USE OF A MODIFIED GOMPERTZ EQUATION TO MODEL NONLINEAR SURVIVAL CURVES FOR LISTERIA-MONOCYTOGENES SCOTT-A

The heat resistance of Listeria monocytogenes was determined in 0.1 M KH2PO4, buffer at three temperatures (50, 55, and 60 degrees C), three pH levels (5, 6, and 7), and three NaCl concentrations (0, 2, and 4%). Survival curves were fit using nonlinear regression with a modified Gompertz equation. The Gompertz equation is capable of fitting survival curves which are linear, those which display an initial lag region followed by a linear region, and those which are sigmoidal. Parameter estimates were used to describe the lag region, death rate, and the tailing region of a survival curve. These estimates were also used to predict single and interactive effects of temperature, pH, and percentage of NaCl on the log surviving fraction (LSF) of bacteria Interactions among these variables significantly (P<.05) affected the LSE Generally,increased pH or NaCl concentration lead to an increased (P<.05) LSF, whereas increased time or temperature lead to a decreased (P<.05) LSF. AII multiple factor interactions significantly (P<.05) affected the LSF. These interactions differed depending on the heating medium and the region of the survival curve. The correlation of observed LSF and predicted LSF (R(2)=.89) indicated that the Gompertz equation was in close agreement with the observations. This study demonstrated that the Gompertz equation and nonlinear regression can be used as an effective means to predict survival curve shape and response to heat of L. monocytogenes in many different environmental conditions.