Oregano Oil Emulsion Coatings as a Natural Approach to Extend the Shelf Life of Atlantic Bonito (Sarda sarda) Fillets

Atlantic bonito (Sarda sarda) is a commercially important fish species with high nutritional value that is widely harvested along the Portuguese coast; however, it is highly susceptible to lipid oxidation and microbial deterioration during refrigerated storage. This study investigated the effectiveness of oregano oil (OO) emulsion coatings, with and without fish gelatin, to preserve quality and increase shelf life of Atlantic bonito fillets stored at 5 °C for 10 days. Emulsions were produced by high-intensity ultrasound treatment and subsequently, applied as surface coatings. Physicochemical, microbiological, and quality-related parameters, including pH, water activity, lipid oxidation, protein oxidation, color, and texture, were evaluated throughout storage. Water activity of the coated fillets remained stable (p > 0.05), whereas pH values remained below 6.0 until day 7. Coated fillets exhibited reduced oxidative deterioration and delayed microbial growth compared with uncoated fillets, with the 10% OO emulsion coating (OO10) showing the higher effectiveness in maintaining color stability and limiting oxidation. The OO10 emulsion coating with fish gelatin (FO10) improved antimicrobial performance, although the gelatin matrix appeared to modulate the release of bioactive compounds from OO. Overall, OO emulsion coatings represent a promising natural preservation approach for extending the shelf life and maintaining the quality attributes of refrigerated fish products.