Study On Double Extrusion Processing Of Instant Rice With Twin-screw Extruder

Rehydration characteristic and edible quality of instant rice is one of the sticking points that restricted its development since it was invented, therefore people made unremitting efforts for its improvement. Based on broken rice as the main material and other cereal flours, food additive and micronutrient, the double extrusion processes were studied with twin-screw extruder in order to develop a new kind of Nutrition-Fortification Instant rice and aims at providing reference to industrial production.Firstly, extrusion cooking is a mechano-chemical modification process which is completely different from normal cooking.The physicochemical properties of extruded rice was significantly related to extrusion parameters and the porosity microstructure was observed by SEM. Then a response surface method was conducted to get the following optimal parameters: material moisture content was 33.8%, and screw speed was 195.2 r/min, the the three cooking temperature was 30℃, 50℃, 127.2℃respectively. Through response surface analysis, the cooked material with high quality was produced.Texture analysis of rehydrated instant rice with twin-screw extruder, using TA.XT 2i, showed that its quality was improved to a certainty extent, which the cooked rice's hardness and adhesiveness decreased. These results revealed that it was feasibility of processing instant rice with the double extrusion processes. The response surface analysis and the analysis results showed that the optimal processing condition of extrusion were as following: gelatinization degree(68%), moisture content(32%), screw speed(201 r/min), the three cooking temperature( 30℃, 50℃, 50℃respectively).Instant rice with twin-screw extruder was subjected to convective hot-air, microwave and combined microwave-hot-air dehydration. Three microwave power (210, 300, 560 W) and three air temperature (70, 80, 90℃) settings were investigated to find the drying kinetics, rehydration kinetics and colour change. Combined microwave-hot-air drying decreased the drying time required when compared to drying with either hot-air or microwave energy alone. Predictive models were developed to describe dehydration and rehydration kinetics. Dehydration rate, rehydration rate and total colour change of rehydrated product generally increased with microwave level and air temperature. Combination drying with MW=300 W and T=80℃was optimal in terms of drying time, rehydration time and colour.The results of texture analysis and sensory evaluation revealed that instant rice made in double extrusion processing could completely rehydrate for 3-5min, basically excelled the traditional one except for shape, was close to the fresh hybrid cooked rice.