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Survival of foodborne pathogens during frozen storage of cheese made from artificially inoculated milk

机译：冷冻接种人工接种牛奶制成的奶酪后食源性病原体的存活

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As cheese is often frozen prior to microbiological analysis, the aim of this study was to determine the survival of foodborne pathogens to freezing in cheese. A semi-soft cheese was produced, in independent triplicate for each pathogen mix, using milk inoculated with two pathogen mixes: Listeria monocytogenes + Staphylococcus aureus and Escherichia coli O157:H7 + Salmonella typhimurium. Three different strains were used for each pathogen, except for E. coli O157:H7 for which two strains were used. Cheeses were manufactured, wrapped in plastic bags and frozen at -20 A degrees C. For the E. coli O157:H7 + S. typhimurium pathogen mix, the effect of different freezing conditions on survival was studied. In all cases, the numbers of starter bacteria and pathogens were monitored in fresh cheese and after 2, 7, 30 and 90 days of storage. Two methods of thawing were compared after 30 days (14 h at 4 A degrees C and 4 h at 20 A degrees C). The numbers of L. monocytogenes, S. aureus and starter bacteria did not change significantly during frozen storage at -20 A degrees C, but E. coli and S. typhimurium decreased significantly after 2 days. There was no significant (p > 0.05) influence of the thawing method. Freezing of cheese at -80 A degrees C or flash freezing in liquid nitrogen only facilitated survival of E. coli O157:H7 and S. typhimurium for 1 day. The study shows that cheese samples should not be frozen prior to analysis for detection or enumeration of E. coli or S. typhimurium.
机译：由于奶酪通常在微生物分析之前先冷冻，因此本研究的目的是确定食源性病原体在奶酪中冷冻的存活率。使用接种两种病原体混合物的牛奶，分别为每种病原体混合物制作一个半软奶酪，一式三份，分别是：单核细胞增生李斯特菌+金黄色葡萄球菌和大肠杆菌O157：H7 +鼠伤寒沙门氏菌。除使用两种菌株的大肠杆菌O157：H7外，每种病原体使用三种不同菌株。制造奶酪，将其包装在塑料袋中，并在-20 A摄氏度下冷冻。对于大肠杆菌O157：H7 +鼠伤寒沙门氏菌病原体混合物，研究了不同冷冻条件对存活率的影响。在所有情况下，在新鲜奶酪中以及储存2天，7天，30天和90天后，都要监测起始细菌和病原体的数量。 30天后（在4 A摄氏度下14 h和20 A摄氏度下4 h）比较了两种解冻方法。在-20°C冷冻保存期间，单核细胞增生李斯特菌，金黄色葡萄球菌和入门细菌的数量没有明显变化，但大肠杆菌和鼠伤寒沙门氏菌在2天后显着下降。解冻方法没有显着影响（p> 0.05）。奶酪在-80 A摄氏度下冷冻或在液氮中速冻仅能促进大肠杆菌O157：H7和鼠伤寒沙门氏菌存活1天。研究表明，奶酪样品不应在进行分析以检测或计数大肠杆菌或鼠伤寒沙门氏菌之前进行冷冻。

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