Development of models to determine non-thermal inactivation of bacteria in cheese

Cheese has become a popular food product worldwide. Consumes are constantly demanding for cheese with a greater range of flavours and styles. To meet this demand, many producers are now interested in making cheeses from unpasteurised or ‘raw’, milk in which many believe to have an unparalleled depth of flavour. Using raw milk is also thought to retain the regional character and give the cheese distinctive flavour. However, raw milk can contain high levels of pathogenic bacteria. These bacteria could survive during cheese production and cause illnesses. While cheeses made from both raw and pasteurised milk have been implicated in food-borne outbreaks, several reports have indicated that raw milk cheeses represent a higher risk to public health than analogous pasteurized milk products on a risk-per-serving basis. There is great interest in government and industry to be able to assess the microbiological safety of cheeses. As such, the New Zealand Food Safety Authority (NZFSA) funded this project to develop tools to predict the fate of pathogens during cheese production. These tools will enable cheese manufacturers to begin safely producing a wider range of raw milk cheeses. A preliminary study was undertaken to investigate the inactivation kinetics of L. monocytogenes and E. coli in a simple broth system that emulate cheese and in commercial cheese at different temperatures (25°C, 20°C and 10°C). This presentation will discuss results, including potential use of broth system to mimic bacterial response in cheese and relationship of the data obtained to existing models for pathogens in fermented foods. In particular, the rates of non-thermal inactivation of both species were strongly dependent upon temperature rather than strain, species and environment. The results suggest that the fate of pathogens could be predicted simply from a small number of factors.