Effects of gelatinization and moisture content of extruded starch pellets on morphology and physical properties of microwave-expanded products

Extruded pellets were prepared from normal corn starch using a corotating t win-screw extruder (25:1 L/D ratio, 31 mm diameter screw), and then expande d by heating in a conventional microwave oven for 70 sec. The effects of ge latinization level and moisture content of the extruded pellets on the morp hology and physical properties of the microwave-expanded products such as p uffing efficiency, expansion bulk volume, and bulk density were investigate d. The expanded shape and air cell structure differed according to the degr ee of gelatinization of the pellets. Maximum puffing efficiency and expansi on volume with the pellets containing 11% moisture were achieved at 52% gel atinization. For this level of gelatinization, starch was extruded at 90 de greesC barrel temperature. In addition, the moisture content of the pellets critically affected the expansion behavior. The maximum puffing efficiency and expansion volume were achieved in a moisture range of 10-13%. For opti mum product shape and uniform air cell distribution, the pellets should und ergo sudden release of the superheated vapor during the microwave-heating, The expansion by microwave-heating was optimized at approximate to 50% gela tinization.