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.
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Acknowledgments The corresponding author would like to thank FONDECYT (11075053) Fund from CONICYT (Chile) for the support of this research.
- Rice starch
- Starch extrudates
- Waxy maize starch