The effect of pH, salt concentration and temperature on the survival and growth of Listeria monocytogenes

Factorially designed experiments have been used to study the growth and survival of Listeria monocytogenes in different combinations of pH and salt concentrations at ambient and chill temperatures. Survival at low pH and high salt concentration was strongly temperature dependent. The minimum pH values that allowed survival after 4 weeks from an initial 10(4) cells were 4.66 at 30 degrees C, 4.36 at 10 degrees C and 4.19 at 5 degrees C. These limits were salt dependent, low (4-6%) salt concentrations improved and higher concentrations reduced survival at limiting pH values. The lowest pH that allowed a 100-fold increase in cell numbers within 60 d was 4.66 at 30 degrees C but this was increased to 4.83 at 10 degrees C. At 5 degrees C growth occurred at pH 7.0 but not at pH 5.13. Simple predictive models describing the effect of hydrogen-ion and salt concentration on the time for at least a 100-fold increase in numbers at 10 degrees C and 30 degrees C were constructed after analysis of the results for a least squares fit to a quadratic model. The interactions between salt and hydrogen-ion concentration on growth were found to be purely additive.