Submitted to: Journal of Food Safety
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/2/2006
Publication Date: 8/26/2006
Citation: Uhlich, G.A., Luchansky, J.B., Tamplin, M.L., Molina-Corral, F.J., Anandan, S., Porto Fett, A.C. 2006. Effect of storage temperature on the viability of listeria monocytogenes in queso blanco. Journal of Food Safety. 26:202-214. Interpretive Summary: Although there have been several outbreaks of listeriosis due to consumption of Hispanic-style cheese contaminated with Listeria monocytogenes, there have only been few studies to evaluate or model the growth of this important food borne pathogen in soft cheese. In this study, we compared the growth of a pool of 5 strains of L. monocytogenes inoculated onto vacuum packaged slices of a commercially-prepared Hispanic-style soft cheese called Queso Blanco. The growth rate, maximum growth achieved, and duration of onset of growth were all determined for L. monocytogenes at five different temperatures (5, 10, 15, 20, and 25 deg C) which represent both proper and improper storage conditions. The results revealed that Queso Blanco provides a very suitable environment for growth of L. monocytogenes. In fact, the bacterium grew to the same level/density in cheese at all 5 temperatures tested. These findings provide valuable information which may be used to estimate growth of L. monocytogenes in Queso Blanco at a variety of storage temperatures and to possibly devise ways to prevent it from causing illness.
Technical Abstract: A five-strain cocktail of Listeria monocytogenes was inoculated onto vacuum-packaged slices (ca. 50 g) of a commercial, Hispanic-style cheese, that being Queso Blanco. Growth was determined at appropriate intervals during storage at 5, 10, 15, 20, and 25 deg C. In general, as the incubation temperature increased, a shorter lag phase duration (LPD) and a faster growth rate were observed. The LPD at 5, 10, 15, 20 and 25 deg C were 65.3, 19.9, 2.1, 8.4 and 11.4 hours, respectively. The growth rates were 0.011, 0.036, 0.061, 0.090, and 0.099 log10 cfu/h at 5, 10, 15, 20 and 25 deg C, respectively. There were no statistical differences in LPD at 10, 15, 20, and 25 deg C. However, the LPD during growth at 5oC was statistically longer than at all other temperatures (P<0.05). The growth rates at 20 and 25 deg C were not significantly different; however, the growth rates at 5, 10 and 15 deg C were each significantly different from the rates at the other temperatures (P<0.05). The maximum population density showed relatively little variation over the range of storage temperatures tested, with an average of 8.38 log cfu/g (std. dev. = 0.33). The results of this study indicate that L. monocytogenes replicates in commercially-prepared Queso Blanco at a temperature range of 5 to 25 deg C and that proper storage and handling procedures are required to preclude the bacterium from contaminating the product and/or for controlling its growth.