Valorization of onion and chicory by-products and polyphenols recovery using liquid submerged fermentation by selected microbial strains

Onions and chicory are valuable and economically-important vegetable crop, produced commercially in many countries worldwide. Onion skins and chicory leaves are residual biomass, now recycled as organic fertilizers or for animal feed, could be used as a substrate in environmental-friendly bioprocesses to recover high-value bioactive compounds and food ingredients. In this study, a bioprospecting method was carried out using bacterial and yeast strains from several agri-food and environmental niches to verify their ability to use these feedstocks as unique nutrients source. Red and yellow onion skins, as well as green and red chicory leaves were assessed by newly selected starter-driven liquid submerged fermentation assays mainly aimed at the release and modification of polyphenols through microbial activities. Fermented products were also investigated as inexpensive favorable sources of microbial enzymes. In red and yellow onion skins, the treatment with selected bacterial and yeast strains produced an increase of the quercetin aglycone content of about 25% and 60% in comparison with the not fermented material, respectively. After green chicory and red radicchio leaves fermentation, beside a significant modification in the profile of mainly identified polyphenols, the use of bacterial strains promoted an average increase of polyphenols. In particular, in green chicory samples the polyphenols amount increased of about 67%; while in red radicchio leaves fermentation, the highest content of total polyphenols (> 50%) was obtained by the use of a selected yeast strain. In conclusion, the proposed microbial pre-treatment method can be a potential strategy to re-use onion skins and chicory leaves as a fermentation substrate, and as a first step in the development of a bio-refinery process to produce high value-added products. This approach is a part of the funded BBI-JU PHENOLEXA project (Grant Agreement no. 101023225 H2020-BBI-JTI-2020) addressed to build a bio-refinery for recovering polyphenols from agro-food wastes.