Fermentation of Common Nettle Extracts by Ligilactobacillus salivarius: New Avenue for the Development of Added-Value Bioactive Products

The medicinal plants industry generates approximately 30 million tons of by-products annually, most of which remain underutilized. The common nettle (<i>Urtica dioica</i> L., <i>Urticaceae</i>) is a valuable medicinal plant, rich in polyphenols, vitamins, and essential fatty acids, widely used in food and pharmaceutical applications. Its by-products still lack sustainable valorization strategies. This study aimed to valorize nettle tea by-products and flowers using green extraction techniques and microbial biotransformation. Lyophilized aqueous/ethanolic extracts were fermented with <i>Ligilactobacillus salivarius</i> ATCC 11741 to assess whether fermentation could enhance the content and bioavailability of phenolic compounds while maintaining probiotic viability. The results showed that fermentation significantly increased phenolic content and antioxidant activity, with chlorogenic acid concentrations increasing up to 4-fold and caffeic acid derivatives up to 2.5-fold. <i>L. salivarius</i> remained viable during fermentation, demonstrating the potential for the production of added-value extracts and probiotic biomass. These findings indicate that nettle by-products can be efficiently converted into functional ingredients through low-energy, environmentally friendly processes, supporting sustainable production and waste valorization.