Film functionality of prolamins extracted from barley and sorghum brewers' spent grain

Abstract

Commercial production of bioplastics is limited due to their high cost and inferior properties compared to synthetic plastics. Brewer’s spent grain (BSG) is a low-cost brewery by-product, rich in prolamin protein and fibre with potential as a raw material for protein-based bioplastics. However, the functional properties of BSG prolamins are poorly understood. This study characterised prolamins extracted from commercial BSG from barley malt (hordein), and from laboratory prepared BSG from waxy (high amylopectin) and high protein digestibility sorghum grain (kafirin). Cast films made from these prolamins were investigated as a model bioplastic and compared with films made from commercial zein and standard kafirin. Barley BSG gave a lower prolamin extraction yield (9%) than sorghum BSG (37-43%). This was presumably caused by greater disulphide-bonded polymerisation, as shown by SDSPAGE, of the hordein. This was due to industrial drying of the BSG, and because aqueous ethanol is not the optimum solvent for hordein extraction. BSG kafirin protein purity (81-88%) was higher than BSG hordein (71%) because sorghum BSG was washed to remove residual starch, thus semi-purifying it. SDS-PAGE indicated that kafirin disulphide-bonded polymerisation occurred during the sorghum brewing and BSG drying but to a lesser extent than barley. Stereomicroscopy and scanning electron microscopy revealed that all films remained intact after soaking but developed pores. Hordein films had the lowest buffer uptake (28%), followed by the kafirins extracted from different sorghum lines (39-96%). Commercial zein films absorbed the most buffer (105%). These findings reflected the degree of polymerisation of the different prolamins, hordein being the most polymerised and commercial zein the least. Tensile properties of plasticised films revealed that hordein films were similar in strength to commercial zein films but the least extensible (126% strain). Kafirin films were the strongest with intermediate strain values. v Composite kafirin-zein films were made with the aim to produce bioplastics of intermediate functionality between the individual prolamins. Buffer uptake for kafirin-zein films increased from 50% with a high kafirin proportion to 85% with a high zein proportion because of zein’s lower hydrophobicity. All composite films were less strong than kafirin films and less extensible than zein. As the proportion of kafirin increased, film tensile strength increased, and extensibility reduced due to kafirin’s greater disulphide-cross-linking. This study shows that BSG prolamins can produce bioplastics with higher resistance to moisture uptake and greater tensile strength than commercial zein bioplastics. BSG hordein bioplastics are better moisture barriers than BSG kafirin bioplastics but are less strong and less extensible. Furthermore, BSG kafirin and kafirin-commercial zein composites can make bioplastics with better moisture resistance and extensibility than zein and kafirin, respectively.