Soybean (Glycine max) hull valorization through the extraction of polyphenols by green alternative methods

•Soybean hull is a source of polyphenols not currently exploited.•An alkaline green method was optimized for polyphenols extraction.•A. oryzae fermentation and enzymatic treatment enhanced polyphenols liberation.•Phenolic acids, flavonoids and isoflavones were recovered. Soybean is one of the greatest crops in the world, with about 348.7 million tons being produced in 2018. Soybean hull is a by-product produced during the processing of soybean to obtain flour and oil. Though not being actually exploited, it is a source of polyphenols with antioxidant activity. Here, the extraction of polyphenols from soybean hull was performed by means of an alkaline hydrolysis treatment, which was optimized by the response surface methodology. At the optimal region, a total phenolic content of 0.72 g gallic acid equivalents per 100 g of soybean hull was obtained with an antioxidant activity of 2.17 mmoles of Trolox equivalents. Polyphenols responsible for the antioxidant activities were identified by LC–MS, including phenolic acids, anthocyanins, stilbenes, and the two main isoflavones of soybean, daidzein and genistein, in their non-glycosylated form. Other alternative extraction methods based on Aspergillus oryzae fermentation and α-amylase hydrolysis are also proposed.