Development of Thermal Surrogate Microorganisms in Ground Beef for In-Plant Critical Control Point Validation Studies

Abstract

In search of a suitable surrogate microorganism for in-plant critical control point validation, we compared the rates of thermal inactivation of three bacteria, Enterococcus faecium B2354, Pediococcus parvulus HP, and Pediococcus acidilactici LP, to those of Listeria monocytogenes and Salmonella. Ground beef samples containing 4 and 12% fat were inoculated with E. faecium, L. monocytogenes, and Salmonella Senftenberg 775W and heated at 58, 62, 65, or 68°C. The decimal reduction times (D-values) for E. faecium B2354 in 4 and 12% fat ground beef were 4.4 to 17.7 and 3.6 to 14.6 times greater, respectively, than those for L. monocytogenes or Salmonella Senftenberg 775W at all temperatures tested, with the greatest differences in D-values occurring at 58 and 62°C. Higher fat content protected bacteria from thermal inactivation in general, especially at temperatures lower than 68°C. The heat resistance in a broth medium at 62°C of two food-grade bacteria, P. parvulus HP and P. acidilactici LP, was compared with that of the three strains under study. The D-values of P. parvulus HP and P. acidilactici LP were lower than those of E. faecium B2354 but 4.1 and 2.5 times greater, respectively, than those of Salmonella Senftenberg 775W, the most resistant pathogen. These results indicate that thermal treatments of ground beef at 58 to 68°C that kill E. faecium B2354 will also kill Salmonella and L. monocytogenes, and the two Pediococcus isolates may serve as alternate surrogates for validation studies when a less heat-resistant surrogate is desired. However, additional studies in ground beef are needed with the Pediococcus strains in the desired temperature range intended for validation purposes.