Development and Validation of the Effects of Interventions and Processes on Persistence of Listeria Monocytogenes on Queso Fresco Cheese

APPROACH: Listeria monocytogenes (LM) will be added at various steps of the cheese making process to examine the effects of pH through addition of food grade acids, temperature adjustment, starter cultures, adjunct cultures, and processing technique on reduction or elimination of the pathogen through processing. Queso fresco will be manufactured according to a generic protocol. Initially, we will monitor the fate of LM during manufacture and storage of Queso Fresco using the general protocol. It is anticipated that the pathogen will be added to the raw milk, to the curd at the cutting, salting, and milling stages, and/or onto the surface of the finished product prior to vacuum packaging to mimic likely points of contamination in the industry. LM viability, pH, proximate composition of cheese and levels of indigenous lactic acid bacteria will also be monitored. We will investigate the effects of process adjustments in the cheese making process and also evaluate the feasibility of using high pressure processing (HPP), considered a non thermal process, at pressures in the range from 200 to 800 MPa, at temperatures from 10 to 30 degrees C, and for holding times ranging from 1 to 20 min, in the manufacture of the cheese as a step following packaging of the cheese. The fate of LM will be monitored throughout shelf life for a minimum of 30 d at both refrigeration (4 degrees C) and abuse temperatures (10 degrees C). At least two trials will be conducted for each treatment and three samples will be ananlyzed at each sampling interval. We will also expand the study as appropriate to monitor and/or to evaluate other pathogens. The fate of LM will also be determined on low-salt Cheddar Cheese of various salt contents for comparison to Queso Fresco and the study expanded to monitor growth of other pathogens.