Effects of Fermentation and Oxidative Degradation on the Composition, Antioxidant Activity, ACE Inhibitory Activity, and In Vitro Neuroprotective Potential of Soybean-Derived Kefir Polysaccharide-Rich Extracts

Kefir is a probiotic beverage produced by symbiotic bacteria and yeasts. Polysaccharide-rich extracts from yellow and black soybeans (S and B) were obtained and subsequently fermented to produce S-F and B-F. The fermented extracts were further subjected to oxidative degradation using ascorbic acid and hydrogen peroxide to generate S-FD and B-FD. Physicochemical analyses revealed distinct differences in composition, phenolic profiles, and molecular weight among S-F, S-FD, B-F, and B-FD. Fourier transform infrared (FTIR) spectra indicated that oxidative degradation altered specific functional group intensities without disrupting the fundamental polysaccharide framework. Fermentation enhanced angiotensin-converting enzyme (ACE) inhibitory activity, and subsequent oxidative degradation further improved this effect. Both fermented and degraded extracts exhibited antioxidant activities, including 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity, ferrous-ion chelating ability, and reducing power, with degraded samples showing greater activity. The effects of the extracts on SH-SY5Y human neuroblastoma cells were evaluated in vitro. No cytotoxicity was observed at concentrations up to 400 μg/mL. Treatment at 200 μg/mL increased cell viability and reduced apoptosis in rotenone (ROT)-treated cells. Multivariate analysis further indicated that oxidative degradation enhanced antioxidant and ACE inhibitory activities but may reduce the protective effects observed in SH-SY5Y cells. Overall, soybean-derived kefir polysaccharide-rich extracts show potential as functional ingredients for applications related to blood pressure regulation and antioxidant activity, while their protective effects in neuronal cell models warrant further investigation.