Microwave foaming of starch-based materials (I) dielectric performance

Extruded pellets from starch-based materials have been heated and foamed under microwave radiation. The foaming mechanisms and effects of some additives on the dielectric properties of the materials have been investigated using a microwave calorimeter. A rapid increase in dielectric loss factor (ɛ″) has been found for all the tested materials foamable under microwave heating and the onset of the ɛ″ increase is considered to be correlated to glass transition. It has been found that extruded starch materials can be foamed at 15℃/min but the expansion level reduced compared with that of the same material foamed at higher heating rate. The incorporation of organic additives in starch-based materials generally led to significant decrease of their microwave foamability when glycerol and polyvinyl alcohol are used as the additives in the extruded pellets.