Microbial contamination, community diversity and cross-contamination risk of food-contact ice

Ice is widely used in the food industry, as an ingredient (edible ice) directly added to food or as a coolant (food-contact ice) for fresh food preservation along the cold chain. However, it has been shown that food-contact ice are easily polluted by pathogens, potentially endangering the public's health. In the present study, the hygiene status of food-contact ice collected from various sources (local farmer markets, supermarkets, and restaurants) was evaluated through the quantitative estimation of total bacterial counts and coliform counts as well as the prevalence of foodborne pathogenic bacteria (Staphylococcus aureus, Vibrio parahaemolyticus, Salmonella, Listeria monocytogenes, Shigella). The average levels of total bacterial counts in the ice for preserving the aquatic products, poultry meat and livestock meat are 4.88, 4.18 and 6.11 log₁₀ CFU/g, respectively. Over 90 % of the food-contact ice were positive for coliforms. The detection rate of S. aureus in all the food-contact ice samples was highest, followed by Salmonella, V. parahaemolyticus and L. monocytogenes, and Shigella was not detected. In addition, the bacterial community diversity of food-contact ice was analyzed with high-throughput sequencing. The dominant bacteria taxa in food-contact ice are heavily dependent on the environment of sampling sites. The predicted phenotypes of biofilm forming, oxidative stress tolerance, mobile element containing and pathogenesis were identified in the bacteria taxa of food-contact ice, which should be carefully evaluated in future work. Finally, the cross-contamination models of pathogen transfer during ice preservation were established. The results showed that the transfer rates of ice-isolated S. aureus between food and ice were significantly higher than that of V. parahaemolyticus. The binomial distribution B(n, p) exhibited a better fitness to describe the pathogen transfer during ice preservation when the transfer rate was low, in turn, the transfer rate-based probability model showed a better fit to the data when the transfer rate was high. Monte Carlo simulation with Latin-Hypercube sampling was carried out to predict the contamination levels of S. aureus and V. parahaemolyticus on food as the result of cross contamination during ice preservation ranging from -2.90 to 2.96 log₁₀ CFU/g with a 90 % confidence interval. The findings of this work are conducive to a comprehensive understanding of the current hygiene status of food-contact ice, and lay a theoretical foundation for the risk assessment of cross-contamination during ice preservation.