Soybean Hull Fermentation Into Tempe Modifies Dietary Fibre Composition and Improves In Vitro Antidiabetic Properties

Soybean (Glycine max) processing generates large quantities of soybean hulls that are often underutilized despite their high dietary fiber content. However, the predominance of insoluble fiber, low solubility, poor palatability, and limited physiological functionality restrict their application in food formulations. Fermentation is a promising approach to improve the nutritional and functional properties of legumes, yet its effects on soybean hulls remain unclear. This study evaluated the effects of fermentation during tempe processing, including spontaneous lactic acid bacteria fermentation (SHL) and Rhizopus-inoculated fermentation/soybean hull tempe (SHT), on the physicochemical, prebiotic, and in vitro antidiabetic properties of soybean hulls compared with an unfermented control (SHC). Fermentation significantly increased protein content (23.33% in SHC to 29.78% and 30.27% in SHL and SHT, respectively) while reducing carbohydrate content (62.95% to 53.86% and 56.88%). Soluble dietary fiber and viscosity also increased, accompanied by disrupted and porous fiber structures observed through scanning electron microscopy. These modifications improved the in vitro antidiabetic potential, as indicated by higher glucose adsorption capacity (0.63 to 0.78 and 1.04 g/g) and stronger inhibitory activities against α-amylase (55.51% to 74.34% and 78.42%) and α-glucosidase (52.74% to 52.98% and 57.61%). Fermented soybean hulls also exhibited positive prebiotic activity by selectively promoting the growth of Lactobacillus paracasei LPC-37 while limiting Escherichia coli ATCC 25922 proliferations, with SHT showing the highest prebiotic activity score. These findings demonstrate that traditional fermentation can transform soybean hulls into a promising functional food ingredient with potential benefits for glycemic regulation.