Effects of Rhizopus oligosporus-Mediated Solid-State Fermentation on the Protein Profile and α-Glucosidase Inhibitory Activity of Selenium-Biofortified Soybean Tempeh

Solid-state fermentation (SSF) enhances the nutritional profile of legumes. This study evaluated Rhizopus oligosporus-mediated SSF for selenium (Se) biofortification in soybean tempeh (a traditional Southeast Asian food), assessing the effects of selenate and selenite (0&ndash:60 mg kg&minus:1) on R. oligosporus growth, substrate consumption, mycelium morphology, and Se speciation in tempeh. Selenium supplementation at 18&ndash:24 mg kg&minus:1 reduced soybean protein content by 9.4~13.8% relative to the protein content of the Se-free fermented tempeh (control group, 19.85%) and significantly promoted proteolysis. Higher concentrations (48&ndash:60 mg kg&minus:1) restored protein levels to control values (19%), indicating concentration-dependent regulation of protein stability. Selenate at 42 mg kg&minus:1 significantly increased the levels of flavor amino acids (e.g., glutamate, aspartate), essential amino acids, and total amino acids in tempeh. In contrast, selenite showed no significant improvement in amino acid content and even reduced non-essential amino acids (e.g., alanine, glycine) at high concentrations (42 mg kg&minus:1). Selenium biofortification converted selenate to selenomethionine (SeMet) and Se(VI), but transformed selenite into methylselenocysteine (MeSeCys), selenocystine (SeCys2), and SeMet. Fermented Se-tempeh demonstrated potent &alpha:-glucosidase inhibition (IC50 values ranging from 1.66 &plusmn: 0.05 to 2.89 &plusmn: 0.03 mg mL&minus:1), suggesting Se-enriched soybean tempeh could be considered a promising blood-sugar-friendly food. Thus, developing soybean-based functional foods via co-inoculation of R. oligosporus with inorganic Se is a promising way to enhance tempeh bioactivity.