Twin-screw Extrusion Combined with Low Temperature Impact Mill Improves the Quality of Rice Polishing Powder

In order to solve the problem of rancidity of rice polishing powder and improve its physicochemical properties, the effects of twin-screw extrusion under different extrusion temperatures on the storage stability and nutritional properties of rice polishing powder were studied. Under the optimum extrusion temperature, the rice polishing powder (SR-1, SR-2, SR-3, and SR-4) with different particle sizes was prepared by low temperature impact mill (LTIM) under different air classifier speeds (300, 600, 1200, and 1800 r/min). The effects of the air classifier speed on volatile aroma compounds and physicochemical properties of rice polishing powder were studied. The results showed that the lipase activity of rice polishing powder decreased significantly from 12.94 mg/g to 2.18 mg/g after extrusion treatment. Stored at the 150th day, the fatty acid value of untreated rice polishing powder was 255.57 mg KOH/100 g, while the fatty acid value of the extruded sample was only 121.90 mg KOH/100 g. After extrusion treatment, dietary fiber, γ-aminobutyric acid, and γ-oryzanol in rice polishing powder were also well retained. LTIM promoted the release of volatile aroma compounds in rice polishing powder, and SR-3 had the highest concentration. Based on the partial least square discriminant, 9 differentially labeled components were screened according to the importance projection of variables, and it was confirmed that SR-3 had the strongest flavor. With the decrease of particle size, the specific surface area of rice polishing powder gradually increased, and the bulk density showed a decreasing trend. The whiteness index increased significantly from 66.08 to 76.61. Water absorption ability and solubility increased significantly from 1.77 g/g and 24% to 2.37 g/g and 31%, respectively. Fourier transform infrared spectroscopy analysis showed that LTIM destroyed hydrogen bonds in starch particles, resulting in more free hydroxyl group exposure. The results can provide a theoretical basis and technical support for the processing of high-quality rice polishing powder.