Abstract: This study was intended to evaluate the effect of oregano essential oil against Pseudomonas fluorescens and the quality of salmon during cold storage. The minimal inhibitory concentration (MIC) of oregano essential oil against P. fluorescens was determined in vitro, as well as its effect on the growth curve, membrane fluidity, microstructure, electrical conductivity and alkaline phosphatase (AKPase) activity of P. fluorescens. Salmon fillets were inoculated with P. fluorescens, dipped in oregano essential oil and then stored at 4 ℃. Those not treated with oregano essential oil were served as control. Changes in the quality indicators including sensory quality, bacterial count, total volatile basic nitrogen (TVB-N) content, biogenic amines and T2 parameters of nuclear magnetic resonance were studied during storage. The results showed that the minimum inhibitory concentration (MIC) of oregano essential oil against P. fluorescens was 0.062 5% (V/V). The bacterial cells treated with oregano essential oil at MIC exhibited increased electric conductivity and lose their original morphology. The fluorescence intensity of the cell membrane of P. fluorescens treated by oregano essential oil was higher than that of the control, indicating that oregano essential damaged the cell membrane during cold adaptation leading to the leakage of intracellular compounds and consequently reduced ability of P. fluorescens to regulate membrane fluidity. At the end (day 12) of storage, the total bacterial count of the treated sample decreased by 1.34 (lg(CFU/g)), the TVB-N content by 7.48 mg/100 g, and the contents of putrescine and cadaverine by 48.5% and 33.5%, respectively as compared to the control. In addition, oregano essential oil inhibited transformation of immobilized water to free water, thereby inhibiting the growth of P. fluorescens and maintaining the quality of salmon fillets.

Key words: oregano essential oil, Pseudomonas fluorescens, cold storage, salmon, preservation