Listeria monocytogenes is a ubiquitous pathogen that can cause morbidity and mortality in the elderly, immune compromised, and the fetuses of pregnant women. The intrinsic properties of fresh cheese-high water activity (aW), low salt content, and near-neutral pH-make it susceptible to L. monocytogenes contamination and growth at various points in the production process. The aim of this study was to investigate the ability of lactose oxidase (LO), a naturally derived enzyme, to inhibit the growth of L. monocytogenes in fresh cheese during various points of the production process. Lab-scale queso fresco was produced and inoculated with L. monocytogenes at final concentrations of 1 log CFU/mL and 1 CFU/100 mL. LO and LO sodium thiocyanate (TCN) combinations were incorporated into the milk or topically applied to the finished cheese product in varying concentration levels. A positive control and negative control were included for all experiments. When L. monocytogenes was inoculated into the milk used for the cheese-making process, by day 28, the positive control grew to above 7 log CFU/g, while the 0.6 g/L treatment (LO and LO + TCN) fell below the limit of detection (LOD) of 1.3 log CFU/g. In the lower inoculum, the positive control grew to above 7 log CFU/g, and the treatment groups fell below the LOD by day 21 and continued through day 28 of storage. For surface application, outgrowth occurred with the treatments in the higher inoculum, but some inhibition was observed. In the lower inoculum, the higher LO and LO-TCN concentrations (0.6 g/L) reduced L. monocytogenes counts to below the LOD, while the control grew out to above 7 log CFU/g, which is a >5 log difference between the control and the treatment. These results suggest that LO could be leveraged as an effective control for L. monocytogenes in a fresh cheese.
Keywords: Listeria; enzymes; lactose oxidase; queso fresco.