aided model using finite elements to analyze non-linear diffusion in milled rice during soaking at 60 °C. The contour plots of the moisture distribution showed that variation in moisture content was between 35 and 50% at 30 min of soaking, whereas at 60 min the variation was between 40 and 51%. The average mass diffusivity changed from an initial value of 6.4 x 10-7 m2/hr to 4.2 x 10-7 m2/h in 10 min of soaking. This period corresponded to intervals of maximum water uptake, after which the water uptake slowed down as did the change in mass diffusivity. Average mass diffusivity decreased from 6.4 x 10-7 m2/h to 3.0 x 10-7 m2/h as the moisture content increased from 13 to 50%, which indicates that mass diffusivity is a function of concentration.

The diffusion coefficients of brown rice at soaking temperature of 30 °C are 1.78-3.68 x 10-6 cm2/min (Song et al., 1988), 3.51 x 10-6 cm2/min (Kim et al., 1984a), 5.52 x 10-6 cm2/min (Steff and Singh, 1980), and 2.31 x 10-6 cm2/min (Han et al., 1996). These results suggest that the diffusion coefficient ofbrown rice is fairly constant regardless of the variety. The hydration rate of brown rice at 60 °C was reported to be 0.0394-0.0552 min-1/2 (Lee and Kim, 1994), that at 100 °C being 0.0743-0.1419 min-1/2 (Kim and Suh, 1990). Lee and Kim (1994) reported that the water absorption rate of brown rice had a positive correlation with numbers and thickness of aleurone layers of the kernel, implying that the differences in water absorption among brown rice are due to the differences in bran structure of the kernel.

Yoon (2002) demonstrated that moisture gain of brown and milled rice soaked in water for 20 min at 30 °C showed a linear relationship with degree of milling and the slope was essentially the same among three rice cultivars. An increase of degree of milling by 1 % resulted in an increase of moisture gain by 1.06 times. Moisture gain was highly significantly related inversely to protein and fat contents.

Table 19.2 Diffusion coefficients of milled rice at soaking temperature of 30 °C

D x 104

Number in






Cho et al. (1980)



Steff and Singh (1980)



Suzuki et al. (1977)



Lee et al. (1983)



Kim et al. (1984a)



Song et al. (1988)

The water uptake rate constants of brown and milled rice linearly decrease as storage time increases when rice in laminated film pouch is stored at 430 °C for up to five months (Cho and Kim, 1990, 1993; Han et al, 1996). The change in water uptake rate is more pronounced in milled rice and at elevated temperatures. The EMC remains fairly constant for both brown and milled rice, but the time to reach EMC increases as storage time is prolonged and temperature is elevated (Cho and Kim, 1990, 1993).

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