Antimicrobial Activities Of Essential Oils Against Special Spoilage Bacteria Isolated From Marine-farming Fish And Evaluation Of Their Fresh-keeping Effects

Aquatic products are highly susceptible to spoilage and deterioration due to enzymatic autolysis,oxidation,and microbial growth.Currently,aquatic products are mainly preserved refrigerated or frozen.Modified atmosphere packaging,irradiation,ultra-high pressure or the addition of preservatives have been use to extened the shelf-life of chilled aquatic products,but usually with a limited ability to inhibit bacteria,or caused sensory deterioration issue.With the increase of consumers’ demand for natural green food,there is an urgent need to find a safe and efficient natural alternatives to replace synthetic chemicals in fish preservation,with an aim to facilitate the distribution and processing of aquatic products.Essential oils(EOs)are volatile,aromatic,fat-soluble natural mixtures extracted from aromatic plants and are recognized as Generally Regarded as Safe(GRAS)by FDA.Studies have demonstrated that most plant essential oils have antibacterial effects,and some have antioxidant properties.Therefore,EOs have been regarded as a potential natural fresh-keeping agent for aquatic products in recent years.This study aimed to investigate the preservation effects of essential oils on chilled large yellow croaker and turbot.The main contents of this study are summerized as follows:(1)Isolation and identification of the dominant spoilage bacteria isolated from large yellow croaker and turbot during chilled storage,and analysis of their spoilage ability.Thirteen strains of bacteria were isolated from spoiled large yellow croaker and turbot storage at 4oC,and identified by sequence similarity analysis of the 16 S rDNA genes and physiological & biochemical test.The results showed that a toal of 13 strains of bacteria were identified as eight strains of Shewanella putrefaciens,two strains of Pseudomonas sp.,one strain of Brochothrix thermosphacta,one strain of Aeromonas sobria and one strain of Acinetobacter harbinensis.The dominant spoilage bacteria of chilled yellow croaker were S.putrefaciens,A.sobria and B.thermosphacta were also involved in the spoilage activity of fish;the predominant spoilage bacteria of chilled turbot were S.putrefaciens,followed by Pseudomonas sp.(2)Antibacterial activity and mechanism of action of essential oils against the spoilage bacteria isolates.The paper disc diffusion method and microbroth dilution technique were employed to determine the antimicrobial activity of 14 essential oils.The mode of action of Litsea cubeba oil and cinnamaldehyde against spoilage bacteria was studied by scanning electron micrograph,measurement of release of intracellular lactate dehydrogenase(LDH)and 260 nm absorbing material.Additionally,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 14 essential oils showed different antimicrobial activities against spoilage bacteria isolates,of which cinnamaldehyde had the strongest antibacterial activity with a minimum inhibitory concentration(MIC)of 0.125-1 μL/mL,followed by eugenol and thymol with a MIC of 0.125-2 μL/mL.Litsea cubeba oil and cinnamaldehyde can disrupt the cytomembrane of spoilage bacteria isolates and cause the leakage of protease and nucleic acid.The flavour of large yellow croaker fillets treated with eugenol,thymol,citral,Cymbopogon citratus and Litsea cubeba oil were found to be acceptable.(3)Fresh-keeping effects of essential oils on large yellow croaker and turbot during chilled storage at 4oC.Based on the results of antimicrobial activity and sensory acceptance of essential oils,cinnamaldehyde and litsea cubeba oil were screened for the subsequent fresh-keeping test.To examine the preservation effect of cinnamaldehyde and Litsea cubeba oil on large yellow croaker and turbot fillets,the total microbial load,Pseudomonas,H2S-producing bacteria and Enterobacteriaceae,total volatile bases nitrogen(TVB-N),thiobarbituric acid(TBA)and bioamine content were detected.The results showed that after dipping into a solution of 2 μL/mL cinnamaldehyde and 4 μL/mL Litsea cubeba oil,the total microbial load,Pseudomonas,H2S-producing bacteria,Enterobacteriaceae,TVB-N,TBA,and biogenic amin content of the fillets were significantly reduced.In conclusion,cinnamaldehyde,eugenol,thymol,citral,Cymbopogon citratus,and Litsea cubeba oil showed high antimicrobial activity against the dominant spoilage bacteria isolates from chilled large yellow croaker and turbot,and had no adverse effects on the organoleptic properties of the fish fillets.Essential oils had the potential to be used as natural preservatives to enhance the shelf-life of seafood during chilled storage.