Elucidating the Multifunctional Roles of Olive Oil By-Products: Phenolic Profiles, Antioxidant Capacity, Probiotic Growth-Promoting and Antimicrobial Activity

The olive oil industry generates substantial by-products, notably olive pomace and stones, which present environmental challenges yet harbour untapped bioactive potential. This study investigated the phenolic profiles, antioxidant capacity, and probiotic growth-promoting and antimicrobial activities of olive pomace (OP), olive pomace extract (OPE), olive stones (OS), and olive stone extract (OSE). Phenolic composition, determined by HPLC-DAD/MS, revealed a richer profile and higher phenolic levels in OPE than in OSE, consistent with antioxidant capacity, which was 56.8% higher in OPE (as Trolox equivalents). Probiotic growth promotion was assessed using Lactiplantibacillus plantarum IMC 509 and Lacticaseibacillus rhamnosus IMC 501®, monitoring growth curves, short-chain fatty acid (SCFA) production, total phenolic content (TPC), and antioxidant levels in supplemented culture fluids. Antimicrobial activity was examined against opportunistic pathogens such as Escherichia coli, Pseudomonas aeruginosa, Bacillus cereus, and Staphylococcus aureus. All substrates supported probiotic proliferation, and fermenting fluids from L. plantarum IMC 509 increased or sustained SCFA production after 28–30 h. OP and OPE significantly enhanced TPC and antioxidant capacity for both strains, whilst OPE and OSE inhibited the tested pathogens. These findings identify olive oil by-products as promising functional food and nutraceutical ingredients, providing preliminary in vitro evidence whilst offering a route to valorise a problematic waste stream.