Development of hydrolysates with antioxidant effects from brewers’ spent grain proteins

Abstract

Brewers’ spent grain (BSG), the most abundant brewing by-product contains up to 24% (w/w) of protein on dry basis. Despite of a rich protein source, it is still mainly used as low value animal feed. This study was conducted to develop hydrolysates with antioxidant effects from BSG proteins by enzymatic hydrolysis, and study BSG peptide antioxidant activities in relation to molecular structures. The BSG proteins were extracted by alkaline method with a protein content of 62.6% (w/w) and a protein recovery of 46.3%. The proteins were then hydrolyzed by selected proteases or protease combinations having different specificities. Resulting hydrolysates were analyzed for their molecular structures such as hydrolysis degree, amino acid composition, molecular weight and conformation, and their antioxidant effects were measured by reducing power, DPPH radical scavenging, ferrous chelating and superoxide radical scavenging assays. Hydrolysates prepared by alcalase and alcalase combined treatments with neutrase, flavourzyme and everlase showed the highest DPPH radical scavenging activities ranged between 72.6-74.9% at the hydrolysate concentration of 1.0 mg/mL. It could be due to specific activity of alcalase to produce amino acid sequences with greater DPPH radical scavenging potential. Similarly, high proton donation ability of the imidazole ring present in the side chain of histidine may have strong ability to scavenge the DPPH radical. The highest superoxide radical scavenging activity of 19.3% was observed in the hydrolysate resulted from alcalase and flavourzyme combined treatment and the value increased with increasing the sample degree of hydrolysis. Highly hydrolyzed proteins can be expected to have more positive charges available on the amine groups to form electrostatic interactions iii with negative charges on superoxide radicals. Everlase and FoodPro PHT combined treatment was the most effective in producing ferrous ion chelating peptides. The ferrous ion chelating ability was negatively correlated to degree of hydrolysis, suggesting that longer peptides are more likely to form compact structures to trap ferrous ions. These results suggest good potential of BSG protein hydrolysates as antioxidants. The commercial potential of BSG protein hydrolysates was analyzed by comparing the highest antioxidant activities with selected commercial antioxidants. The best DPPH radical scavenging activity reported by alcalase and neutrase combined treatment (74.9%) at 1.0 mg/mL was comparable to that of BHT (42.4%) and ascorbic acid (76.3%) at 0.1 mg/mL. The highest superoxide radical scavenging activity observed at 2.0 mg/mL was greater than the activity of ascorbic acid (11.4%) at 0.01 mg/mL. None of the hydrolysates were effective in chelating ferrous ions compared to EDTA, which is a strong metal chelator. All the hydrolysates at 1.0 mg/mL showed significantly (p < 0.05) higher reducing power activity compared to that of ascorbic acid at 0.01 mg/mL (0.16). There is a growing interest in using peptides as natural antioxidants due to their ability to suppress oxidative species through multiple mechanisms. The BSG protein hydrolysates can be incorporated into food formulations to retard lipid oxidation, or suppress oxidative stress and associated negative effects. Similarly, they can be potentially developed as animal feed additive to reduce oxidative stress in livestock and consequently improve animal performance, feed efficiency and productivity. Although BSG protein hydrolysates showed comparable antioxidant effects to commercial antioxidant reagents at high dosage levels, they may be added in food and feed iv formulations at higher concentration with minimum impact on the product sensory quality. This study is expected to bring benefits to breweries, barley growers, food and feed industries. Also, this study gives insight to molecular structures of peptides affecting different antioxidant mechanisms.