Extraction optimization and antioxidant activity evaluation of se‐enriched walnut proteins

In this study, the proteins of selenium-enriched walnuts produced in three seleniferous areas of Enshi (Yesanguan (YS), Lichuan (LC), and Sancha (SC)) were extracted by the alkaline method. Response surface methodology (RSM) was used for the optimization of protein extraction. The total selenium (Se) content of walnut proteins was 1.3345 mg/kg in YS, 0.6219 mg/kg in LC, and 0.4219 mg/kg in SC, and about 82–85% was organic Se. The major Se speciation was selenocystine and selenomethionine. All walnut proteins showed antioxidant activities in a Se-concentration-dependent manner. The antioxidant activity of YS walnut was stronger than that of other proteins, which exhibited high DPPH radical (69.5%) and moderate hydroxyl radical (46.5%) scavenging activities at 1.0 mg/ml. After enzymatic hydrolysis, protein hydrolysates displayed significantly higher antioxidant capacities than undigested proteins at all tested concentrations. The overall results indicated that Se-enriched walnuts have great potential as functional food additives for a dietary supplement. Novelty impact statement Organic selenium accounts for more than 82% of the total Se in walnut proteins, in which SeCys2 and SeMet are major organic forms of selenium. The Se content in walnut proteins is positively correlated with the antioxidant ability. Walnut protein hydrolysates exhibit better antioxidant activities than proteins.