Modification of kafirin film properties by microwave heating

Abstract

Kafirin, the prolamin of sorghum, is hydrophobic and produces biodegradable and/or edible films of good tensile and barrier properties. However, the diverse packaging requirements necessitate modification of the functional properties of kafirin films. Microwave heating is known to cross-link proteins and has been used to modify the functional properties of whey protein films. Here, microwave heating was applied to kafirin before film casting and to cast kafirin films. The effects of microwaving on the tensile and barrier properties, digestibility, biodegradability, microstructure and protein conformation and cross-linking of kafirin films were investigated. Microwaving of kafirin powder before film casting had no effect on the film functional properties. Microwave heating of wetted kafirin to 96oC for 2 min and microwaving of cast films at 80 W for 2 min increased tensile strength, decreased strain and slowed the biodegradability of the films. Microwaving before film casting decreased film water vapour permeability (WVP), while microwaving after film casting had no effect. The microstructure of the films indicated a general decrease in the number and size of pores with microwaving. Microwaving decreased the digestibility of kafirin, films made from microwaved kafirin and non-plasticised films cast in aqueous ethanol (AE) but had no effect on the digestibility of plasticised films cast from AE and films cast from glacial acetic acid (GAA). The GAA cast films had a higher digestibility than AE cast films, suggesting GAA mediated deamidation. FTIR, SDS-PAGE and SS-bond assay were used to study the changes in the conformation and cross-linking of kafirin after microwave heating. Microwave heating increased the ƒÒ-sheet conformation of the kafirin in films microwaved before and after casting. However, the kafirin in the films microwaved before casting was predominantly ƒÒ-sheet while that of the films that were microwaved cast was predominantly ƒÑ-helical in conformation. It is postulated that exposure of kafirin to moisture (in microwave heating before film casting) and to GAA (in microwave heating after film casting) altered the kafirin conformation before the application of the microwaves. SDS-PAGE indicated microwave-induced intermolecular SS and/or non-SS covalent cross-linked kafirin oligomers. The oligomers seem to be responsible for the reduced digestibility of kafirin and kafirin films after microwave heating. SS bond assay confirmed that microwaving induced SS cross-linking. The increase in glass transition temperature of non-plasticised films after microwaving further suggests microwave induced cross-linking. It is proposed that the microwave heating induced SS cross-links created a dense protein network in the kafirin films, which resulted in the increased tensile strength, reduced strain and digestibility and slowed biodegradability of kafirin films. The conformational changes following microwaving seem to have facilitated the cross-linking of the kafirin. Although microwave heating of kafirin can modify kafirin film properties, the kafirin must be microwaved wet. Microwave heating after film casting was more effective than microwaving before film casting. The increased tensile strength and decreased WVP of microwaved kafirin films suggests that they could provide increased protection of e.g. coated fruits against mechanical damage and moisture loss.