Optimization and experimental study of tempering process parameters during hot air drying of paddy rice

Paddy rice is the second largest grain crop in the world, and it needs to be dried to a safe moisture content post-harvest for long-term storage. The traditional continuous hot air drying has many problems such as low drying efficiency, serious additional crack percentage, and poor edible quality and nutritional quality. For this reason, the tempering operation is generally added in the drying process of paddy rice. Tempering drying is a periodicity drying technology, in which the continuous heating for paddy rice is stopped, and the moisture in paddy rice is redistributed under the action of water potential. When the drying heat is obtained again, the moisture diffusion rate and surface evaporation rate increase significantly. The parameters of tempering drying mainly include the drying section and tempering section and their matching parameters. However, most of letters have explored the variety of macro indexes of paddy rice for the parameters of the drying section and some of the tempering section such as tempering time. There is a lack of systematic analysis on the conditions of tempering, such as tempering time, tempering temperature and tempering time. And the influence of different parameters on the nutritional quality of paddy rice also needs to be further studied. Aiming at the problem of insufficient research on the correlation between drying technology and quality indicators, this research explores the influence of tempering temperature, starting time of tempering, duration of tempering, and number of tempering cycles on the additional crack rate, head rice rate, protein content, and fatty acid value of paddy rice based on previous research results. Firstly, the change trend of paddy rice drying quality with tempering process parameters was analyzed by a single-factor test, and the weight of additional crack percentage, head rice rate, protein content and fatty acid value were all greater than 20 %, which were the key indexes of paddy rice tempering drying. Secondly, the main factors affect the drying quality of paddy rice and their corresponding levels were determined by the comprehensive evaluation method of the membership function model, including the moisture content of paddy rice at the beginning of tempering, tempering temperature and tempering time. Eventually, in this study, the Central-Composite experimental optimum design methods were used. The tempering temperature, the moisture content of paddy rice at the beginning of tempering, and tempering time as independent variable, and the additional crack percentage, protein content, fatty acid value as response indexes, using response surface to analyze the relationship between the test factors and quality indexes and explaining the causes of the results. The results showed that, the optimal operating parameters were determined as the moisture content of paddy rice at the tempering temperature of 45℃, the moisture content of paddy rice of 21% at the tempering beginning, the tempering time of 1.61 h. Under these conditions, the predicted value of the additional crack percentage of paddy rice after drying was 6.63%, the protein content was 5.39%, the fatty acid value was 11.68%. The results of verification experiments showed that the additional crack rate was 6.86%, the protein content was 5.35%, and the fatty acid value was 11.14%. The average error between the test value and the software optimization parameter value was 2.97%. The results showed that the optimized tempering drying process significantly improved the drying quality of paddy rice, which would provide the theoretical basis for production practice and in-depth exploration of the mechanism of paddy rice quality change.