Modeling the heat inactivation of foodborne pathogens in milk powder: High relevance of the substrate water activity

Due to the ability of foodbome pathogens to survive in low moisture foods, the decontamination of these products is an important issue in food hygiene. Up to now, such decontamination has mostly been achieved through empirical methods. The intention of this work is to establish a more rational use of heat treatment cycles. The effects of thermal treatment cycles on the inactivation of dried Salmonella Typhimurium, Salmonella Senftenberg, Cronobacter sakazakti and Escherichia colt were assessed. Bacteria were mixed with whole milk powder and dried down to different water activity levels (0.11, 0.25, 0.44 and 0.58). The rate of inactivated bacteria was determined after thermal treatment at 85 degrees C, 90 degrees C, 95 degrees C and 100 degrees C, from 0 s to 180 s in closed vessels, in order to maintain a(w), during treatment. In a first step, logarithmic bacterial inactivation was fitted by means of a classical loglinear model in which temperature and a, have a significant effect (p < 0.05). DTA., values were estimated for each z(T), a(w), condition and the results clearly showed that a,, is a major parameter in the thermal decontamination of dried foods, a lower a,, involving greater thermal resistance. In a second step, Bigelow's law was used to determine z(T), a classical parameter relative to temperature, and y(aw) values, a new parameter relative to a(w), resistance. The values obtained for z(T) and y(aw), showed that the bacterium most resistant to temperature variations is Salmonella Typhimurium, while the one most resistant to a, variations is Escherichia coil. These data will help design decontamination protocols or processes in closed batches for low moisture foods.