Effects of Saccharomyces boulardii Fermentation on Free and Bound Phenolics, In Vitro Bioaccessibility, Antidiabetic and ACE Inhibitory Activities of Chestnut Inner Shell

Chestnut inner shell (CIS) is a phenolic-rich by-product with potential for valorization through microbial fermentation. This study investigated the effects of autoclaving and subsequent fermentation with Saccharomyces boulardii S10 on free and bound phenolics, antioxidant activity, and α-glucosidase, α-amylase, and angiotensin-converting enzyme (ACE) inhibitory activities. Furthermore, simulated in vitro gastrointestinal digestion was performed to evaluate phenolic bioaccessibility and changes in individual phenolic compounds during digestion. Autoclaving increased free total phenolic content (TPC) from 139.09 to 178.61 mg GAE/g DW. During fermentation, the free phenolic fraction exhibited the highest TPC at 48 h (190.81 mg GAE/g DW), accompanied by the lowest TPC in the bound fraction (34.89 mg GAE/g DW). Similarly, α-glucosidase inhibitory activity was highest at 48 h (IC50 = 21.44 µg/mL), whereas ACE inhibitory activity progressively increased throughout fermentation, reaching its maximum at 72 h (IC50 = 10.08 µg/mL). Following in vitro gastrointestinal digestion, gallic and ellagic acid concentrations increased during the intestinal phase. Notably, the 48 h fermented sample exhibited the highest intestinal TPC (218.86 ± 5.02 mg GAE/g DW) among all samples, along with the highest TPC-based bioaccessibility (96.97%) and individual phenolics-based bioaccessibility (16.93%) among the fermented samples. These findings highlight the potential of fermentation to modify phenolic distribution and bioactivities while demonstrating the importance of gastrointestinal digestion in evaluating the bioaccessibility of CIS phenolics.