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Title: Growth of Listeria monocytogenes on Queso Fresco during storage at 4 deg and 10 deg C

item Porto-Fett, Anna
item Call, Jeffrey
item Shoyer, Brad
item Kwoczak, Raymond
item Van Hekken, Diane
item Tomasula, Peggy
item Luchansky, John

Submitted to: American Society for Microbiology
Publication Type: Abstract Only
Publication Acceptance Date: 1/20/2009
Publication Date: 5/21/2009
Citation: Leggett,L.,Porto-Fett,A.,Call,J.,Shoyer,B.,Kwoczak,R.,Farkye,N.,Van Hekken,D.,Tomasula,P.,Luchansky,J. 2009.Growth of Listeria Monocytogenes on Queso Fresco during storage at 4 deg and 10 deg C [abstract].American Society of Microbiology Annual Meeting.Philadelphia,PA. p.1.

Interpretive Summary:

Technical Abstract: Queso Fresco (QF) is a white, high-moisture, slightly-crumbly, and salty Hispanic-style cheese that has been implicated in several outbreaks of listeriosis. The relatively high pH (>6.0) and high moisture content (>50%) coupled with the labor intensive practices of QF production may lead to contamination with Listeria monocytogenes (Lm) and may cause this cheese to be associated with recalls. In this study, we evaluated the survival of Lm both in and on QF slices during storage at 4 deg and 10 deg C for up to 15 days. Briefly, raw milk was standardized to 3.5% fat, pasteurized at 72 deg C for 15 sec, and homogenized. The milk was then heated to 32 deg C in a cheese vat and 0.1% CaCl2 was added. Next, chymosin was added and the milk was allowed to coagulate for ca. 30 min. The resulting curds were cut and cooked for 30 min at 39 deg C. About 60% of the whey was drained and 1.1% salt (wt salt/wt milk) was mixed into the slurry for ca.10 min before the remaining whey was drained. The curds were cooled and then milled. About 8 kg of curd was packed into a mold while another 8 kg was inoculated with Lm prior to packing into a mold. The QF was held at 4 deg C overnight before the molds were removed and the QF was sliced (40 to 80 g each slice). Both the salted curds and the surface of non-contaminated slices of QF were inoculated with about 3.0 log10 CFU/g of a rifampicin-resistant Lm cocktail and individually vacuum packaged. In each of two trials, three slices of QF were separately tested at each sampling interval. Pathogen numbers on slices wherein Lm was inoculated into the salted curd increased from 3.54 to 6.65 log10 CFU/g when stored at 4 deg C and increased to 7.77 log10 CFU/g when stored at 10 deg C. When inoculated directly onto the surface of QF slices, pathogen numbers increased from 2.64 to 5.88 log10 CFU/g when stored at 4 deg C and increased to 7.82 log10 CFU/g when stored at 10 deg C. These results show that proper refrigeration (4 deg C) slows, but does not prevent Lm growth, and that QF provides an environment conducive to growth of this pathogen.