Thermomechanical glass transition of extruded cereal melts

Cereal melts were produced via extrusion cooking using a twin-screw extruder at in-barrel moisture contents ranging from 25 to 42% (wb). Dynamic mechanical thermal analysis was performed to determine the plasticizing effect of water on the glass transition (T-g) for melts obtained from degermed corn meal, oat flour, pregelatinized corn starch and pregelatinized waxy maize. For degermed corn meal: T-g was found to decrease with increasing water contents up to about 30% (wb). Above 30% moisture content, T-g was found to be essentially constant and approximately equal to 270K. Thus, W-g' was estimated to be 30% moisture content. Por samples of moisture content less than W-g', loss tangent vs. temperature plots in the glass transition region were observed to be broader and of smaller amplitude than those above W-g. Glass transitions ranged from 270 to 321K for sample moisture contents ranging from 36.3 to 22.2% (wb), respectively. For an oat flour, T-g values ranging from 270 to 303K were observed for moisture contents of 39.2 to 26.1% (wb), respectively. Melts formed from pregelatinized corn starch and pregelatinized waxy maize had glass transitions similar to corn meal at a similar moisture content of 27%. The significance of these results, in terms of extrusion cooking of cereals, is discussed in this work. (C) 1997 Academic Press Limited.