Water-absorption rate equation of rice for brewing sake

This study was undertaken to analyze the kinetics of water absorption and to derive an equation for the rate at which water is absorbed by rice for brewing sake. We used two rice varieties: Gin-oumi, commonly used as a staple food, and Gohyakumangoku, a variety used particularly for brewing sake. The water-absorption rate equations of Gin-oumi and Gohyakumangoku were postulated based on the following equations. For Gin-oumi (water content, 11.5%), dx/dθ=k(1-x)n, n=1, k=(2×10−9)exp(0.0604×(t+273.15)). For Gohyakumangoku (11.5%), dx/dθ=k(1−x)n(x+a), n=1, a=0.29, k=(2×10−8)exp(0.0534×(t+273.15)). Here, x, θ (min), and t (°C) are the water absorbing ratio, time, and temperature, respectively. The result shows that the values of the temperature-dependence parameter k (min−1), as well as its curves, are different; a typical rice grain has a monotonically smooth curve, whereas that suitable for sake brewing has an S-shaped curve.