Material Properties of Starch and Cereal Protein Films and Foams
Materials that are renewable and biodegradable are demanded for environmental reasons. Biopolymeric materials, such as materials from starch and cereal proteins, are both renewable and biodegradable. However, starch and cereal protein materials are generally rather brittle and therefore need to be plasticized to become more flexible.
Investigations were made of physical properties, such as thermomechanical, tensile and barrier properties, of starch and proteins from maize, oats and sorghum with different levels of plasticizers. Avenin was weaker than the other two cereal proteins but more extensible at low plasticizer contents. Zein was the most extensible of the films with high plasticizer content. The manufacturing conditions of films made of high amylose maize starch affected the molecular structure of the starch as determined by light microscopy and size exclusion chromatography, which in turn affected the film forming properties. The mechanical properties were not significantly affected by the manufacturing conditions, however, and the films were found to be equally strong and extensible as zein and kafirin films with similar plasticizer contents. The mechanical properties of native starches were highly dependent on their amylose content, and to some extent also on their phosphate content. Films from pure amylose were the strongest, even stronger than cereal protein films at a similar plasticizer content. Films made of potato starch with a low phosphate content seemed to possess an optimal combination of amylose and phosphate content since they displayed superior extensibility and wet resistance. Kafirin and zein exhibited approximately the same barrier properties, whereas avenin was more permeable.
Due to the troublesome fragility of cereal proteins, the plasticization effect of water, glycerol and 2-mercaptoethanol in zein films were investigated in thermomechanical and spectroscopy (infrared and dielectric) studies. It was found that the mechanisms of water and glycerol did not differ significantly from each other by the hydrogen bonding interaction with the protein. Neither had any greater effect on the protein conformation. 2-mercaptoethanol seemed only to have a small plasticizing effect, completely different from the effects of water and glycerol.
glass transition temperature
water vapour permeability