Phenolic Composition Profile and Antidiabetic Activities of Jack Bean [Canavalia ensiformis (L.) DC.] Tempeh

This study examined the effects of fermentation with Rhizopus oligosporus on cell wall structure and permeability, phenolic compound bioavailability, and antidiabetic potential of jack bean [Canavalia ensiformis (L.) DC.] tempeh. Fermentation significantly increased the total phenolic (∼21%) and flavonoid (∼152%) contents, mainly through the release of bound phenolics. Confocal fluorescence scanning microscopy with calcofluor white staining and fluorescein isothiocyanate-dextran probe permeation revealed the disruption of cellulose linkages in jack bean tempeh, which increased permeability and facilitated the diffusion of larger molecules. LC-ESI-QTOF-MS/MS identified 102 compounds across the free, conjugated, and bound phenolic fractions, with the conjugated fraction showing the most diverse composition. Certain bioactive compounds, such as D-(+)-3-phenyllactic acid, apigenin-7-O-glucoside, luteolin-7-neohesperidoside, kaempferol, arbutin, and oleacein, were tentatively linked to α-amylase and α-glucosidase inhibition. The free phenolic fraction exhibited the strongest α-amylase inhibition [half maximal inhibitory concentration (IC₅₀)=35.75±0.91 µg/mL], whereas the conjugated and bound fractions were most effective against α-glucosidase (IC₅₀=353.70±119.79 and 56.33±10.40 µg/mL, respectively). Correlation analysis revealed positive associations between phenolic content and α-amylase inhibition (r>0.8, P<0.05). In summary, fermentation improved cell wall permeability, the release of bound phenolics, and antidiabetic potential, suggesting that jack bean tempeh, rich in bioaccessible phenolics, is a potential functional food for glycemic regulation.