Ultrasonic processing and fermentation temperature influence protein structure, isoflavone extractability, and antioxidant properties of tempe

Tempe is a fermented soybean product rich in phytoestrogens that shows promise as a dietary intervention for postmenopausal women. However, conventional processing limits its bioactivity. This study investigated the effects of ultrasound-assisted soaking and cooking, combined with fermentation at 30 °C and 36 °C using Rhizopus oligosporus , on the nutritional composition, protein molecular structure, and antioxidant properties of tempe. Fermentation at 36 °C decreased L* and h° by 3% and 3.6%, respectively, while markedly increasing a* by 92%. FTIR analysis revealed structural modifications, including reductions in Amide II (-21.9%), Amide III (-14.1%), and amino acid side chains (-28.3%). Notably, genistein increased over 6-fold (from 32.21 ± 2.01 µg/g to 206.66 ± 8.61 µg/g). Compared to the control tempe, the ultrasound pretreatment, particularly during both soaking and cooking, enhanced color alterations, maximized the ABTS antioxidant capacity, and increased daidzein and genistein concentrations. The effects of ultrasound pretreatment appear to increase recoverable isoflavone aglycone equivalents after extraction/saponification, and antioxidant potential, with larger end-point differences observed at 36 °C under the present processing conditions. These findings provide mechanistic insights into tailoring traditional fermentation processes with emerging technologies to enhance the nutritional and functional properties of tempe.