Effect of aggregation properties of zein dispersions on film preparation

Zein is the major storage protein of corn. Due to its unique amino acid composition, zein is insoluble in water, while it is soluble in alkaline solutions and in certain mixtures of water and organic solvents. Over the past decades, zein has shown promising industrial applications, including the manufacturing of biodegradable films and plastics for packaging [1]. Despite the large number of published studies aimed to evaluate the effect of additives and the preparation methods on zein film features, it appears that many authors neglected some fundamental properties of zein solvent dispersion known since the beginning of the last century [2]. Specifically, it is known as temperature and solvent composition markedly affect the “solubility” of zein dispersion. In the last 10 years, such changes were interpreted as modification of zein secondary and tertiary structure [3] and/or supramolecular aggregates formation [4]. In this work the effect of temperature and solvent composition on the aggregation state of zein dispersions were analysed through turbidity and dynamic light scattering (DLS) and the obtained results related with the ability of zein to produce films. The condition analysed were selected among those commonly used in coating applications, particularly the solvent used was an ethanol/water mixtures where the ethanol amount varied between 70 and 90%. The analyses of turbidity and DLS allowed a deep insight on zein aggregation in ethanol/water mixtures. Zein molecules existed as aggregates in all the conditions analysed, although zein dispersions displayed very different properties according with the size of aggregates themselves. As a general rule, the increase of temperature and ethanol amount and the reduction of zein concentration were associated with a sized reduction of zein aggregates. However, strong interactions between parameters were found. Thus, the analysis of each single factor can not be performed without taking into considerations the others. For example, in 90% ethanol, at low zein concentration (1%) oligomers were the prevalent species at all the temperatures, while at higher zein concentration (5%) they were predominant only at high temperatures. Casted films were prepared at different conditions keeping constant the zein concentration at 5%. Visual and microscopy examination revealed as homogenous films could be prepared only at 90% of ethanol and at high temperatures. This is the conditions when oligomers are the prevalent species in the ethanol/water dispersion. In all the others cases the films showed different degree of inhomogeneity up to samples where the inner part of the films showed a typical gel consistency. The presence of inhomogeneity determined a higher variability of the mechanical properties of the films, moreover, despite their lower stiffness, lower elongation at break and ultimate strength were recorded. Temperature, solvent composition and zein concentration affect the zein aggregation state in a complex manner due to their strong interactions. The aggregation state results also to be crucial in film preparation, in fact, only when small aggregates are present homogeneous film can be prepared. Such results should be borne in mind anytime zein films are prepared or the effect of additives is investigated, to avoid any effect due to the incorrect casting procedure.