Novel two-step fungal and lactic acid bacteria fermentation as an innovative strategy for soy okara valorization: improvement of sensory properties and bioactive functionality

Abstract

Soy okara is a dietary fiber-rich side stream residue from tofu production, but its limited digestibility and sensory properties restrict its application in food systems. This study investigates a novel two-step fermentation strategy combining solid-state fermentation with Cyclocybe aegerita and subsequent submerged fermentation with five food-associated lactic acid bacteria (LAB) strains (Lacticaseibacillus rhamnosus, Lacticaseibacillus casei, Lactiplantibacillus plantarum, Pediococcus acidilactici, and Pediococcus pentosaceus) to improve nutrient accessibility, sensory properties, and bioactive functionality of soy okara. Fungal fermentation served as an active bioconversion step, improving nutrient accessibility for subsequent bacterial fermentation. C. aegerita reduced total fiber content by approximately 22% and total carbohydrates by approximately 70%, while increasing reducing sugars 3.8-fold, indicating extensive matrix degradation. Consequently, LAB growth increased by 2–3 log₁₀ in CFU/g compared to LAB-fermented soy okara. GC-MS analysis revealed that the two-step fermentation diversified the aroma profile, where fungal fermentation primarily promoted the formation of methyl-branched fatty acids, whereas subsequent LAB fermentation generated acetoin, and diacetyl. In addition, fermentation was associated with significant shifts in taste perception, including reduced bitterness and enhanced umami perception compared to non-fermented soy okara. Furthermore, sequential fermentation significantly enhanced the release of phenolic compounds, resulting in a 4.4-fold increase in antioxidant capacity and approximately 17% stronger anti-inflammatory effects in cell-based assays. Overall, the results demonstrate that fungal bioconversion combined with LAB fermentation enables nutrient accessibility and significant enhancements on sensory, and bioactive properties of soy okara, providing a promising strategy for the valorization of soy-based food side streams.