Sopade: The plasticisation effect of glycerol and water on the gelatinisation of wheat starch

In the extrusion manufacture of starch-based thermoplastics such as biodegradable packaging materials glycerol is an effective additive as a plasticiser, that is, to diminish the brittle nature of the product and provide the desired extent of flexibility. However, the addition of glycerol may also affect the gelatinisation behavior of the starch-water mixture, and hence the required processing conditions for producing a homogeneously gelatinised starch-based material. The effect of glycerol on the gelatinisation of wheat starch is studied using differential scanning calorimetry (DSC). As expected, water acted as a plasticiser in that the onset temperature for gelatinisation (T0) decreased with increasing moisture content. Glycerol, however, increased T0. It is shown that the T0 of starch-glycerol-water mixtures may be predicted on the basis of the effective moisture content of the starch fraction of these mixtures resulting from the relative speed of moisture absorption by glycerol and starch respectively. Moisture sorption kinetics of wheat starch and glycerol in 100 % relative humidity were determined and used to predict the preferential water absorption by glycerol in starch-glycerol-water mixtures and the resulting T0 of the system.