Moisture Diffusivity in Rice Components During Absorption and Desorption

Abstract

Moisture diffusivity values of different rice kernel components, namely endosperm, bran and husk, are required to solve mathematical models describing absorption and desorption processes. In addition to the rice variety and temperature, the moisture diffusivity also depends on its instantaneous moisture content or water activity (a w ) and whether rice is absorbing or desorbing moisture. This research was undertaken to determine moisture diffusivity values of rough rice components in different a w ranges during absorption and desorption. Experiments were performed to measure sorption rates of different rice forms, including white rice, brown rice, and rough rice kernels. Mathematical models were developed to predict their moisture distribution during moisture sorption processes. These models were solved by finite element method using Comsol Multiphysics 1 simulation program. Moisture diffusivity values of different rice components were calculated and found to be different during absorption and desorption. Diffusivity of rice endosperm was higher during desorption than absorption at a w higher than 0.20 and increased with an increase in a w in 0.20-0.80 a w range. Diffusivity of bran remained almost the same with a w while diffusivity of husk decreased with an increase in a w . Results obtained in this research demonstrated that the moisture diffusivity of different rice components varies significantly with the change in water activity or moisture and should be accounted in the mathematical models.

Dynamic vapor sorption isotherms of medium grain rice varieties

“…For RR and BR the differences between SSS and DVS were higher at a water activity range of 0.40e0.80 than the lower or higher water activities. Although Prakash et al (2011) reported the highest diffusion coefficient of WR at a water activity range of 0e0.20, it was interesting to note that for WR as the RH increased, the difference between DVS and SSS methods decreased.…”

“…Keeping in mind that since diffusion coefficients of different forms of rice were smaller than 10 À9 m 2 /s (Prakash et al, 2011), a difference between SSS and DVS methods could already be anticipated. We have observed that at 0 and 98% RHs for all forms of rice there were approximately 8e11% and 7e9% differences, respectively, between DVS and SSS methods (Fig.…”

“…It was shown by Prakash, Bingol, and Pan (2011) that at different RH the moisture diffusivity values of medium grain rice ranged from 10 À11 to 10 À12 m 2 /s. Moreover, Arlabosse, Rodier, Ferrasse, Chavez, and Lecomte (2003) stated that when the apparent diffusion coefficient of a material is less than 10 À9 m 2 /s, a difficulty occurs to reach the thermodynamic equilibrium with the SSS method.…”

Recent advances in drying at interfaces of biomaterials

“…Modelling work clearly lags behind, mainly due to the multitude of processes that need to be captured. Simple diffusion models based on Fick's law are commonplace, for example to determine the effective film diffusivity [38,163].…”

Determination of external mass transfer coefficients in dynamic sorption (DVS) measurements

“…However, only few studies have taken this effect into account. Oliver et al [24] and Peer reviewed manuscript, accepted in Drying Technology 12 May 2017 (DOI 10.1080/07373937.2017.1331239) Prakash et al [25] estimated the external mass transfer coefficient by calculating the Sherwood number, while Anderberg and Wadsö [16], Wadsö et al [26] and Roca et al [27] performed measurements of the mass transfer resistance related to their experimental setups and subsequently used these values as corrections in dynamic sorption experiments. These measurements were made by eliminating the inner transport of the sample by measuring the mass loss rate from either a wet surface or a water droplet.…”