Prediction of Listeria spp. growth as affected by various levels of chemicals, pH, temperature and storage time in a model broth.

The effects of concentration of NaCl (0.5 to 12.5%), methyl paraben (0.0 to 0.2%), sodium propionate (0.3%), sodium benzoate (0.1%), potassium sorbate (0.3%), pH (> 5.9) temperature (4 to 30 degrees C), storage time (up to 58 d) and inoculum (> 10(5) to > 10(-2) per ml) on the log10 probability percentage of one cell of Listeria spp. to initiate growth in a broth system were evaluated in a factorial design study. At pH 5.96 and temperature ranging from 4 to 30 degrees C the concentrations of sodium propionate, potassium sorbate, and sodium benzoate examined allowed growth of L. monocytogenes with lag phases at 4 degrees C of 18, 27 and 21 days, respectively. For 0.1 and 0.2% methyl paraben growth of all Listeria spp. was initiated at 8 degrees C and 30 degrees C, respectively. At pH 6, concentration of 12% NaCl supported the growth of L. monocytogenes at 8 to 30 degrees C, whereas 12.5% inhibited all Listeria species. Four regression equations were derived relating probability of growth initiation to temperature, concentrations of NaCl and preservatives storage time, and Listeria species specific effects. From these equations, the number of cells needed for growth initiation can be calculated. The impact of this type of quantitative study and its possible application on the development of microbial standards for foods is discussed.