EFFECTS OF ACID TYPE AND ALTA(TM)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 ALTA(TM)2341 (ALTA). The cheese was prepared by direct acidif ication (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 chee se coagulated with citric or malic acids and stored at 4 degrees C, bu t decreased slightly in cheese coagulated with acetic acid At 20 degre es C, counts oft, monocytogenes increased in cheeses acidified with ci tric 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 co mpared with counts in cheese that did not contain ALTA. However, at 20 degrees C populations of L. monocytogenes increased in cheese contain ing ALTA regardless of acid type. Additional studies compared the effe cts of acetic acid, alone or in combination with 0.6 or 2.5% ALTA, aga inst low (10(2) CFU/g) and high (10(6) CFU/g) inoculum levels. When in oculum levels were low, pathogen counts decreased by >1.1 log(10) CFU/ g in all formulations at 4 degrees C. After 7 days at 20 degrees C, pa thogen counts increased in the queso blanco type of cheese prepared wi th 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 ch eese with or without ALTA. At 20 degrees C, counts increased in the qu eso blanco type of cheese prepared with 0.6% ALTA, but decreased appre ciably in cheese prepared with 2.5% ALTA. These results demonstrate th at acetic acid is significantly more effective than malic or citric ac ids for controlling L. monocytogenes in queso blanco, and that inclusi on of ALTA can provide added protection against the pathogen.