Devitrification of the amorphous fractions of starch during gelatinisation

Gelatinisation of waxy maize starch (WMS) in excess glycerol was investigated by a temperature modulated DSC in quasi-isothermal and non-isothermal mode. By using the measured specific heat capacities (c(p)) at different stages of gelatinisation and the hypothetical enthalpy of melting for 100% crystalline amylopectin, a method was developed to reveal the amorphous fraction which remains glassy in the presence of excess glycerol what can be called "rigid amorphous". It was suggested that the amorphous part of starch was plasticised at around 40 degrees C whereas, rigid amorphous fraction underwent a second stage glass transition which showed itself by an increase in c(p) under the gelatinisation endotherm. It was also shown that holding WMS in excess glycerol at 100 degrees C (10 degrees C above the gelatinisation onset temperature) for 10 days was not enough to gelatinise the whole WMS implicating that the most stable crystals did not melt before the required threshold temperature was reached. (C) 2012 Elsevier Ltd. All rights reserved.