Effects Of Different Drying Methods On The Quality Of Glutinous Rice

In recent years,with the improvement of people’s living standards and the continuous adjustment of dietary structure,the market demand for glutinous rice has increased year by year.The scale of glutinous rice planting has been expanded,therefore,glutinous rice has become a characteristic industry that cannot be ignored in China’s grain production.The demand for glutinous rice in China reached 5.5 million tons in 2020.When glutinous rice is harvested,the moisture content is high.If it is not dried in time,there will be quality deterioration phenomena such as heat,mildew and germination.With the development of modern mechanical harvesting technology,the harvesting time has been greatly shortened.Natural drying has been unable to meet the drying needs of glutinous rice.The drying rate of mechanical drying is very fast to satisfy the needs of production,however the quality of glutinous rice is greatly affected by mechanical drying parameters.Too fast drying speed will seriously influence taste,processing and appearance of glutinous rice,so under the condition of large drying rate,it is particularly important to ensure the quality of glutinous rice.Based on the results of single-factor experiments,this study has determined the optimal process of hot air and far-infrared drying of glutinous rice through the experimental design of response surface,and has explored the changes of quality,structure and properties of glutinous rice in the drying process under the optimal drying process parameters,in order to provide theoretical support for the drying process of glutinous rice.The main test results are as follows.(1)Based on the three single-factor tests of drying temperature,tempering time and tempering temperature,the head rice yield is used as the response value.The drying rate is fast,and the optimal drying process of hot air and far-infrared drying of glutinous rice is determined by Box-Behnken test.The results showed that the optimized process parameters of hot air drying were drying temperature of 50°C,tempering temperature of 60 °C,and tempering time of 71 min,at which time the head rice yield of rice was64.03% and the drying rate was 2.22 %/h.The optimized process parameters of farinfrared drying were drying temperature of 54°C,tempering temperature of 60 °C,and tempering time of 80 min,at which time the head rice yield was 63.91% and the drying rate was 2.31 %/h.(2)Under the optimal drying parameters after the optimization of the response surface,the effects of four different drying methods in the non-tempering group(hot air drying group,far-infrared drying group)and the tempering group(hot air dryingtempering group,far-infrared drying-tempering group)on the moisture distribution,color,crack rates,head rice yield,texture characteristics and volatile substances of glutinous rice are explored during the drying process.The results showed that the hotair drying-tempering group had the lowest crack rates and the highest head rice yield,the hardness,adhesion and chewiness,while the hot-air drying group had the slowest growth.There was no significant difference in whiteness and brightness during the drying process of the four drying methods.Through principal component analysis,the9 indexes in glutinous rice could be reduced to 2 principal components,and the cumulative contribution rate reached 91.55%.The comprehensive score of glutinous rice in the drying process of different drying methods was calculated.It was found that the hot air drying-tempering group of glutinous rice has better quality.Correlation analysis showed that the drying time of the four drying methods was negatively correlated with the combined water peak area and polishing rice rate,and was positively correlated with the free water peak area,whiteness,brightness,burst rate,hardness,adhesiveness and chewiness.A total of 39 volatile components were detected in all samples by gas chromatography,including 10 aldehydes,9 alcohols,5 ketones and 10 esters,and 15 substances.They were screened from the volatile components of all samples by relative odor activity method,which had a significant influence on the contribution of glutinous rice flavor.Compared with the drying group,the decrease of aldehydes and the increase of alcohols in the drying-tempering group improved the deterioration of odor to a certain extent.(3)Under the optimal drying parameters after the optimization of the response surface,we explore the effects of the four drying methods on the microstructure,crystallinity,thermal stability and protein secondary structure of glutinous rice.The results showed that with the extension of drying time,the pores of the microstructure of the four drying methods increased and the pore size increased.The protein secondary structure was mainly β-fold and β-turn,and the four drying methods had no significant effect on the protein secondary structure and crystallinity(P>0.05).Compared with the undried samples,the content of short-range ordered structures was reduced in the drying group and the drying-tempering group.The enthalpy of endothermic heat showed an overall growth trend.The growth in the tempering group was greater than that in the non-tempering group.Far-infrared drying is greater than hot air drying in gelatinization starting temperature.