TRANSITIONS IN FROZEN GELATINIZED-STARCH SYSTEMS STUDIED BY DIFFERENTIAL SCANNING CALORIMETRY

Differential scanning calorimetry (d.s.c.) has been used to study the transitions that occur when gelatinized-starch systems are heated from -20-degrees-C to +20-degrees-C. The samples contained from 10 to 70% polymer. With gelatinized suspensions containing less than about 30% starch, a glass transition due to plasticization of amorphous polymer was detected at about -5-degrees-C. The ice in the specimen melted at about the same temperature as pure ice. In the case of samples of intermediate concentration, containing, say, 45% starch, an exotherm due to crystallization of unfrozen water occurred once the glass-transition temperature had been exceeded. This exotherm only had a significant area when the sample was frozen at a relatively high rate. The melting of the ice in these specimens was characterized by a biphasic endotherm. It was established that, when starch is gelatinized in a d.s.c. pan, water evaporates and can condense on the pan lid. Division of moisture between the sample and the lid was found to be responsible for the twin ice-melting endotherms. Experiments showed that concentrated amorphous polymer caused the melting temperature of the ice in the specimen to decrease below that of pure ice, whereas the crystalline domains had less effect. When the concentration of gelatinized polymer was increased still further, for example, to 70%, polysaccharide-water interactions inhibited freezing. and no transitions were detected in the d.s.c. trace.