Growth of Listeria monocytogenes on vacuum-packed cooked meats: effects of pH, aw, nitrite and ascorbate.

Slices of cooked meats were inoculated with Listeria monocytogenes strain Murray B, vacuum-packed and stored at either 0 or 5 degrees C. Decreases in pH (6.9-5.9) and aw (0.993-0.960; adjusted with sodium chloride) of the cooked meats increased the lag time and reduced the growth rate at 5 degrees C. The type of meat (beef, pork, chicken or turkey) had no effect on the growth of the organism after allowance was made for pH. Sodium tripolyphosphate (0.3%) increased growth by increasing the pH of the cooked meat. Sodium nitrite reduced the growth rate and increased the lag time. Three microM of residual undissociated nitrite doubled the time taken for a 3 log increase in numbers of L. monocytogenes. The effectiveness of nitrite was significantly increased by sodium ascorbate (0.042%). In the absence of nitrite, ascorbate had no detectable effect on growth. The extent of growth at 0 degree C was similarly influenced by the interaction of pH, aw, nitrite and ascorbate, and was considerably less than at 5 degrees C. Quadratic equations were developed to describe some of the combined effects of pH, aw and residual nitrite on lag, growth rate and time for a 3 log increase in numbers of L. monocytogenes.