| At present, chilled pork was favored by consumers in the market, because it was rich in nutritional and its sensory quality was good, besides, it was safety and health. All in all, it possesses the advantages of fresh meat and frozen meat, meanwhile excludes their defects. The production, processing, distribution, sales and so on of chilled pork were in the cold chain system, so the growth of most microbes were restrained, but there were still some psychrophilic microorganisms, such as Brochothrix thermosphacta, which is a facultative anaerobic Gram-positive bacillus and can grow good under conditions of low oxygen and4℃. Astrain of Lactococcus lactis subsp.lactis was isolated from forest soil, which exhibited a broad bactericidal activity against seven spoilage and food-borne pathogens bacteria commonly related to pork, including B. thermosphacta and Pseudomonas fluorescens. The results were as follows:In this study, three strains were isolated and purified from the chilled pork sold in supermarkets using repeated plate streaking on STAA solid medium plate. The strains were be identified as B. thermosphacta based on their individual and colony morphology features, physiological and biochemical characteristics, the sequence features of the16SrDNA and the phylogenic tree, respectively, and named as CMS1， CMS2，CMS3.In order to explore the spoilage characteristics, three B. thermosphacta strains were inoculated into the chilled pork at4℃. Then B. thermosphacta count, pH value and sensory evaluation score were deter mined every2days, respectively. This study showed that B. thermosphacta possessed with significantly stronger spoilage capability. In comparison, under the low temperature the growth rate of stains CMS3was the fastest, acid production was the most and spoilage degree was the highest among three stains, which led to difference among their spoilage capability. This study shows that the growth ability obvious differences among three B. thermosphacta strains under4℃in chilled pork.Using the B. thermosphacta strain CMS3as indicator bacteria,25strains which had antibacterial activity against strain CMS3were obtained from soil samples in early screening, and one of them had antibacterial activity against seven kinds of indicator bacteria, including B. thermosphacta, Listeria monocytohenes, Lactococcus lactis, Staphyloccocus aureus, pathogenic Escherichia coli, P. fluorescens and Salmonella paratyphi. We further studied the strain’s individual and colony morphology features, physiological and biochemical characteristics, the sequence features of the16S rDNA and the species-specific N-acetylmura minidase gene （acmA）. Consequently, it was identified as L. lactis subsp. lactis, named as NFL.Fermentation filtrate of strain NFL was low-level sensitivity to trypsin, proteinase K and pepsin. After disposed by these enzymes, the inhibition zone diameter was narrowed, but still had obvious antibacterial activity. Fermentation filtrate of strain NFL could resistant to high temperature and high pressure of121℃for90min, its antibacterial activity basically did not change. Antibacterial activity under acidic conditions was good, especially when the pH value of3.0, the antibacterial activity was the best. With the increase of pH, antibacterial activity decreased until the minimum stable state. We ruled out the inhibition due to lactic acid and hydrogen peroxide. Considering these characteristics, the antibacterial substance produced by strain NFL was initially deter mined as a kind of bacteriocin-like substance.Fermentation filtrate of strain NFL was sterilized at121°C for20min, and then used to immerse chilled pork. Physiological saline served as control. Then aerobic bacterial count, pH value and sensory evaluation score were deter mined every2days. Results showed that the fermentation filtrate could extend the shelf life of chilled pork and had a good preservation effect on chilled pork, especially significantly reduced rancidity. So, Lactococcus lactis NFL was a potential food preservative-producing strain, not only for the preservation of chilled pork, but also for certain application prospects in other kinds of food. |
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