Behavior of Listeria monocytogenes during the manufacture, ripening, and cold storage of Afuega'l Pitu cheese

Afuega'l Pitu is an artisanal acid-coagulated cheese manufactured in Asturias (northern Spain) and mainly consumed between the 3rd and the 30th day of ripening. Six cheese-making trials were performed in a pilot plant by using pasteurized whole milk inoculated with Listeria monocytogenes (strain L2 [serotype 1/2a], L39, or L41 [serotype 4b]) to ca. 2.7 log CFU/ml. A starter containing three strains, Lactococcus lactis subsp. lactis IPLA 947, Lactococcus lactis subsp. lactis biovar diacetylactis IPLA 838, and Leuconostoc citreum IPLA 616, grown separately in milk and combined in the volumetric proportion 3:1:1.3 was used. During the acidification L. monocytogenes counts increased 2.78- to 7.03-fold, depending on the strain, and remained within the curd; from this time counts decreased abruptly and were not detected in cheeses beyond 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 the tolerance limit for L. monocytogenes and probably caused cell damage. Although moisture, a(w), and NaCl levels were not limiting for the growth and survival of L. monocytogenes, salt content must be considered 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 Afuega'l Pitu cheese from pasteurized milk and the design of a specific starter from the autochthonous lactic microbiota can lead to a safer product that can be consumed after very short ripening periods.