Increased Heat Resistance of Salmonellae in Beef with Added Soy Proteins

Raw beef inoculated with cells of a composite of five Salmonella strains was heated at 54 or 60°C. Survivors were enumerated by plating samples in plate count agar (PCA), XL agar or PCA followed by an XL agar overlay. Best differential recoveries of salmonellae were effected by incubation of PCA plates for 4 h at 37°C followed by overlay with XL agar and incubation for an additional 44 h. D-values of salmonellae at 54 and 60°C were increased significantly when ground beef was supplemented with 30% textured soy protein, soy protein concentrate or isolated soy protein. Increased heat resistance appeared to be caused by an increase in the pH of beef from 5.8-5.9 to 6.1 upon addition of the soy proteins. After adjusting the pH of mixtures of beef and soy proteins with hydrochloric or lactic acid to 5.8 to 5.9, survival of salmonellae to heat was reduced to the level of survival in beef alone. The pH of beef with added structured soy isolate was the same as beef, and heat resistance of salmonellae was not increased in this product. In the pH range 5.9 to 7.1, the maximum heat resistance of salmonellae in beef containing textured soy protein occurred at pH values of 6.5 to 6.8.