Effect of high-pressure processing on reduction of Listeria monocytogenes in packaged Queso Fresco

Abstract

The effect of high-hydrostatic-pressure processing (HPP) on the survival of a 5-strain rifampicin-resistant cocktail of Listeria monocytogenes in Queso Fresco (QF) was evaluated as a postpackaging intervention. Queso Fresco was made using pasteurized, homogenized milk, and was starter-free and not pressed. In phase 1, QF slices (12.7×7.6×1cm), weighing from 52 to 66g, were surface inoculated with L. monocytogenes (ca. 5.0 log10 cfu/g) and individually double vacuum packaged. The slices were then warmed to either 20 or 40°C and HPP treated at 200, 400, and 600MPa for hold times of 5, 10, 15, or 20min. Treatment at 600MPa was most effective in reducing L. monocytogenes to below the detection level of 0.91 log10 cfu/g at all hold times and temperatures. High-hydrostatic-pressure processing at 40°C, 400MPa, and hold time ≥15min was effective but resulted in wheying-off and textural changes. In phase 2, L. monocytogenes was inoculated either on the slices (ca. 5.0 log10 cfu/g; ON) or in the curds (ca. 7.0 log10 cfu/g; IN) before the cheese block was formed and sliced. The slices were treated at 20°C and 600MPa at hold times of 3, 10, and 20min, and then stored at 4 and 10°C for 60d. For both treatments, L. monocytogenes became less resistant to pressure as hold time increased, with greater percentages of injured cells at 3 and 10min than at 20min, at which the lethality of the process increased. For the IN treatment, with hold times of 3 and 10min, growth of L. monocytogenes increased the first week of storage, but was delayed for 1 wk, with a hold time of 20min. Longer lag times in growth of L. monocytogenes during storage at 4°C were observed for the ON treatment at hold times of 10 and 20min, indicating that the IN treatment may have provided a more protective environment with less injury to the cells than the ON treatment. Similarly, HPP treatment for 10min followed by storage at 4°C was the best method for suppressing the growth of the endogenous microflora with bacterial counts remaining below the level of detection for 2 out of the 3 QF samples for up to 84d. Lag times in growth were not observed during storage of QF at 10°C. Although HPP reduced L. monocytogenes immediately after processing, a second preservation technique is necessary to control growth of L. monocytogenes during cold storage. However, the results also showed that HPP would be effective for slowing the growth of microorganisms that can shorten the shelf life of QF. © 2014 American Dairy Science Association.