Primary Fermentation in Wine Production Influence on Phenolic Retention and Valorization Potential of Berry Skin By-Products

Berry skins are rich in phenolic compounds but are commonly discarded as low-value waste during berry wine production. The present study evaluated how primary alcoholic fermentation affects the retention and transformation of phenolics in berry skins of blackcurrant (<i>Ribes nigrum</i> L.), black chokeberry (<i>Aronia melanocarpa</i> L.), lingonberry (<i>Vaccinium vitis-idaea</i> L.), rowanberry (<i>Sorbus aucuparia</i> L.), and cranberry (<i>Vaccinium macrocarpon</i> L.). Non-fermented and fermented skin fractions were analysed using Folin-Ciocalteu and HPLC to determine total and individual phenolic profiles. Primary fermentation induced significant species-dependent changes in phenolic composition. Blackcurrant, lingonberry, and rowanberry skins exhibited substantial decreases in total phenolics (-66%, -26%, and -57%, respectively), driven by strong losses of flavan-3-ols and hydroxycinnamic acids. In contrast, cranberry and chokeberry skins showed net increases in phenolic content (+47% and +18%, respectively), associated with the release of bound phenolics and the appearance of new low-molecular-weight phenolic acids such as gallic acid. Across all species, fermentation enhanced biotransformation into simpler phenolics while reducing major native anthocyanins and catechins. These results demonstrate that the influence of primary fermentation on berry skins is not uniform but dictated by their inherent phenolic architecture. Berries rich in polymeric or conjugated phenolics benefit from fermentation through increased phenolic extractability. The findings provide a comparative basis for optimizing fermentation and post-processing strategies to enhance the valorization potential of berry by-products in food and nutraceutical applications.