Material Properties of Cereal Protein Films
Materials that are renewable and biodegradable are demanded due to environmental aspects. Biopolymeric materials, such as materials from prolamins, fulfil both of these requirements. Prolamins are cereal storage proteins. Examples of such are zein, gluten, kafirin and avenin from maize, wheat, sorghum and oats, respectively. Prolamin materials are generally rather brittle and need therefore to be plasticized in order to become more flexible.
Physical properties, such as thermomechanical, tensile and barrier properties, of kafirin, zein and avenin films, with different levels of plasticizer, were investigated. Zein and kafirin were plasticized by a mixture of glycerol, lactic acid and polyethylene glycol, whereas avenin was plasticized by sole glycerol. Avenin was weaker than the other two, but more extensible at low plasticizer contents. Zein was the most extensible of the high plasticizer content films. Kafirin and zein exhibited approximately the same barrier properties, whereas avenin was more permeable. Compared to synthetic polymers, these prolamin materials are not as water resistant and extensible as polyethylene, but can be made stronger and more resistant to oxygen. Other synthetic polymers, such as polyvinyl alcohol are better oxygen barriers, but worse water vapour barriers than these prolamin materials.
The plasticization of prolamin materials is an important issue. By studying the plasticization mechanism we can better understand and control the properties of these materials. Therefore the plasticization effect of water, glycerol and 2-mercaptoethanol in zein films was investigated by a combination of thermomechanical and spectroscopy (infrared and dielectric) studies. It was found that the mechanisms of water and glycerol do not differ significantly from each other by the hydrogen bonding interaction with the protein. Neither one has any greater effect on the protein conformation. 2-mercaptoethanol was likely evaporated already during film casting, as all samples containing this substance displayed the same glass transition temperature. However, a small portion of the 2-mercaptoethanol seems to have broken the disulphide bonds in the protein before evaporation, which is a totally different plasticizing mechanism compared to the ones of water and glycerol.
water vapour permeability
glass transition temperature