Original papersChanges 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 60-80 min 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 × 10−8 m2 min−1 with an active energy of 6.547-36.913 kJ mol−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 60-80 min for less than 10% damage to kernels.