Bioaccessibility and Microbial Metabolism of (Poly)phenol-Rich Dietary Fiber Fractions from Frozen Mixed Berries in an In Vitro Model

Berries are rich in (poly)phenols and dietary fiber (DF), both known for their health-promoting properties. However, little is known about how these compounds interact within the berry matrix. This study characterized berry fiber fractions as value-added ingredients and assessed their effects on (poly)phenol bioaccessibility and fecal microbiota metabolism. Three fractions were obtained from freeze-dried berries (FDB): insoluble fiber (IFF), soluble fiber (SFF), and (poly)phenol-rich extract (PRE). IFF had the highest DF content and was rich in anthocyanins and ellagic acid. In vitro fecal fermentation showed that all fractions promoted short-chain fatty acid production: SFF generated the most acetate, IFF the most propionate, and FDB the most butyrate. Microbiota-derived metabolites from flavan-3-ols and ellagic acid peaked in IFF at 8–24 h, suggesting gradual release from the fiber matrix. These findings highlight the novelty of using fiber–(poly)phenol complexes as potential prebiotic carriers for controlled delivery of bioactives in the colon.