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Inhibition of Listeria monocytogenes in queso fresco by topical application of lactose oxidase.

K. Jencarelli



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Inhibition of Listeria monocytogenes in queso fresco by topical application of lactose oxidase.
K. Jencarelli*, S. Kozak, M. Lawton, S. Alcaine. Cornell University Ithaca, NY.

Listeria monocytogenes is of major concern in dairy products, especially in fresh cheeses that have high water activity and moisture content, and near-neutral pH. Consumer trends in clean-label foods necessitates the development of strategies to combat pathogens without traditional preservatives. The enzyme lactose oxidase (LO) represents a potential clean-label antimicrobial in dairy products. LO oxidizes lactose to lactobionic acid and produces the antimicrobial hydrogen peroxide. The objective of this research was to determine the efficacy of LO to inhibit L. monocytogenes growth in queso fresco. Since cheese has a reduced level of lactose, a 16% lactose solution was mixed 50/50 with LO before its application to the cheese to ensure adequate substrate levels and produce an antimicrobial effect. A 100 �L aliquot of LO was added at varying concentrations (0.012 g/L, 0.12 g/L, 1.2 g/L and 12 g/L) to 5 � 0.5g of commercial queso fresco inoculated with 2 log cfu/g of L. monocytogenes. The inoculum was allowed to attach for 30 min before LO treatments. Two positive controls, inoculated cheese with and without lactose addition, and a negative, uninoculated, control were included. Cheese was stored at 6�C and enumerated on modified oxford agar on d 0, 2, 4, 7, 14, and 21 post-inoculation. An ANOVA and Tukey tests were used to compare differences in L. monocytogenes growth among treatments and the positive control. By d 21, L. monocytogenes on the positive control reached 7.39 log cfu/g. There was a significant reduction of L. monocytogenes growth in both the 12 g/L and 1.2 g/L LO treatments. The level of L. monocytogenes in the 12 g/L treatment remained below the detection limit of 2 log cfu/g, while the level in the 1.2 g/L treatment reached 3.04 log cfu/g (P < 0.001). L. monocytogenes growth in the lactose control did not differ from the positive control without lactose (P = 0.899). This study shows that LO is effective in inhibiting the growth of L. monocytogenes in the fresh cheese queso fresco at concentrations 12 g/L and 1.2 g/L. Future research will be conducted to study potential sensorial effects of LO in cheese.

Keywords: Listeria, queso fresco.