Effect of Membrane Processing on Amino Acid Composition and Antioxidant Properties of Marble Vine Seed (Dioclea reflexa) Protein Hydrolysate

Abstract

Marble vine (Dioclea reflexa) seed flour was defatted using n-hexane and made into a protein isolate by alkaline solubilization and acid precipitation. The protein isolate was hydrolyzed using alcalase and fractionated into different peptide sizes of <1, 1–3, 3–5 and 5–10 kDa using ultrafiltration membranes. The hydrolysate and its membrane fractions were evaluated for amino acid composition and in vitro antioxidant activities. The results showed that sequential fractionation resulted in higher serine, glutamic acid+glutamine, glycine and tyrosine contents. The 5–10 kDa peptides exhibited significantly (P < 0.05) higher 1,1 diphenyl-1-picryhydradzyl radical scavenging and metal chelation activity when compared to the unfractionated protein hydrolysate and other peptide fractions. Lipid oxidation was significantly (P < 0.05) attenuated by the peptide fractions. We conclude that peptide antioxidant activities were significantly (P < 0.05) improved by membrane fractionation, especially for the <1 kDa and 5–10 kDa fractions. Practical Applications Oxidative deterioration of foods, which limits shelf life, increases economic losses and may even lead to formation of toxic compounds which can be prevented using synthetic antioxidants. However, there is increased consumer preference for natural sources of antioxidants. The protein hydrolysate and membrane fractions produced in this work showed antioxidant ability that could make them candidates to replace potentially toxic synthetic antioxidants in foods. Metal chelating ability of the peptides can be used to prevent metal ion-dependent initiation of toxic free radical formation in foods. Incorporation of the peptides into foods will enhance scavenging of toxic free radicals that may form during storage, thereby improving product freshness and longevity. The linoleic acid inhibition potential of the peptides can be used to prevent lipid peroxidation and fatty acid deterioration in foods. Therefore, the Dioclea reflexa seed protein hydrolysates represent good natural sources of antioxidant food preservatives.