Study On Semi-Dry Milling Process Of Rice

China is a large producing country.In recent years,with the continuous upgrading of the dietary structure,the consumption of rice foods has increased dramatically and has a broad prospect.Most of these rice-based foods are made of rice flour,and the quality of rice flour is greatly affected by the different milling methods during the milling process.The quality of rice flour processed by dry milling is poor.Although the commonly used wet processing technology has good quality rice flour,the cost is very high due to the large amount of water and the production of sewage.The semi-dry milling method is a new rice milling method,which has incomparable advantages compared with the wet milling process because of its simplicity,no wastewater pollution,and better quality of rice flour.In this paper,we use early indica rice as raw material,through the investigation of particle size,tempering process,powder-making process and drying temperature,to determine the semi-dry milling process of rice in a roller mill.The semi-dry milling rice flour produced was compared with dry-milling and wet-milling rice flour and the quality of the rice noodles was compared.The main findings of this paper are as follows:The chemical composition of rice flour with different particle size distributions is the same,but there are significant differences in the rice flour properties and the quality of rice noodles.With the decrease in particle size,the whiteness and damaged starch content of rice flour increased significantly.Regarding the pasting characteristics,the peak viscosity,final viscosity breakdown and setback of rice flour showed a decreased trend,and the enthalpy change value decreased from 11.12 J/g to 8.79 J/g.After the particle size was reduced to 124μm,there was no significant difference in the pasting viscosity and pasting temperature of rice flour.The starch crystal type was an A-type starch structure,and the crystallinity decreases,and the storage modulus and loss modulus decreased.The water absorption,water solubility and swelling increased;the hardness and chewiness of rice flour gel increased,and the elasticity and adhesion decreased.The breaking rate and cooking loss of rice noodles decreased first and then increased,reaching the lowest value when the particle size was 124μm,and the hardness decreased.There was no significant difference in cooking quality and TPA between rice noodles with particle sizes of 156μm and 124μm.The research on the rice tempering process shows that:with the increase of tempering moisture,the content of bound water in the rice grain increased,the free degree of water increased,the hardness decreased significantly,the L^*value and whiteness of the rice flour increased significantly,the b^*and a^*values showed a tendency to decrease,and the particle size and damaged starch content of the rice flour decreased significantly.With the increase of tempering time,the free water content in the seeds was transformed into bound water and semi-bound water,and the hardness was reduced.The whiteness and particle size of the rice flour did not change significantly,and the damaged starch content showed a decreasing trend.After 5 h,it reached equilibrium.With the increase in tempering temperature,the free water content in the seeds decreased significantly,and the hardness decreased.The content of damaged starch in rice flour increased first and then decreased.If the tempering temperature was high and the tempering time was more than 4 h,the microbial growth and reproduction were active,which was easy to cause rice flour deterioration.Therefore,the tempering temperature should not exceed 35℃.The moisture content is 28%,and tempering 5 h at room temperature is suitable as the conditioning parameters of the semi-dry milling process.In the rice semi-dry milling process,when the rice endosperm has been broken into coarse particles after grinding by two tooth rollers,the overall flour yield rate reached about 91%after grinding with four smooth rollers.When the rolling distance of the first tooth roll is 0.40mm,the rice flour extraction rate is about 10%,and when the rolling distance of the second tooth roll is 0.20 mm,the rice flour extraction rate is about 7%.After grinding with two tooth rollers,the remaining 2.4%on the 24GG sieve,and the total rice flour extraction rate of the tooth roller is about 17%.The rolling distances of the four smooth rolls are 0.15 mm,0.13 mm,0.11 mm and 0.09 mm,respectively,and the corresponding single-pass rice flour extraction rates are about 40%,19%,12%and 2%,respectively.According to the above milling parameters,the semi-dry milling process of rice was designed:the rice was milled in six passes,with a total extraction rate of 91%.The D₅₀of rice flour was 91.91μm,and the iodine absorption value of damaged starch was 83.61%.In the study of drying temperature,there were significant differences in the quality of rice flour after drying to about 12%at different drying temperatures.With the increase in drying temperature,the drying rate of rice flour increased significantly,and had no significant effect on the particle size of rice flour,while the whiteness of rice flour showed a slight increase trend,and the content of damaged starch showed an increasing trend.There was no significant effect on the pasting viscosity and pasting temperature of rice flour,and the enthalpy change value showed a decreasing trend.The viscoelasticity of rice flour dried at 70℃is closer to that of rice flour dried at 40℃.The increase in temperature did not change the crystal structure of starch,and the crystallinity was the highest at 60℃.The hydrogen bond strength between rice flour starch molecules decreased after drying at 90℃.Therefore,the drying rate of rice flour is small and the drying time is long at low temperatures.When the temperature is high,the hydrogen bond strength between starch molecules in rice flour is weakened.Drying rice flour at medium drying temperatures（60℃,70℃）not only significantly improves the drying rate,and shortens the drying time,but also has little effect on the quality of rice flour.In summary,the ideal semi-dry rice milling parameters are as follows:the particle size of the rice flour was 156μm,and the moisture content was 28%,tempered at room temperature for 5 h,two tooth rollers（0.4 mm,0.2 mm）and four smooth rollers（0.15 mm,0.13 mm,0.11mm,0.09 mm）were set up for rice semi-dry milling,and then dried at 70°C until the moisture content was about 12%.The prepared rice flour was compared with wet milling and dry milling.The nutrients such as protein and starch in rice flour obtained by wet milling were more lost than those obtained by dry milling and semi-dry milling,while there was no significant difference in the chemical composition of rice flour obtained by dry milling and semi-dry milling.Wet milling rice flour has high whiteness and a small particle size distribution range.Compared with dry milling rice flour,semi-dry milling rice flour is closer to wet milling rice flour in physical and chemical properties.For the rice noodles made of rice flour obtained by the three milling methods,the cooking loss,broken rate and hardness of rice noodles made by dry milling were much higher than those of wet milling and semi-dry milling.There was no significant difference in cooking loss between semi-dry rice noodles and wet-milled rice noodles.