Fungal biotransformation of okara into functional food: Comparative species-dependent metabolomic and antioxidant profiles

Abstract

AbstractOkara, a nutrient-dense by-product of soybean processing, is abundantly produced in many Asian countries and holds significant potential for multiple purposes. Solid-state fungal fermentation represents a promising approach to improve its nutritional quality and functional properties; however, there is currently no research available on how fungal starter selection shapes the metabolites in the final product. This study aimed to investigate and compare the metabolite profiles of okara fermented using five different fungi: three species of Rhizopus (Rhizopus microsporus, Rhizopus oligosporus, and Rhizopus oryzae), Aspergillus oryzae, and Neurospora sitophila. Increased glucan content, which represents the fungal biomass, was observed after all fermentation periods (24 h for Rhizopus spp.; 48 h for A. oryzae; 72 h for N. sitophila). The highest glucan content, at 9.07% w/w, was achieved after fermentation with N. sitophila, representing an 11.20-fold increase, with α-glucan accounting for 6.38%. Meanwhile, R. microsporus yielded the highest β-glucan content at 5.93%, achieving a 20.45-fold increase. The LC-MS results showed that each starter facilitated the expression of different metabolites. A total of 181 metabolites were detected, with Rhizopus spp. notably producing 49–61 unique metabolites, while A. oryzae and N. sitophila distinctively resulted in formation of betaine and L-glutamic acid. In addition, fungal-fermented okara exhibited elevation of phenolic compounds by 2.19- to 4.13-fold and correspondingly possessed superior antioxidant properties compared to non-fermented okara. All told, the present study provides substantial information on considerations in fungal starter selection for fermenting okara.