Study On The Preparation And Characterization Of Instant Millet Powder

In this paper, enzymatic hydrolysate, which was derived from extruded millet, was treated with spray drying process to produce instant millet powder. The influences of extrusion on the physicochemical properties, enzymolysis properties of millet and the spray drying properties were studied. In order to solve the wall deposition problem, maltdextrin(MD) or whey protein isolate(WPI) was added, and limited starch enzyme hydrolyzation method was used. And their influences on spray drying were also investigated. In addition, the processes of extrusion and spray drying were optimized, and the quality of the instant millet powder obtained was analysied.Firstly, extrusion process was adopted in millet thermal treating, and the influences on the physicochemical properties of millet were compared. Then, the extruded millet was subjected to enzyme hydrolysis, and the effects of extrusion on the enzymolysis properties of millets were investigated. The analysis of physicochemical properties the extruded millet was gelatinized completely. Extrusion could decrease the water absorption index(WAI) and increase the water soluble index(WSI) of millet significantly(p<0.05). The pattern of crystal was destroyed by extrusion. Moreover, extrusion decreased the degree of polymerization of the amylopectin chain and increased the content of amylose. The analysis of enzymolysis properties showed that extrusion could improve the soluble solids content and the utilization rate of the hydrolysate(p<0.05). And extrusion could reduce the reducing sugar content in the hydrolysate, reduce the low molecular sugar content of the soluble solids in the hydrolysate, increase the molecular weight of polysaccharide and improve the glass transition temperature of the soluble solids.Nextly, the influences of extrusion on the spray drying properties of millet hydrolysate were also investigated. Results indicated that, extrusion result in a higher recovery rate of spray drying. The color of the powder obtained with extrusion was more close to that of the raw millet powder. It was also found that the moisture content, water activity and hygroscopicity of the powder obtained with extrusion were lower. The antioxidant capacity of the powder with extrusion was lower while the value of the Zeta potential after being dissolved was higher compared with that of the control.In the following, the influences of the MD addition and limited starch enzyme hydrolyzation on the properties of spray drying were investigated. Results showed that, relatively lower dextrose equivalent(DE) value, higher addition amount of MD and lower degree of starch hydrolysis would result in a higher recovery rate, glass transition temperature and buck density, while a lower dispersibility, hygroscopicity, moisture content, water activity and caking capacity of instant millet powder. When MD was used as drying carrier, DE8 had a better spray drying effect than DE20. Under the condition of the same DE value of hydrolysate, properties of the instant millet powder produced by limited starch enzyme hydrolyzation was superior to the powder produced with MD except for the recovery rate. In addition, the microstructure of the particles was also influenced by treatment method.Besides, the effects of protease hydrolysation and WPI addition on the properties of spray drying were also investigated. Results showed that, there was no significantly effects on spraying drying properties when protease hydrolysation was adopted, whereas the recovery rate, moisture content, water activity and hygroscopicity of the powder were increased and buck density and caking capacity was decreased with WPI added, and these effects were enhanced with the addition amount increased.Finally, the process parameters of extrusion and spray drying were optimized and quality of the instant millet powder was analysied. The optimal process parameters were obtained. Extrusion parameters: water addition 15%, screw speed 80 r/min, extrusion temperature 130 °C. Then the obtained utilization rate was 80.98%. The spraying drying parameters: inlet air temperature 170 °C, feeding speed 42.9 m L/min, compressed air flow 140 NL/min. Then the obtained powder recovery was 53.82% and the dispersion was 32 s.