Changes in moisture effective diffusivity and glass transition temperature of paddy during drying

A modified moisture diffusion equation fits for the relationship between moisture ratio of paddy samples and drying time.The geometry of paddy kernels was assumed as homogenous finite cylinders, their moisture effective diffusivity increased with the raise of drying temperature.Most of the five paddy varieties had similar desorption rates at drying temperatures of 45-70°C.When MC of the paddy samples decreases from 20% to 13% at drying temperatures of 50-70°C, drying time should be 60-80min for less than 10% kernel damage. Moisture effective diffusivity and glass transition temperature (Tg) of paddy during drying were investigated by drying five varieties of paddy with an initial moisture content of 21.1-24.4% wet basis and a temperature range of 45-70°C in a constant temperature and blast oven. With an increase in drying temperature, the desorption rate and moisture effective diffusivity of paddy kernels increased and drying duration decreased. At a drying temperature of 45°C, the curve of the desorption rate changed slowly and drying duration was longer. Most of the five paddy varieties had similar desorption rates at drying temperatures of 45-70°C, and their moisture effective diffusivities were in the range of 2.638×10-9-2.514×10-8m2min-1 with an active energy of 6.547-36.913kJmol-1. Tg values of long-grain variety 'Zhunliang you' and two medium-grain varieties 'Zhongjia zao' and 'Dianjiang' determined using the differential scanning colorimeter method increased from 38.7°C to 51.2°C when their moisture contents decreased from 20% to 13%. Under similar conditions, differences in Tg values of the three varieties of paddy were observed. When the moisture content of the three varieties of paddy decreases from 20% to 13% at drying temperatures of 50-70°C, drying time should be 60-80minfor less than 10% damage to kernels.