Changes in Polyphenols and Antioxidant Properties of Pomegranate Peels Fermented by Urolithin A‑Producing Streptococcus thermophilus FUA329

Studies investigating the bioactivity application of Streptococcus thermophilus FUA329, a potential urolithin A-producing strain isolated from human breast milk in a previous study, are insufficient. We here investigated the changes in polyphenols and antioxidant activities of pomegranate peels before and after fermentation by S. thermophilus FUA329. Liquid chromatography–mass spectrometry was used to detect the polyphenols in the fermentation broth. Total phenolic and flavonoid contents were used as indicators of the antioxidant activity of the broth. The results revealed that the total phenolic and flavonoid contents of the fermented pomegranate peels peaked at 48 h (122.54 ± 0.93 mg GAE/100 mL and 26.09 ± 1.11 mg RE/100 mL, respectively), and urolithins, especially urolithin A, produced by the strain during fermentation were considered a new and primary metabolite that enhances the antioxidant ability of the fermentation broth. Thus, as a potential probiotic, S. thermophilus FUA329 can transform ellagic acid in pomegranate peels to urolithin A and improve the antioxidant power of natural foods.