Listeria cross contamination levels in raw ice cream mix can serve as a predictor of their potential presence as heat-injured cells

Abstract

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.