Polyphenols, as common components with various functional activities in plants, have become a research hotspot. However, researchers have found that the bioavailability and bioactivity of plant polyphenols is generally low because they are usually in the form of tannins, anthocyanins and glycosides. Polyphenol-rich fermented foods (PFFs) are reported to have better bioavailability and bioactivity than polyphenol-rich foods, because polyphenols are used as substrates during food fermentation and are hydrolyzed into smaller phenolic compounds (such as quercetin, kaempferol, gallic acid, ellagic acid, etc.) with higher bioactivity and bioavailability by polyphenol-associated enzymes (PAEs, e.g., tannases, esterases, phenolic acid decarboxylases and glycosidases). Biotransformation pathways of different polyphenols by PAEs secreted by different microorganisms are different. Meanwhile, polyphenols could also promote the growth of beneficial bacteria during the fermentation process while inhibiting the growth of pathogenic bacteria. Therefore, during the fermentation of PFFs, there must be an interactive relationship between polyphenols and microorganisms. The present study is an integration and analysis of the interaction mechanism between PFFs and microorganisms and is systematically elaborated. The present study will provide some new insights to explore the bioavailability and bioactivity of polyphenol-rich foods and greater exploitation of the availability of functional components (such as polyphenols) in plant-derived foods.

To investigate the changes in phenols and antioxidant capacity in fermented grains during different stages of the fermentation process (Xiasha, Zaosha, and single-round stages) of Maotai-flavored liquor, the total phenolic contents of 61 samples, collected in different stages, were analyzed via the Folin–Ciocalteu method, and the phenolic compounds were then identified by high-performance liquid chromatography (HPLC). Subsequently, the antioxidant activities were determined using the DPPH free radical scavenging rate and ABTS and FRAP antioxidant capacities. The correlations among the total phenolic contents, individual phenolics, and three antioxidant activities of the samples were analyzed. The results show that the total phenolic contents of the fermented samples did not change significantly in the Xiasha and Zaosha stages but showed an upward trend in the single-round stage. A total of 12 phenol acids were identified in the fermented grains, including 5 phenolic acids (e.g., ferulic acid and caffeic acid), 4 flavonoids (e.g., luteolin and apigenin), and 3 proanthocyanidins (e.g., apigeninidin), for which the DPPH free radical scavenging rates and ABTS and FRAP antioxidant capacities of all of the fermented grain samples ranged from 78.91 ± 4.09 to 98.57 ± 1.52%, 3.23 ± 0.72 to 13.69 ± 1.40 mM Trolox, and 5.06 ± 0.36 to 14.10 ± 0.69 mM FeSO₄, respectively. The total phenolic contents of the fermented grain samples were significantly and positively correlated with the ABTS and FRAP (p ≤ 0.05), while no significant correlations were found between total phenolic content and DPPH. In general, the total phenolic content, phenolic substances, and antioxidant capacity of the fermented grains exhibited changes during the fermentation process in liquor production, and the phenolic components contributed more to the antioxidant properties of the fermented grains. The present study provides a theoretical reference for analyzing the dynamic changes and antioxidant properties of functional phenolic components in fermented grains.