Microbiological Safety of Blue and Cheddar Cheeses Containing Naturally Modified Milk Fat

Milk containing naturally modified fat was obtained by feeding lactating dairy cows a Control diet and two experimental diets containing either extruded soybeans or sunflower seeds. Milk from cows fed the experimental diets contained higher levels of both long chain (C_18-C18:2) and unsaturated fatty acids than the milk from cows fed the Control diet. Each milk was pasteurized, standardized to 3.6% milk fat, and inoculated with Listeria monocytogenes (strains Scott A and V7), Salmonella typhimurium and Salmonella senftenberg , before manufacturing into Blue or stirred-curd Cheddar cheeses. Populations of L. monocytogenes and Salmonella spp. were monitored during manufacture and aging using Oxford and Xylose Lysine Desoxycholate agars, respectively. During the manufacture of Blue and Cheddar cheese, and during the aging of Blue cheese, behavior of Salmonella spp. and L. monocytogenes in the experimental cheese was similar to the Control cheese. During aging of Cheddar cheese, the rate and extent of decline of Salmonella spp. and L. monocytogenes varied among the cheeses. Declines correlated with the accumulation of specific fatty acids, namely C₁₂, C₁₄, C_18:1 and C_18:2. These fatty acids were also found to be inhibitory to S. typhimurium and L. monocytogenes when incorporated into tryptic soy agar plates at 37°C. Therefore, the natural fat modification of Blue and Cheddar cheeses enhanced the safety of these cheeses.