The role of the glass transition in extruded cereal snack character

This research was an investigation of the role the glass transition of cereal melts play in the determination of direct expanded extruded cornmeal snack character. The investigation began with the successful development of a thermal analysis method to identify glass transitions in extruded cornmeal puffs at cereal melt moistures (approximately 19%db).;Isotherm and DSC data resulting from a study of the plasticization of water on Tg and texture indicated that extruded cornmeal puffs undergo glass transition at 24°C at a water activity of 0.87, corresponding to a moisture content of 20.8%db. Experimental texture results of the samples supported the isotherm and DSC data used to determine the water activity and moisture content sufficient to depress Tg to room temperature.;Investigation of cornmeal components utilized dynamic vapor sorption (DVS), a new technology, to generate sorption isotherms and rapidly condition samples. Component isotherm data allowed for the accurate prediction of cornmeal sorption behavior. As expected, cornmeal isotherm and Tg behavior was similar to the behavior of native dent starch. Analysis of data indicated that the components contributed to cornmeal Tg values on a weighted average basis.;Cornmeal was extruded in a 4 x 2 factorial design to produce samples covering a range of product character. The extrudates were characterized by extruder response, physical properties, texture properties, and polymeric properties.;As the severity of the extrusion scenario increased, expansion properties increased and the texture properties were more acceptable for this type of product. As SME values increased, both molecular weight and Tg values decreased in agreement with glass transition theory.;It was proposed that the glass transition plays a role in the development of product structure as a result of its relationship to melt viscosity. It was proposed here that the location of Tg relative to the cereal melt temperatures (DeltaT) determines the viscosity of the melts (High DeltaT equates to low melt viscosity). The viscosity of the cereal melt controls the rate and extent of expansion that occurs upon expulsion from the die. Experimental results supported the proposed mechanisms of effect of the glass transition on cornmeal snack character.