Development and validation of a dynamic growth model for Listeria monocytogenes in fluid whole milk

Listeria monocytogenes, a psychrotrophic microorganism, has been the cause of several food-borne illness outbreaks, including those traced back to pas teurized fluid milk and milk products. This microorganism is especially imp ortant because it can grow at storage temperatures recommended for milk (le ss than or equal to 7 degrees C). Growth of L. monocytogenes in fluid milk depends to a large extent on the varying temperatures it is exposed to in t he postpasteurization phase, i.e., during in-plant storage, transportation, and storage at retail stores. Growth data for L. monocytogenes in steriliz ed whale milk were collected at 4, 6, 8, 10, 15, 20, 25, 30, and 35 degrees C, Specific growth rate and maximum population density were calculated at each temperature using these data. The data for growth rates versus tempera ture were fitted to the Zwietering square root model. This equation was use d to develop a dynamic growth model (i.e., the Baranyi dynamic growth model or BDGM) for L. monocytogenes based on a system of equations which had an intrinsic parameter for simulating the lag phase. Results from validation o f the BDGM for a rapidly fluctuating temperature profile showed that althou gh the exponential growth phase of the culture under dynamic temperature co nditions was modeled accurately, the lag phase duration was overestimated. For an alpha(o) (initial physiological state parameter) value of 0.137, whi ch corresponded to the mean temperature of 15 degrees C, the population den sities were underpredicted, although the experimental data fell within the narrow band calculated for extreme values of alpha(o). The maximum relative error between the experimental data and the curve based on an average alph a(o) value was 10.42%, and the root mean square error was 0.28 log CFU/ml.