BEHAVIOR OF LISTERIA-MONOCYTOGENES DURING THE MANUFACTURE, RIPENING, AND COLD-STORAGE OF AFUEGAL-PITU CHEESE

Afuega'l Pitu is an artisanal acid-coagulated cheese manufactured in A sturias (northern Spain) and mainly consumed between the 3rd and the 3 0th day of ripening. Six cheese-making trials were performed in a pilo t plant by using pasteurized whole milk inoculated with Listeria monoc ytogenes (strain L2 [serotype 1/2a], L39, or L41 [serotype 4b]) to ca. 2.7 log CFU/ml. A starter containing three strains, Lactococcus lacti s subsp. lactis IPLA 947, Lactococcus lactis subsp. lactis biovar diac etylactis IPLA 838, and Leuconostoc citreum IPLA 616, grown separately in milk and combined in the volumetric proportion 3:1:1.3 was used. D uring the acidification L. monocytogenes counts increased 2.78- to 7.0 3-fold, depending on the strain, and remained within the curd; from th is time counts decreased abruptly and were not detected in cheeses bey ond the 7th day. The average pH in the curd was 4.43, and it decreased to around 4.0 in 5- to 7-day-old cheeses. These pH values were near t he tolerance limit for L. monocytogenes and probably caused cell damag e. Although moisture, a(w), and NaCl levels were not limiting for the growth and survival of L. monocytogenes, salt content must be consider ed as a contributing factor in L. monocytogenes inactivation. Finally, the L2 strain grew better in curd and was slightly more resistant to low pH and refrigeration than strain L39 or L41. The manufacture of Af uega'l Pitu cheese from pasteurized milk and the design of a specific starter from the autochthonous lactic microbiota can lead to a safer p roduct that can be consumed after very short ripening periods.