Bioprocessing of Olive Mill Wastewater Sludge-Derived Subcritical Water Extracts (SWE) by Lactic Acid Bacteria

Olive mill wastewater (OMW) and the sludge accumulated in evaporation ponds represent major environmental challenges in Mediterranean olive oil-producing areas because of their high organic load, phytotoxicity and complex composition. Despite their potential as sources of phenolics and fermentable substrates, lactic fermentation of phenolic-rich aqueous extracts obtained from OMW sludge through subcritical water extraction (SWE) remains scarcely investigated. This study evaluated their suitability as fermentation substrates. The selected extract combined the highest phenolic enrichment, representing a high-value but potentially inhibitory matrix, with the greatest reducing sugar content as an endogenous carbon source. Preliminary assays with Lactiplantibacillus plantarum under non-limiting carbon conditions identified a 75% (v/v) extract dilution as the best compromise between microbial compatibility and phenolic retention. Enzymatic conditioning with pectinase, xylanase and cellulase mobilised endogenous carbohydrates, reaching approximately 50 g glucose equivalents/L without external glucose supplementation. Fermentations at 37 ºC for 180 h with L. plantarum and a wheat-associated Pediococcus pentosaceus strain showed rapid acidification, reaching final pH values of approximately 4.0 and 3.75, respectively, sustained viability at 10⁸ - 10⁹ CFU/mL and partial utilisation of reducing sugars during the first 24 h. Total phenolic content remained stable at approximately 2.5 mg GAE/mL, while strain-dependent functional and physicochemical changes, including matrix clarification with P. pentosaceus, were observed. These findings support an integrated extraction-conditioning-fermentation strategy to convert OMW sludge into phenolic-rich, fermentable matrices for the sustainable development of value-added bioproducts.

ACKNOWLEDGEMENTS

This work was supported by Grant DIN2024-014118-2, funded by MICIU/AEI/10.13039/501100011033. Sergio Martínez-Terol acknowledges the contractual support received through this grant.