Modification of the submerged coil to prevent microbial carryover error in thermal death studies

A submerged coil unit generates death rate data for foodborne pathogens through precise computer-controlled sequential sampling rather than the usual manually timed, labor-intensive single sampling associated with other approaches. Our work with Yersinia pseudotuberculosis and Listeria monocytogenes Scott A using the submerged coil unit indicated non-log-linear death rates with large degrees of tailing. Varying degrees of cell adhesion to the surface of the exit port resulted in carryover that was likely the primary cause of these non-log-linear kinetics. This carryover also resulted in erroneously high measured levels of thermal resistance for both organisms. To address the carryover problem, modifications were made to the exit port of the submerged coil unit to ensure continuous and uniform heat treatment. These modifications resulted in a 2-fold decrease in measured D-values for L. monocytogenes Scott A and a 10-fold decrease in measured D-values for Y. pseudotuberculosis. D-values measured with the modified machine for L. monocytogenes Scott A were similar to those found in the literature. Slight tailing in survival curves persisted with the modified method, particularly for Y. pseudotuberculosis. These results indicate that kinetic data for microbial death rates obtained using an unmodified submerged coil unit must be viewed with suspicion in light of the significant potential for carryover.