Research On Process Optimization And Model Of Rice Drying At Variable Temperature

Newly harvest rice has a high moisture content and needs to be dried to the target moisture before it can be stored or processed.Therefore,drying is a necessary process after rice harvest.Rice is a heat-sensitive materials,it is easy to lead to the degradation of rice quality after drying and reduce its nutritional value and economic value by the unreasonable drying process.Hot air drying is the main drying method of rice,but the contradiction between drying rate and drying quality is still a difficult problem.In this paper,glass transition theory is used to explain the mechanism of rice cracking,and the drying method of rice with variable temperature is put forwards,and the changing law of drying rate and drying quality of rice with variable temperature is explored.The effects of variable temperature gradient,hot air speed,initial moisture content and other factors on the drying rate and quality of rice after drying were studied,and compared with the constant temperature drying.Design-Expert software was used to analyze the mapping relationship between each factor and the response index(average drying rate,explosive waist increase rate,milling rice rate,fatty acid value).Through optimizing the process parameters and experimental verification,the optimal process parameters of variable temperature drying of rice were finally obtained.The main conclusions are as follows:(1)In this paper,the rice variety is Daohuaxiang "Wuyoudao No.4",which is used as experimental raw material to explore the relationship between rice glass transition temperature and initial moisture content based on the glass transition theory.The results show that the glass transition temperature is negatively correlated with water content.According to the fitting curve of rice glass transition temperature,the drying parameters of rice with variable temperature are formulated to ensure the same state(rubber state and glass state)of rice during drying.(2)Through the comprehensive experiment of rice drying under the condition of rubber drying at variable temperature,the drying rate increases with the increase of variable temperature gradient,hot air speed and initial moisture content;When the initial moisture content are 22.3% and 24.3%,the drying temperature increases in the late drying period(rice moisture content is 18%),but the dryin rate still keeps decreasing.When the initial moisture content is 25.7%,Variable temperature gradient increases in the later drying period(rice moisture content was 21%),but the drying rate doesn’t increase significantly.When the initial moisture content are 22.5%,23.9% and 26.2%,the drying rate remains low when the water content is less than 17%,18% and 20% respectively.The results show that when the moisture content is more than 21%,the drying rate of constant temperature is the fastest,followed by the drying rate of rubber state and the drying rate of glass state.The effective water diffusion coefficient increases with the increase of variable temperature gradient,hot air speed and initial water content,and the drying rate increases.However,when t initial moisture content of rice is at a high level,the drying time increases.By comparing six thin-layer drying mathematical models,The results show that: the Two term exponential model is the best model for rice variable-temperature drying and is suitable for describing rice variable-temperature drying characteristics.(3)Through the comprehensive experiment of rice drying at variable temperature,the study show that under the condition of rubber drying at variable temperature,the increase rate of cracking ratio increased with the increase of variable temperature gradient and hot air speed,but the change of head rice yield was opposite.When the temperature change gradient was 4 ℃and 6℃,the fatty acid value doesn’t change significantly,and the germination percentage of rice remained high level.When variable temperature gradient is 8℃,the fatty acid value increases slightly and the germination percentage decreased.Under the glassy drying condition,the increase rate of cracking ratio is lower than 3%,and head rice yield was higher than 75%.The fatty acid value doesn’t increase significantly,and the germination percentage was higher.Comparing the quality of rice after variable temperature drying with that after constant temperature drying,the quality of rice after variable-temperature drying in glass state is the best.Under the condition of rubber temperature change drying,the temperature change gradient is 4 ℃,6℃,and the quality after drying is better than constant temperature drying.When the temperature change gradient is 8℃,the increase rate of cracking ratio increases,t head rice yield decreases,and the drying quality decreases.(4)On the basis of comprehensive test,response surface optimization was carried out,and the optimal parameters of rice rubber drying process under variable temperature gradient are obtained as temperature gradient of 5℃,hot air speed of 1.6m/s and initial water content of 22.5%.At this time,the average drying rate of rice is 0.176%/min,the crack additional rate is 5.7%,the head rice yield is 66.8%,and the fatty acid value is 17.78mg/100 g.The optimum parameters of the glass drying process of rice under variable temperature are the variable temperature gradient of 6℃,the hot air speed of 1.8m/s and the initial water content of 24.3%.At this time,the average drying rate of rice is 0.089%/min,the crack additional rate is 0.3%,the head rice yield is 78.6%,and the of fatty acid value is 16.86mg/100 g.In conclusion,the application of the glass transition theory in rice drying at variable temperature can effectively increase the drying rate and improve the quality of rice after drying.It indicates that it is scientific and feasible to use the glass transition theory to guide the drying practice of rice at variable temperature.