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Inhibition of Listeria monocytogenes by lactose oxidase in UHT skim milk.

B. Flynn



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Inhibition of Listeria monocytogenes by lactose oxidase in UHT skim milk.
B. Flynn*, S. Kozak-Weaver, M. Lawton, S. Alcaine. Cornell University Ithaca, NY.

Listeria monocytogenes is a ubiquitous pathogen that grows at refrigeration temperature and can cause mortality in immunocompromised individuals. Hispanic-style fresh cheeses are susceptible to the growth of L. monocytogenes due to their high water activity, low salt content, and near neutral pH. Traditionally, these cheeses are produced using raw milk which increases the risk of L. monocytogenes contamination due to the lack of a pasteurization step. The use of antimicrobials in raw milk is a potential way to control L. monocytogenes growth in processes without a thermal kill step. Microbial-based enzymes offer a clean-label approach for control of L. monocytogenes. Lactose oxidase (LO) is a microbial-derived enzyme with antimicrobial activity. It oxidizes lactose into lactobionic acid and generates hydrogen peroxide. This study investigated the utilization of LO for the control of L. monocytogenes growth in UHT skim milk as a model system for future applications in raw milk. Three concentrations of LO, 0.006, 0.012, and 0.12 g/L, were evaluated for their ability to inhibit L. monocytogenes growth at 6�C over 21 d. UHT skim milk samples containing these LO concentrations, and a control with no LO, were inoculated with either 4 log cfu/mL or 2 log cfu/mL of a 5-strain cocktail of L. monocytogenes. Samples were enumerated for growth of L. monocytogenes on d 0, 2, 4, 7, 14, and 21. Analysis of Variance and Tukey's Honest Significant Difference tests were performed individually for each time point and log differences between the control and treatments were determined. By d 2, all treatments showed significant differences (P < 0.001) from the control in both challenge experiments. Significant differences between treatments and the control continued throughout the 21-d trials at each time point. By d 21 the control grew to over 7 log cfu/mL while all treatments remained at 1 log cfu/mL. These results suggest that LO is a potential control strategy for L. monocytogenes growth in milk, and future studies will investigate its antimicrobial efficacy in raw milk and cheese applications.

Keywords: Listeria, enzymatic preservation, milk.