Prediction of the Glass Transition Temperature on Extruded Waxy Maize and Rice Starches in Presence of Glycerol

Polymer science approach has shown to be useful to understand the structural complexity of food systems. This work looks at the effect of glycerol (5%, 10%, and 20% (dry weight basis)) on the glass transition temperature (Tg) of a starchy matrix determined by DSC and its prediction using Ten-Brinke-Karasz equation. Waxy maize starch (WMS) and rice starch (RS) systems were prepared by thermomechanical extrusion. A decrease in Tg, from ~170 to ~25 °C, was detected for both starches (0% glycerol) when the moisture increased from 5% to 25% (wet weight basis (wb)). When glycerol was added, a further decrease in Tg was obtained. Tg was reduced from 120 to 60 °C and 45 °C for 10% and 20% glycerol, respectively, at a moisture content of 10% (wb). A direct comparison between starches with similar composition showed equal Tg (P > 0.05). The modeling for the control samples (0% glycerol) gave a ΔCp ~0.4 Jg ^-1 K ^-1 for WMS and RS and ~1.9 Jg ^-1 K ^-1 for water. These values can be explained by the well-known Tg dependence on degree of polymerization and molecular weight. When the glycerol concentration was increased, an increase was observed, ΔCp ~0.6 Jg ^-1 K ^-1, for both starches and ~2.8 Jg ^-1 K ^-1 for water. This behavior would indicate an overestimation in the plasticizing effect of glycerol due to the possible occurrence of polyol-rich fractions in the extruded samples.