Oligosaccharide Production from Olive Oil Industry By-Products: A Comparative Review with Other Agro-Industrial Substrates

Olive industry by-products, particularly olive pomace (OLP), extracted olive pomace (EOLP), and olive stones (OLS), are promising alternative substrates for oligosaccharide (OS) production, as their polysaccharides can be converted into OS by physicochemical treatment, enzymatic hydrolysis, or direct fermentation. However, OS mixtures and one-step strategies integrating enzyme production and substrate conversion remain underexplored. This review evaluates OLP, EOLP, and OLS as raw materials for producing XOS-rich and POS-related oligosaccharides, as well as OS mixtures with prebiotic and technological potential, emphasising direct fermentation. A structured literature mapping was used to compare substrate types, product classes, and production strategies. Conventional physicochemical and enzymatic strategies are compared with the emerging direct fermentation strategy in terms of substrate accessibility, OS yield, degree of polymerisation, product composition, downstream requirements. Physicochemical processes enable rapid solubilisation but require careful control to limit over-depolymerisation, whereas enzymatic hydrolysis provides greater selectivity but depends on substrate accessibility and enzyme inputs. Direct fermentation integrates enzyme production and substrate conversion, although product control and recovery remain challenging. Human evidence is strongest for XOS, while POS and OS mixtures require direct validation. Further progress requires linking substrate composition and process selection to product standardisation, biological validation, and application-oriented testing.