Thermal and mechanical behavior of laminated protein films

The whey protein and zein films plasticized by glycerol and olive oil were prepared by casting method and then were laminated. The thermal behavior of the whey protein, zein and whey protein-zein laminated films was investigated by the dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC). Both techniques showed that the films containing olive oil had higher glass transition temperature (Tg) than the films containing glycerol (e.g.∼115 and ∼88 °C in the zein-olive oil and zein-glycerol films, respectively). As well as, the laminated films had higher Tg than the whey protein films (e.g.∼82 and ∼31 °C in the whey-zein-glycerol and whey-glycerol films, respectively). The Tg values obtained from two different methods were close. The results showed that the Tgs of the zein-glycerol films predicted by Couchman and Karasz equation were very close to the values obtained by DSC experiments. The tensile tests showed that the laminated films had higher tensile strength than the single whey protein films and the single zein films plasticized by olive oil (∼260% and ∼200% in the whey-zein-glycerol and whey zein-olive oil films in comparison to single whey-glycerol films, respectively).