Effect of transglutaminase crosslinking on solubility property and mechanical strength of gelatin-zein composite films

The solubility (WS) of gelatin films in water limits its packaging application of high moisture content foods. Crosslinking by transglutaminase (TGase) and addition of zein were reported to reduce WS of gelatin films, but it is a challenge to incorporate TGase in a system of two immiscible materials of gelatin (water soluble) and zein (ethanol soluble) as well as in the environment of low pH when aqueous acetic acid is used to as a commonly used solvent. Zein in ethanol was added to gelatin in aqueous solution at a controlled rate. The final concentration of ethanol was 8.5% and this film forming solution provided the optimal pH for TGase while keeping both proteins soluble. Crosslinking was confirmed by the formation of higher molecular weight protein in SDS-PAGE. The optimum crosslinking degree (65%) was with 95% gelatin/5% zein. Tensile strength of the TGase treated films was improved from 5 to 36 times and proportional to the gelatin-zein crosslinking degree, while water solubility (~98% insoluble in water) depended on the type of crosslinking. The presence of zein improved the thermal properties (melting temperature increased up to 19 °C) but reduced the transparency as compare to pure gelatin film. TGase modified gelatin/zein composite films with improved water insolubility, water vaper permeability and mechanical properties are potential candidates for high moisture food packaging application.