Factors affecting water uptake of rice grain during soaking

The soaking process of rice (raw, dehulled or bran and white) in plain water was investigated between 25 degreesC and 65 degreesC. The corresponding effective diffusion coefficients were evaluated by fitting experimental absorption curves to Fick's second law for diffusion out of sphere. Effective diffusion coefficients for raw, brown and white rice varied between 1.56 x 10(-11) and 7.20 x 10(-11) m(2)s(-1); 2.22x10(-11) and 8.82x10(-11) m(2)s(-1) and 20.5x10(-11) and 47.0x10(-11) m(2)s(-1), respectively, for the temperature range investigated. The amount of total solids leached increased with the increase of temperature and time; the highest corresponded to white rice. The rates of absorption of raw rice in aqueous solutions of ClH, CH3COOH and PO4H3 at 25 degreesC were quite similar and much lower than those in plain water at the same temperature. Raw rice soaked in NaOH aqueous solutions and Na2CO3 aqueous solutions gave, in terms of the diffusion coefficients, higher absorption rates than raw rice soaked in plain water. Absorption rates for raw rice soaked in NaOH concentrations were higher than those found in Na2CO3 concentrations. When raw rice was soaked in NaOH aqueous solution at 55 degreesC the rate of absorption was coincident to that found during soaking of dehulled rice in plain water. (C) 2004 Published by Elsevier Ltd. on behalf of Swiss Society of Food Science and Technology.