Growth of Listeria monocytogenes as a function of dynamic environment at 10°C and accuracy of growth predictions with available models

A combination of a factorial design and two central composite designs was used to assess quantitatively the effects and interactions of water activity (1-0.95) and pH (5.6-9.5) variations on the growth of Listeria monocytogenes in a meat broth at 10 degrees C. At inoculation or at the beginning of the exponential phase, the cells were exposed to the addition of NaCl and acetic acid or NaCl and NaOH. The effects of abrupt fluctuating conditions on the generation and lag times were analysed using turbidity measurements. The data indicated that the cells exposed to osmotic and acid or alkaline variable conditions from the time of inoculation were less affected than cells exposed at the beginning of the exponential phase. in this last case, a lag phase could be induced and the growth recovery was different from those expected in the new environment. Generation time values were estimated by three available predictive models which describe the effects of temperature, salt concentration and pH on L. monocytogenes growth to highlight the potential problems of variable conditions. (C) 2000 Academic Press.