Optimization of Olive Paste Acidification with Ascorbic and Malic Acids via RSM to Maximize Oil Phenolic and Volatile Composition

Phenolic compounds, particularly secoiridoids derived from oleuropein and ligstroside, are the main determinants of the antioxidant capacity and health-promoting properties of virgin olive oil, yet their content is strongly affected by processing conditions. This study aimed to enhance phenolic enrichment in Picual olive oil through mild acidification of the paste. Four olive samples were processed under a Central Composite Design varying malaxation time (40–80 min), acid concentration (0.02–0.08 mol/kg paste), and acid type (ascorbic or malic), across two maturity indices (MI) per acid, and evaluated by Response Surface Methodology. Ascorbic acid outperformed malic acid for most of the evaluated responses, with the majority of the monitored parameters exhibiting progressive improvements with increasing acid concentration across the tested range. Extraction efficiency reached 75.8–80.0%, increasing with ripening, malaxation time, and acid dose. Acidification did not affect standard quality parameters but enhanced pigment retention (up to 18.9 mg/kg carotenoids; 28.9 mg/kg chlorophylls) and selectively increased oleuropein- and ligstroside-derived secoiridoids. Antioxidant capacity correlated with phenolic content, reaching 1177.9 µmol Trolox equivalents/kg at high acid concentration and medium–high malaxation times. The optimal acid dose depended on MI, with higher doses favoring riper fruit. Overall, in the Picual cultivar, mild acidification is an effective strategy to enrich the antioxidant fraction of olive oil without compromising its quality.