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Effect of Microwave Heating of Kafirin on the Functional Properties of Kafirin Films

¹Department of Food Science and Technology, Makerere University, PO Box 7062, Kampala, Uganda. ²Food Science and Technology Programme, CSIR Bio/Chemtek, PO Box 395, Pretoria 0001, South Africa. ³Department of Food Science, University of Pretoria, Pretoria 0002, South Africa. ⁴Corresponding author. Phone: +27 12 420 4296. Fax: +27 12 420 2839. E-mail:jtaylor@postino.up.ac.za

To improve the functional properties of cast kafirin films, dry kafirin, extracted with an aqueous ethanol-based solvent at 70°C, was microwave-heated. No effect on film tensile properties was found. Two strategies were employed to improve the effect of microwaving: extraction of kafirin using an aqueous tert-butanol-based solvent at ambient temperature to minimize temperature-induced denaturation and wetting the kafirin to increase its dielectric properties. Microwave heating this kafirin to 90 or 96°C and holding for 1–2 min more than doubled maximum tensile strength and Young's modulus, and decreased strain by about one-third compared with films made from nonmicrowaved kafirin. Film water vapor permeability was reduced by at least one-third. Digestibility of microwaved kafirin and films was also substantially decreased, and film biodegradability was slowed slightly. Microwave heating gave a film microstructure with fewer and smaller size pores. SDS-PAGE showed microwave-induced intermolecular cross-linking of the kafirin monomers, which was possibly responsible for the modification of film properties. Microwave heating of kafirin can be used to modify kafirin film properties, but the kafirin must be microwaved wet and be as close as possible to its native state.