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 (Tg) for melts obtained from degermed corn meal, oat flour, pregelatinized corn starch and pregelatinized waxy maize. For degermed corn meal, Tgwas found to decrease with increasing water contents up to about 30% (wb). Above 30% moisture content, Tgwas found to be essentially constant and approximately equal to 270K. Thus, Wg′ was estimated to be 30% moisture content. For samples of moisture content less than Wg′, loss tangent vs. temperature plots in the glass transition region were observed to be broader and of smaller amplitude than those above Wg. Glass transitions ranged from 270 to 321K for sample moisture contents ranging from 36·3 to 22·2% (wb), respectively. For an oat flour, Tgvalues 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.