Effect of the Hydrostatic High Pressures (HHP) on the Microbiological and Physicochemical Properties of ‘Picual’ cv. and ‘Morisca’ cv. Olive Pomaces

Olive pomace (alperujo) is a major by-product of the virgin olive oil industry with a substantial environmental impact but remarkable potential as a health-promoting functional food ingredient. However, its high moisture and organic load necessitate immediate stabilization to prevent degradation. This study evaluated the efficacy of High Hydrostatic Pressure (HHP) processing (400 and 600 MPa for 5 min) as a non-thermal technology to stabilize ‘Picual’ and ‘Morisca’ olive pomaces harvested at two distinct ripening stages (green and ripe). HHP treatments achieved complete microbial inactivation, reducing aerobic mesophiles, molds, and yeasts below detection limits (<1.0 log CFU g−1) regardless of cultivar or ripeness, while exceptionally preserving the macroscopic color profile with total color differences (ΔEHHP) remaining at about or below the human visual perception threshold (<3.0). Conversely, endogenous polyphenol oxidase (PPO) exhibited significant baroresistance, remaining statistically unaltered and active after processing. The pressure-induced cellular disruption, combined with this residual PPO activity, triggered a selective oxidative cascade: ortho-diphenols (hydroxytyrosol, oleuropein, and the verbascoside+rutin fraction) significantly decreased, whereas monophenols (tyrosol and p-coumaric acid) and flavonoid glycosides remained highly stable throughout processing for ‘Morisca’ but exhibited reductions in ‘Picual’, due to their lack of a reactive catechol moiety. Despite these slight oxidative changes, a strong linear correlation (p < 0.001) between total phenolic content and antioxidant activity was maintained. Furthermore, HHP preserved individual profiles of unsaturated fatty acids, such as oleic and linoleic acids. Multivariate Principal Component Analysis (PCA) confirmed that HHP effectively safeguards the macro-physicochemical and nutritional identity of the matrix while ensuring microbiological safety. These findings demonstrate that HHP is a robust bio-stabilization tool, particularly when applied to early-harvested (green) matrices, enabling the sustainable upcycling of olive pomace into safe, lipid-stable, and antioxidant-rich functional food ingredients.