| The antibacterial activity of thymol, carvacrol, cinnamaldehyde, eugenol, citral, geraniol and linalool, these seven major antibacterial components in spices essential oils to the main pathogenic bacteria Salmonella, Staphylococcus aureus, Escherichia coli, Vibrio Parahemolyticus and Pseudomonas fluorescens, which is the specific spoilage organism(SSO) during aerobic and cold storage in the tilapia was investigated with inhibition cycle and determination with minimum inhibitory concentration(MIC) and minimum bactericidal concentration(MBC) with broth dilution method.Every isolated essential oil componet showed antibacterial activity. Thymol and carvacrol showed strong antibacterial activity, which had the largest inhibition zone and showed MBCs of no more than 250 mg/L(μL/L) against each bacteria except for Pseudomonas fluorescens. Other components showed antibacterial activity too. Thymol, carvacrol and cinnamaldehyde were selected for the storage assay of tilapia fillets because they showed best in the inhibition zone test and had MBCs of no more than 1500μL/L to all the selected pathogenic bacteria.After processing by essential oil components, the growth of bacteria in tilapia during storage at 4℃ was inhibited. The content of total volatile basic nitrogen(TVBN) increased more slowly. The shelf life was extended. Evaluated by total aerobic count with Gompertz equation, the shelf life was predicted, which was 65,135,142,149h,162,105h and 101h for the control group, thymol 1 group(lg/kg), thymol 2 group(2g/kg), cinnamaldehyde 1(1mL/kg) group,cinnamaldehyde 2 group(2mL/kg), carvacrol 1 group(1mL/kg) and carvacrol 2 group(2mL/kg), respectively. Cinnamaldehyde groups and thymol 1 group showed obvious inhibition of producing TVBN, which was less than 20mg/100g, while the concentration of which in the control group was larger than 30mg/100g. The method of detecting the concentration of the three essential oil components with GC-MS was established. The content of each component in tilapia fillets was detected, showing a value of 379.6,560.1,107.5,645.5,244.7,219.6mg/kg(μL/kg) for thymol 1 group, thymol 2 group, cinnamaldehyde 1 group, cinnamaldehyde 2 group, carvacrol 1 group and carvacrol 2 group, respectively.The killing curve of thymol aganist Pseudomonas fluorescens showed that 400% of MIC of thymol was able to decrease the total count of Pseudomonas fluorescens by two orders. The assay of succinatedehydrogenase activity showed that subinhibitory concentration of thymol damage the permeability of the membrane, causing the leakage of K+ and proteins, which resulted in the death of cells. Subinhibitory concentration of thymol induced an increase of the content of arachidic acid and a decrease of the content of palmic acid in membrane, which decreased the negative effect on membrane. The activity of succinatedehydrogenase was inhibited when high concentration of thymol was added, so that the growth of the bacteria was affected and cells died finally. |
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