Effects of acid type and ALTA 2341 on Listeria monocytogenes in a queso blanco type of cheese

The behavior of listeria monocytogenes was evaluated during storage of a queso blanco type of cheese produced with acidulants (citric, malic, or acetic acids) and a commercial lactic acid bacterium fermentation product, ALTA2341 (ALTA). The cheese was prepared by direct acidification (final pH 5.2), with and without 0.6% ALTA, inoculated with 10(6) CFU/g of L. monocytogenes, and stored at 4 or 20 degrees C for 42 and 7 days, respectively. Levels of L. monocytogenes increased in cheese coagulated with citric or malic acids and stored at 4 degrees C, but decreased slightly in cheese coagulated with acetic acid. At 20 degrees C, counts of L. monocytogenes increased in cheeses acidified with citric or malic acid, but counts did not increase appreciably in cheese acidified with acetic acid. When cheese was stored at 4 degrees C, the presence of 0.6% ALTA resulted in lower counts of L. monocytogenes compared with counts in cheese that did not contain ALTA. However, at 20 degrees C populations of L. monocytogenes increased in cheese containing ALTA regardless of acid type. Additional studies compared the effects of acetic acid alone or in combination with 0.6 or 2.5% ALTA, against low (10(2) CFU/g) and high (10(6) CFU/g) inoculum levels. When inoculum levels were low, pathogen counts decreased by > 1.1 log10 CFU/g in all formulations at 4 degrees C. After 7 days at 20 degrees C, pathogen counts increased in the queso blanco type of cheese prepared with acetic acid alone. In contrast, in the presence of 0.6 or 2.5% ALTA, 7-day counts were less than the initial inoculum. With high inoculum levels at 4 degrees C, counts of L. monocytogenes were less than the initial inoculum in the acetic acid-coagulated queso blanco type of cheese with or without ALTA. At 20 degrees C, counts increased in the queso blanco type of cheese prepared with 0.6% ALTA, but decreased appreciably in cheese prepared with 2.5% ALTA. These results demonstrate that acetic add is significantly more effective than malic or citric acids for controlling L. monocytogenes in queso blanco, and that inclusion of ALTA can provide added protection against the pathogen.