USE OF A MODIFIED GOMPERTZ EQUATION TO PREDICT THE EFFECTS OF TEMPERATURE, PH, AND NACL ON THE INACTIVATION OF LISTERIA-MONOCYTOGENES SCOTTA HEATED IN INFANT FORMULA

The heat resistance of Listeria monocytogenes was determined in infant formula for all possible combinations of temperature (50, 55, and 60 degrees C), pH level (5, 6, and 7), and NaCl concentration (0, 2, and 4%). Survival curves were fit using nonlinear regression with a Gomper tz equation. The Gompertz equation was flexible enough to fit the thre e most commonly observed survival curves: linear curves, those with an initial lag region followed by a linear region, and sigmoidal shaped. Parameter estimates obtained by the method of nonlinear least squares were used to describe the effect(s) of different heating treatments o n the lag region, death rate, and tailing region of survival curves. T hese estimates were further used to predict single and interactive eff ects of temperature, pH, and percentage of NaCl on the log of the surv iving fraction (LSF) of bacteria. Interactions among these variables s ignificantly (P less than or equal to .05) affected the LSE Generally, increased pH or NaCl concentration lead to an increased LSF, whereas increased time or temperature lead to a decreased LSE All multiple-fac tor interactions significantly (P less than or equal to .05) affected the LSE The correlation of observed LSF versus predicted LSF (R(2) = . 92) indicated that the estimated Gompertz equation was in close agreem ent with the observation. This study demonstrated that the Gompertz eq uation and nonlinear regression can be used as an effective means to p redict survival curve shape and response to heat of L. monocytogenes u nder many different environmental conditions.