Harnessing the photothermal properties of table olives: A near-infrared light strategy for enhancing the shelf-life of low-salt naturally fermented black olives

Abstract

This study introduced a novel, non-chemical strategy for enhancing the microbial safety of low-salt, naturally fermented black table olives by harnessing their intrinsic light-to-heat conversion properties. Spectroscopic analysis revealed broad absorbance in the visible and near-infrared (NIR) regions, confirming that the olives acted as light-to-heat converters due to their rich phenolic and pigment content. When exposed to NIR light sources olive samples reached surface temperatures up to 80 °C within 7 min, providing the conditions required for localized photothermal inactivation of surface microorganisms. Artificial contamination experiments with Clostridium butyricum at an initial inoculum of ∼10⁸ CFU/mL showed a ∼ 4-log reduction after a 4-min NIR light treatment, with microbial counts reduced below the detection limit (<1 log CFU/g) in some samples. Listeria monocytogenes viability decreased by ∼2.5 log units when irradiated on olive surfaces (∼10⁶ CFU/g), but no reduction was observed when the bacterial suspension alone was irradiated, confirming that inactivation occurred through an olive-mediated photothermal mechanism. SEM analysis revealed membrane disruption in treated cells, supporting heat-induced rather than non-thermal damage. Physicochemical analyses confirmed that the NIR light treatment did not significantly alter key quality attributes, including pH, color, firmness, dry matter, oil content, acidity, and total phenolic content. Sensory evaluation showed no significant differences in bitterness, acidity, firmness, fibrousness, or crunchiness, with only a minor reduction in perceived saltiness. A six-month ambient storage study (22–25 °C) revealed that a single pre-treatment with NIR light reduced total viable bacterial and mold counts by up to 2-log units compared to untreated controls, while preserving all physicochemical and sensory characteristics. These results demonstrated that NIR light irradiation is an effective, clean-label intervention capable of reducing microbial risks and maintaining product quality in low-sodium table olive formulations.