Unlocking phenolic bioactive compounds from olive leaves through lactic acid fermentation.

Olive leaves represent about 10% of the biomass generated during olive oil production and are rich in phenolic compounds with known bioactivity. This study evaluated the effect of fermentation with six lactic acid bacteria (LAB) strains on the release of phenolic compounds with antioxidant and antidiabetic activities in leaves from the 'Picual' cultivar. LAB survival was strain-dependent and influenced by specific phenolics. Fermentation with selected LAB significantly increased key compounds: oleuropein rose by up to 55% with Pediococcus acidilactici and 28% with Leuconostoc mesenteroides, while hydroxytyrosol increased by up to 36% with Lactiplantibacillus plantarum, L. mesenteroides, and P. acidilactici. Antioxidant activity was maintained and correlated with oleuropein, hydroxytyrosol, and certain flavonoids. Moreover, fermentation enhanced in vitro antidiabetic activity by 10-37% through α-amylase inhibition, mainly associated with oleuropein aglycone, hydroxytyrosol, ligstroside, and esculetin. Overall, LAB fermentation effectively improves the bioactive profile of olive leaves, supporting their potential use as a value-added functional ingredient.