Microbial Biotransformation of Agro-Industrial Fibre-Rich By-Products into Functional Beverages

This study explores the development of functional fermented beverages using fibre-rich residues derived from olive pruning, vineyard pruning, chicory root, and red onion, obtained after subcritical water extraction of polyphenols. Two microbial strains, <i>Lactiplantibacillus plantarum and Bacillus subtilis,</i> were evaluated for their fermentation performance across different fibre matrices, with and without sugar supplementation. Key parameters including microbial growth, pH evolution, and reducing sugar content were monitored, and <i>Lactiplantibacillus plantarum</i> showed superior acidification and viability (>9 log CFU/mL), especially in sugar-enriched formulations, while <i>Bacillus subtilis</i> showed a limited performance. Based on fermentation efficiency, three sugar-supplemented formulations were selected to scale-up: olive pruning fibre and vineyard pruning fibre fermented with <i>Lactiplantibacillus plantarum</i> and olive pruning fibre fermented with <i>Bacillus subtilis</i>. Red onion fibre extract was excluded from scale-up experiments due to its high viscosity, which made it impossible to measure reducing sugars, consistent with its high water-holding capacity. Fermentation significantly increased antioxidant capacity, reaching up to 750 µmol Trolox equivalents/L and 18 mg of gallic acid equivalents/L in <i>L. plantarum</i>-fermented samples, confirming microbial release of bound phenolics and formation of bioactive metabolites. The resulting beverages were microbiologically stable (final pH < 4.5), sensorially acceptable, and potentially antioxidant-rich, supporting their role in sustainable food system development and circular bioeconomy.