Antimicrobial effects and mechanism of action of essential oils against dominant spoilage bacteria isolated from large yellow croaker (Larimichthys crocea) during chilled storage

To evaluate the feasibility of essential oils as natural preservatives in extending the shelf life of chilled large yellow croaker, the antimicrobial efficacy of 14 essential oils against the dominant spoilage bacteria from large yellow croaker (Larimichthys crocea) during chilled storage at 4 °C was investigated. The dominant spoilage bacteria were isolated from skin, body cavity, gill and intestine of large yellow croaker, and identified by the sequence similarity analysis of the 16S rDNA genes and physiological and biochemical test. The paper disc diffusion method and microbroth dilution technique were employed for the determination of antimicrobial activity of 14 essential oils. Sensory acceptability of fish treated with different quantities of essential oils were evaluated using a nine-point hedonic scale ranging from 1 (most disliked) to 9 (most liked). The mode of action of Litsea cubeba oil against spoilage bacteria was studied by scanning electron micrograph, measurement of release of intracellular lactate dehydrogenase (LDH), and 260 nm absorbing material. After dipping in a solution containing 4 μL/mL L. cubeba oil, the total microbial load, H₂S- producing bacteria, total volatile bases nitrogen(TVB-N), and histamine content of refrigerated large yellow croaker fillets as a function storage time were determined. The results showed that a total of 6 strains of bacteria were identified as four strains of Shewanella spp., one strain of Brochothrix thermosphacta and one strain of Aeromonas sobria. The 14 essential oils showed different antimicrobial activity against spoilage bacteria isolates, and cinnamaldehyde had thse highest activity with a minimum inhibitory concentration(MIC) of 0.125 μL/mL, followed by eugenol, thymol, citral, Cymbopogon citratus and L. cubeba oil, with MIC≤0.5 μL/mL. The flavours of large yellow croaker fillets treated with eugenol, thymol, citral, C. citratus and L. cubeba oil were found to be acceptable. L. cubeba oil can disrupt the cytomembrane of spoilage bacteria isolates and cause the leakage of protease and nucleic acid. Dipping fish fillets in a solution of 4 μL/mL L. cubeba oil significantly reduced the total microbial load, H₂S-producing bacteria, TVB-N, and histamine content. Eugenol, thymol, citral, C. citratus, and L. cubeba oil showed high antimicrobial activity against the dominant spoilage bacteria isolates from chilled large yellow croaker, and had no adverse effects on the organoleptic properties of the fish fillets. Therefore, it can be concluded that essential oils had the potential to be used as natural preservatives to enhance the shelf-life of seafood during chilled storage.