Calorimetrically Calibrated Ultrasound-Assisted Fermentation Accelerates Isoflavone Aglycone Biotransformation in Astragalus membranaceus

Ultrasound-assisted fermentation (UAF) was evaluated as a calorimetrically calibrated strategy for biotransforming Astragalus membranaceus isoflavones using Lactiplantibacillus plantarum. Water-based calorimetry yielded power-density estimates of 46.51 ± 4.03 and 86.05 ± 4.03 W/L for the fermentation vessel configuration. High-dose UAF accelerated the conversion of isoflavone glucosides into calycosin and formononetin, reaching 99.7 ± 2.4% of the conventional 24 h aglycone level by 18 h with a 50% higher mean formation rate over 10–18 h. No aglycone formation was detected in the sterile ultrasound control under the tested conditions. At the 18 h time-matched sampling point, high-dose UAF yielded higher in vitro antioxidant capacities, stronger DPP-IV and pancreatic lipase inhibition, and greater measured intestinal-phase calycosin recovery (374.80 vs. 314.31 μg/g) than conventional fermentation. At 16 h, extracellular β-glucosidase activity increased dose-dependently from 3.52 ± 0.82 to 11.19 ± 0.45 U/mL (conventional vs. high-dose UAF), whereas intracellular activity remained comparable across treatments. Combined with qualitative SEM and CLSM observations, these findings are consistent with enhanced extracellular enzyme availability and plant matrix disruption, without establishing de novo enzyme synthesis or a specific permeabilisation mechanism.