Midwest Dairy Foods Research Center, Dairy and Food Science Department, Brookings 57007.
Midwest Dairy Foods Research Center, Dairy and Food Science Department, Brookings 57007.
J Dairy Sci. 2018 Nov;101(11):9659-9669. doi: 10.3168/jds.2018-14486. Epub 2018 Aug 29.
Listeriosis is a life-threatening infection caused by foods contaminated with Listeria monocytogenes. Some of the major ice cream recalls in recent years reaffirm the ability of this food-borne pathogen to survive in diverse dairy processing environments and cause cross contamination. Inspection reports revealed certain lapses in implementing adequate hygienic practices for Listeria persistence in the processing environment, leading to cross contamination of ice cream. The higher levels of cross contamination of raw ice cream mix might result in random heat-injured cells when exposed to minimum heat treatment (69°C for 30 min). These heat-injured cells could later recover under abusive storage and handling conditions and pose a health risk. Evidence about the presence of injured cells in ice cream mix may thus prove useful to establish the overall Listeria risk, which was the aim of this study. Challenge studies were conducted to evaluate the dose-dependent presence of heat-injured cells of Listeria. Ice cream mix formulations of 4 different types (36, 40, 42, and 45% total solids) were inoculated at 2.0, 3.0, and 4.0 log cfu/g levels of Listeria innocua (an established surrogate). The dose levels were selected based on a likely cross contamination on the raw side from environmental Listeria, especially due to their resident nature and growth in harborage sites. The samples were exposed to minimum heat treatment (69°C for 30 min) and the survivors, including heat-injured cells, were enumerated using standard protocols. A binary logistic regression model was fitted for evaluating the severity of risk. The influence of total solids, water activity, and pH variability were also studied on Listeria survival. The enrichment protocol, using buffered Listeria enrichment broth, followed by plating on modified oxford agar and Rapid L'mono medium, revealed the random presence of heat-injured cells in buffered Listeria enrichment broth, only at the highest dose level of 4+ logs. Any potential risk from heat-injured cells was thus limited only to the highest levels of cross contamination, irrespective of the type of the mix. Significantly, none of the pasteurized ice cream mix samples supported the recovery of any heat-injured cells of Listeria during 72 h holding at 7°C, even at the highest dose level of 4+ logs, under the conditions of experimentation. The level of cross contamination (dose) emerged as a predictor of the potential presence of heat-injured cells of Listeria exposed to minimum pasteurization treatment.
李斯特菌病是一种由被单增李斯特菌污染的食物引起的危及生命的感染。近年来，一些主要的冰淇淋召回事件再次证实，这种食源性病原体能够在各种乳制品加工环境中存活，并造成交叉污染。检查报告显示，在加工环境中对李斯特菌持续存在实施充分卫生措施方面存在某些失误，导致冰淇淋的交叉污染。生冰淇淋混合料的更高水平的交叉污染可能导致在暴露于最小热处理（69°C 30 分钟）时随机热损伤细胞。这些热损伤细胞以后可能在滥用储存和处理条件下恢复，并构成健康风险。因此，冰淇淋混合料中存在损伤细胞的证据可能有助于确定总体李斯特菌风险，这是本研究的目的。进行了挑战研究以评估依赖于剂量的单增李斯特菌热损伤细胞的存在。将 4 种不同类型（总固体含量为 36%、40%、42%和 45%）的冰淇淋混合料配方接种在单增李斯特菌无害菌（已建立的替代物）2.0、3.0 和 4.0 log cfu/g 的水平。剂量水平是根据从环境李斯特菌，特别是由于其固有性质和在藏匿点生长而在原始侧发生的交叉污染的可能性选择的。样品暴露于最小热处理（69°C 30 分钟），幸存者，包括热损伤细胞，使用标准方案进行计数。拟合二元逻辑回归模型评估风险的严重程度。还研究了总固体、水活度和 pH 变化对李斯特菌存活的影响。使用缓冲李斯特菌增菌肉汤的富集方案，然后在改良的牛津琼脂和 Rapid L'mono 培养基上进行平板培养，仅在最高 4+ 对数剂量水平下，在缓冲李斯特菌增菌肉汤中随机出现热损伤细胞。因此，任何来自热损伤细胞的潜在风险仅限制在最高水平的交叉污染，而与混合料的类型无关。重要的是，即使在最高 4+ 对数剂量水平下，在实验条件下，在 7°C 下 72 小时保持期间，也没有任何巴氏杀菌冰淇淋混合料样品支持李斯特菌任何热损伤细胞的恢复。交叉污染（剂量）水平是暴露于最小巴氏杀菌处理的李斯特菌热损伤细胞潜在存在的预测因子。
