P2-215 Population Dynamics of Listeria monocytogenes during Gouda Cheese Manufacture Using Artificially Inoculated, Unpasteurized Milk

Tuesday, July 11, 2017
Exhibit Hall (Tampa Convention Center)
Joelle K. Salazar , U. S. Food and Drug Administration , Bedford Park , IL
Sartaj S. Narula , IIT Institute for Food Safety and Health (IFSH) , Bedford Park , IL
Christina K. Carstens , U.S. Food and Drug Administration , Bedford Park , IL
Arlette Shazer , U. S. Food and Drug Administration , Bedford Park , IL
Kristin M. Schill , U. S. Food and Drug Administration , Bedford Park , IL
Mary Lou Tortorello , U. S. Food and Drug Administration , Bedford Park , IL
Introduction: Unpasteurized milk poses a food safety concern due to possible contamination by pathogens, including Listeria monocytogenes. Contaminated milk used for cheesemaking has been shown to harbor this pathogen during manufacture and 60 d aging process. The FDA 2014-2016 sampling assignment revealed the presence of L. monocytogenes in raw milk cheeses, especially semi-soft cheeses such as Gouda.

Purpose: This study intended to determine the differences in population dynamics of L. monocytogenes during Gouda cheese manufacture depending on the initial inoculation levels of one or three log CFU/mL.

Methods: Unpasteurized milk was inoculated with one or three log CFU/mL of a three strain cocktail of L. monocytogenes. Milk was heated to 30°C prior to starter culture addition, ripening occurred for 30 min, then rennet was added. After 30 min, the curd was cut, stirred for 20 min, and 1/3 of whey volume was drained. After cooking the curd at 37°C for 20 min, it was molded and pressed overnight. Brining was conducted according to the resulting cheese weight preceding storage at 10°C. Cheese was sampled for plate count assay throughout cheesemaking and 24 h post-brine. Data were computed as log CFU/g or /mL and statistically analyzed by ANOVA, P≤ 0.05.

Results: No significant differences in L. monocytogenes populations were observed during initial manufacturing steps. After curd cutting, L. monocytogenes was significantly more concentrated in the curd (1.38±0.19 and 2.76±0.13 CFU/g for the 1 or 3 log CFU/mL conditions, respectively). After storage at 10°C for 24 h, the L. monocytogenes population increased significantly to 1.85±0.30 CFU/g in the 1 log CFU/mL condition. A significant population increase (6.82±0.03 CFU/g) was also observed after storage for the three log CFU/ml condition.

Significance: The results of the study can aid in the risk assessment of Gouda cheese processing based on initial concentrations of L. monocytogenes in milk.