Identification of Listeria innocua surrogates for Listeria monocytogenes in hamburger patties.

Listeria innocua M1 has been used by many researchers as a nonpathogenic thermal processing surrogate for Listeria monocytogenes. However, L. innocua M1 has been criticized because its thermal survivability characteristics are not as closely parallel to L. monocytogenes as some would like in a variety of foods and processing conditions. The present study was conducted to compare multiple L. innocua and L. monocytogenes strains to validate L. innocua M1 as the ideal surrogate under high-temperature thermal processing conditions for L. monocytogenes. The D- and z-values of L. innocua M1, L. innocua strain SLCC 5639 serotype (6a), SLCC 5640 (6b), SLCC 2745 (4ab), and L. monocytogenes F4243 (4b) were calculated for raw hamburger patties. Hamburger patties were inoculated with 10(7-8) CFU/g of L. monocytogenes or L. innocua. Samples were heat treated at 4 temperatures (62.5 to 70 degrees C). At each temperature, the decimal reduction time (D-value) was obtained by linear regression of survival curves. The D- and z-values were determined for each bacterium. The D-values of L. innocua and L. monocytogenes serotypes ranged from 3.17 to 0.13 min at 62.5 to 70 degrees C, and the z-values of L. innocua and L. monocytogenes were 7.44 to 7.73 degrees C. Two of the 4 L. innocua serotypes used in this experiment have the potential for use as surrogates in hamburger meat with varying margins of safety. L. innocua M1 should serve as the primary nonpathogenic surrogate with the greatest margin of safety in verifying a new thermal process to destroy L. monocytogenes.