Ultrasound cracks the seed coat barrier: a strategy for whole-bean tempeh fermentation from black kidney bean with balanced protein-starch digestibility.

Whole-bean tempeh production is hindered by the compact seed coat of black kidney beans, which restricts fungal colonization and nutrient bioaccessibility. Dehulling, the conventional solution, removes the nutrient-dense outer layer. This study employed ultrasound pretreatment (10-30 min) to modify seed coat structure prior to fermentation with Rhizopus oligosporus, aiming to overcome this barrier while preserving the seed coat. Ultrasound increased seed coat porosity from 0.9% to 4.5%, reduced hardness from 365.7 N to 73.5-116.8 N, and enhanced hydration, thereby facilitating fungal growth and enzyme activity. Ergosterol content, a marker of fungal biomass, increased 9.0-fold over the untreated control. SEC and CLSM further confirmed enhanced protein and starch degradation as well as cell wall disassembly in ultrasound-treated samples. Compared with untreated whole-bean tempeh, ultrasound-pretreated samples achieved 1.4-2.5-fold higher peptide levels, while maintaining moderate starch utilization distinct from the excessive degradation observed in dehulled tempeh. Protein bioaccessibility reached approximately 55%, bridging 37% of the digestibility gap between whole-bean and dehulled samples, yet the estimated glycemic index remained substantially lower (47.3-49.7) than that of dehulled tempeh (67.2). The 20-min treatment provided the best overall performance, demonstrating that controlled ultrasound is a viable strategy for producing nutritionally balanced whole-bean tempeh.