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Effect of intermittent microwave drying on biophysical characteristics of rice

Xu, Fengying, Chen, Zhen, Huang, Mushui, Li, Changyou, Zhou, Weibiao
Journal of food process engineering 2017 v.40 no.6
Oryza sativa, air drying, color, germination, glass transition temperature, hardness, humans, industry, mathematical models, microstructure, microwave drying, mildews, porosity, rice, seeds, staple foods, storage, water content
The dehydration performance of intermittent microwave drying and its effect on the biophysical characteristics of rice (Oryza sativa L.) were quantified. Three pulse ratio levels (PR2, PR3, and PR5) were tested. Two dehydration performances (temperature and drying rate) and seven biophysical quality indices (moisture content, microstructure, glass transition temperature, color, hardness, crack percentage, and germination) of rice were measured. Different semi‐empirical moisture ratio models were optimized to fit the experimental data. The results showed that the temperature of rice treated by PR2, PR3, and PR5 were significantly different from each other during the process. The rice temperature dried by PR5 was the lowest and it tended to be constant at the late stage. The average drying rates of rice treated by PR2, PR3 and PR5 were 0.568, 0.437, 0.265% w.b. gg⁻¹.min⁻¹, respectively. With the increase of pulse ratio, the porosity of kernel glume cross section and the glass transition temperature of dried rice decreased. The moisture content changed significantly (p < .05). However, the hardness and color of rice showed no significant differences (p < .05). The crack quality and germination rate of dried rice treated by PR5 were significantly higher than those of the other treatments (p < .05). Page model was found to be the most fitted model. By considering the drying performance and quality, PR5 was the most suitable microwave drying treatment for rice. PRACTICAL APPLICATIONS: Rice is one of the most important staple food sources of human being. Newly harvested rice are wet and easy to mildew, need to be dried for the sack of long‐term storage. Compared with conventional hot air drying, intermittent microwave drying greatly improved the drying rate of rice. High pulse ratio treatment can significantly reduce the temperature rise speed of rice, and comprehensively improve the bio‐physical quality of rice, for example, crack percentage, germination rate and so on. The application of the mathematical model of rice drying performance provided a reference for the design of drying process. The research results are helpful to the application of intermittent microwave drying technology in rice drying industry.