Effects of water-glycerol and water-sorbitol interactions on the physical properties of konjac glucomannan films

Konjac glucomannan (KGM)-edible films were prepared with different amounts of glycerol or sorbitol as a plasticizer. Films were characterized by moisture sorption isotherm, and following conditioning at different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbitol and glycerol) were found to plasticize KGM-based films with respect to their tensile properties. However, thermal properties and water sorption capacity (WSC) of polyol-plasticized KGM films were found to vary with water activity (a(w)), namely at low a(w) (< 0.6), WSC and melting enthalpy were decreased with increasing in polyol content and the opposite was true at higher a. (> 0.6). This was attributed to extensive interactions between plasticizer and KGM that reduced the available active site (-OH groups) for water adsorption. The presence of polyols at low a. appeared to suppress crystalline structures due probably to restricted molecular mobility. These effects were diminished when the moisture content was > 20%.