Conversion of oleuropein to hydroxytyrosol by lactic acid bacteria fermentation of olive leaves in water solution with reduced glucose content

Oleuropein is the most abundant bioactive phenolic compound olive trees (Olea europaea L.). It is found in all parts of the plant, but especially in the leaves. This study describes the bioconversion of oleuropein to hydroxytyrosol, a polyphenol with antioxidant and antibacterial properties, by the fermentation of olive leaves by lactic acid bacteria (LAB), using a new, more eco-friendly method that is not based on chemical solvent extraction. This method uses an aqueous solution with reduced glucose content to which ground leaves are added and subsequently inoculated with LAB strains. In this experiment, the pH, glucose, and LAB strains are key factors. We tested a total of fourteen LAB strains for beta-glucosidase activity, from which we selected the five with the best demonstrated activity - Lactiplantibacillus plantarum PB22, Fructilactobacillus sanfranciscensis B415, Lactiplantibacillus pentosus B506, Lactiplantibacillus pentosus B307 and Lactiplantibacillus plantarum B329. The bioconversion was monitored over 28 days using a UPLC system coupled with a UV/Vis Photo Diode Array. The best strains for converting oleuropein to hydroxytyrosol were F. sanfranciscensis B415, L. pentosus B506, and L. pentosus B307 and the optimum fermentation time was found to be 3 days. This work proposes an environmentally friendly low-impact method for reusing agricultural plant wastes.