ABSTRACT
The effect of solid fat (melting point 70°C) and liquid fat (melting point -20°C) on the microwave expansion of extruded glassy amylopectin pellets was investigated. Glassy, unexpanded amylopectin extrudates containing ≤10% fat were equilibrated at different water activity levels (aw) and then expanded by microwave heating. Expanded products were characterized by analyzing degree of expansion, structure, and texture. For the aw range studied, expansion increased with increasing aw of the pellets until a maximum at aw = 0.605 and then decreased. Completely dehydrated samples did not expand at all. This demonstrated that moisture was the driving force in the microwave expansion of the pellets. The addition of solid fat contributed greatly to expansion. The highest degree of expansion was obtained for pellets containing 6% solid fat at all aw levels. Due to the effect on expansion, the addition of solid fat also affected the microstructure and texture of the expanded products. Concentrations of solid fat >2% resulted in expanded products with a large number of cells, uniform in size, with thin cell walls. Liquid fat addition had a negative effect on microwave expansion. The expanded products obtained from pellets that contained liquid fat had a lower expansion volume and a coarser structure.
