The effect of sugars on the extrusion of maize grits: I. The role of the glass transition in determining product density and shape

Abstract

The effects of sugars (sucrose, glucose, fructose, xylose, lactose and maltose) on both sectional (radial) and longitudinal expansion and subsequent shrinkage of maize extrudates have been investigated. Sugars reduced the sectional expansion, monosaccharides more than disaccharides. Reduced sectional expansion and increased density with sugar content was interpreted as a combination of a reduction in bubble growth and an increase in the degree of shrinkage on leaving the die. The decrease in bubble growth was considered to be the result of a temperature reduced driving force for bubble growth and reduced bubble wall extension before rupture caused by less starch conversion with increasing sugar content. Shrinkage stops when the temperature decreases to approximately Tg+ 30°C, where Tg is the glass transition temperature. The addition of sugars and water will reduce the glass transition temperature (Tg) of the melt and hence increase the temperature range over which the extrudate will shrink. The driving force for shrinkage is the release of stored elastic energy following bubble rupture and the pressure difference between the interior of closed cells in the product and atmospheric pressure.