Extraction of kafirin and zein and their film properties

Abstract

Kafirin and zein are alcohol-soluble prolamin-type proteins that can be used to make biodegradable, environmental-friendly bioplastic films and coatings. Despite this potential, there is no commercial process for kafirin extraction. Zein, though commercially produced, is expensive. Factors affecting kafirin and zein extraction and bioplastic quality were studied. Specifically, the effects of sorghum and maize grain particle size, inclusion of glacial acetic acid and sodium hydroxide in the aqueous ethanol extractant on extract filtration time, protein preparation purity and yield; as well as effect of heating kafirin and zein films on water uptake and film digestibility were investigated. Sorghum meal with particle size >1 mm gave more rapid extract filtration time than sorghum flour with particle size <250 ?m, due to larger inter-particle spaces. Sorghum flour yielded a kafirin preparation with lower purity (50%) than sorghum meal (58%). Kafirin extracted from distillers dried grains with solubles (DDGS) had a higher purity (78%) than kafirin from sorghum meal (68%) and zein (69%). Washing decorticated sorghum and maize meals to remove starch prior to protein extraction substantially increased protein preparation purity. Inclusion of sodium hydroxide resulted in a slightly higher protein preparation purity and yield than inclusion of acetic acid. SDS-PAGE under non-reducing conditions revealed more dimers in kafirin than zein, while kafirin from DDGS contained polymers, indicating polymerisation due to cross-linking occurring during DDGS drying. All films absorbed water but remained whole on soaking. Heat treated films prepared from kafirin extracted from washed sorghum meal absorbed less water (approximately 42%), compared to zein films (approximately 47%) due to its greater hydrophobicity and heat induced intermolecular disulphide cross-linking. Heating had no effect on DDGS kafirin film water uptake because it was highly cross-linked. In vitro pepsin and total digestion were less for kafirin films than zein films, probably due to the greater hydrophobicity of kafirin. Heating the films resulted in reduced digestibility, probably due to heat induced disulphide bonding. Extraction of kafirin and zein from coarse meal enables more rapid filtration than from fine flour. Acetic acid-ethanol method appears to be a simpler extraction procedure than NaOHethanol method as it does not require protein neutralisation. Washing decorticated sorghum and maize meals prior to protein extraction improves protein preparation purity. Sorghum DDGS shows potential as a starting material for kafirin extraction for use in film making, based on high purity and low film water uptake. Heating the films decreases water uptake properties and reduces film digestibility.