Quality protein maize (QPM) zein : characterisation and functionality

Abstract

Quality protein maize (QPM) is a high-lysine maize type that was developed through breeding, to improve its grain hardness and agronomic performance. Despite QPM’s nutritional advantage, this maize type has not been widely adopted mainly due to lack of a commercial market and probably because the functionality of its prolamin protein (zein) are largely unknown. In this study, zein extracted from white QPM was characterised. Its functional properties concerning the formation of bioplastic films, viscoelastic masses “doughs” and gels were investigated and compared to “total” zein from a related regular white Ethiopian maize, total zein from regular white South African maize and commercial zein. Amino acid analysis and SDS-PAGE confirmed that QPM had high lysine, cysteine and γ-zein contents. Bioplastic films were prepared from the zeins by casting from aqueous ethanol and from glacial acetic acid (GAA). QPM zein films were more opaque compared to other zein films cast from the same solvent. The opaqueness was attributed to the partial solubility of QPM zein when dissolved in the casting solvents. During preparation of viscoelastic masses, zein preparations were either mixed with a solvent (water or acetic acid) or coacervated from solution in GAA (simple coacervation). Neither QPM zein nor total zein preparations from regular maize formed viscoelastic masses. Possibly, the presence of β- or γ-subclasses hindered interaction of α-zein with the solvent, unlike with commercial zein (predominantly α-zein) which formed viscoelastic masses with water and 5.4% acetic acid treatments. Interestingly, QPM zein formed a viscous mass when the prolamin was dissolved in GAA during coacervation. Hence, the firmness of the zeins was further investigated. It was thought that different levels of β- and γ-prolamins would contribute to gelation. Therefore, total kafirin, kafirin without β-kafirin and kafirin without γ-kafirin preparations were used as controls. Compression testing showed that only total kafirin formed a true gel because its maximum compression force was the highest (9.61 N), compared to other zein and kafirin preparations (<1.57 N) which formed either runny liquids, solutions or colloids. Formation of a kafirin gel is attributable to disulphide crosslinking between γ-kafirin. To investigate the effect of disulphide bonding on prolamin gelation, reducing agent (5% 2-mercaptoethanol (BME)) was added to GAA. After inclusion of BME, QPM zein and kafirin without γ-kafirin formed gels, probably due to protein unfolding. Due to the abundant γ-zein, QPM zein is a potential raw material for producing bioplastic films with better barrier properties.