Study on antifreeze effect of oregano essential oil on Japanese flounder (Paralichthys olivaceus) muscle during frozen storage

To address the quality deterioration of Japanese flounder (Paralichthys olivaceus) muscle characterized by the formation of a paste-like texture during frozen storage at −20 °C, we investigated the antifreeze mechanism and optimal concentration of oregano (Origanum vulgare) essential oil. Healthy P. olivaceus with a body mass of (700±50) g were used as experimental subjects, and four groups were established: a control group (Soaked in normal saline) and three oregano essential oil soaking groups at concentrations of 100, 500, and 1000 mg·kg⁻¹ (Mass fractions of the key components: carvacrol ≥5.0%, thymol ≥5.0%, eugenol ≥2.5%, cinnamaldehyde ≥4.0%). The samples were stored at −20 °C for 28 d, and indicators including texture properties, moisture-related parameters, myofibrillar protein oxidation and denaturation, antioxidant capacity, and microstructure were measured every 7 d. The results show that after 28 days of frozen storage, the control group exhibited significant quality deterioration (p<0.05): hardness, springiness, and chewiness decreased by 46.21%, 27.73%, and 38.86%, respectively; the drip loss reached (17.49±1.01)%; water-holding capacity (WHC) decreased to 84.36 ± 0.85%; the carbonyl mass fraction of myofibrillar protein increased by 82.74%; the activities of superoxide dismutase (SOD) and catalase (CAT) decreased by 62.90% and 65.56%, respectively; malondialdehyde (MDA) concentration increased to 3.98 ± 0.19 nmol·mg⁻¹; the muscle fiber structure was disordered with severe degradation of the endomysium; and collagen mass fraction decreased to (0.251±0.021) mg·g⁻¹ (A 58.96% reduction compared with the initial value). The 100 and 500 mg·kg⁻¹ oregano essential oil groups significantly ameliorated the above deterioration (p<0.05), among which the 500 mg·kg⁻¹ group showed the best effect: on 28^th day, its hardness was 24.97% higher than that of the control group, Ca²⁺-ATPase activity increased by 26.69%, and muscle fibers still maintained a partial hexagonal arrangement. The 100 mg·kg⁻¹ group had a 17.56% lower drip loss than that of the control group, and its collagen mass fraction was 26.2% higher than that of the control group (p<0.05). In contrast, the 1 000 mg·kg⁻¹ group exhibited inferior protective effects compared with the medium-low concentration groups. In conclusion, medium-low concentrations (100, 500 mg·kg⁻¹) of oregano essential oil can alleviate the quality deterioration of frozen P. olivaceus muscle, possibly by regulating ice crystal morphology, inhibiting oxidation, and reducing myofibrillar protein degradation.