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Title:   Author  item
                                            Tomasula, Peggyitem
                                            Renye, Johnitem
                                            Van Hekken, Dianeitem
                                            Tunick, Michaelitem
                                            Kwoczak, Raymonditem
                                            Toht, Matthewitem
                                            LEGGETT, LATASHA - US Department Of Agriculture (USDA)item
                                            Luchansky, Johnitem
                                            Porto-Fett, Annaitem
                                            Phillips, John
  Submitted to: Journal of Dairy Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 11/28/2013
                                    Publication  Date: 1/20/2014
                                    Publication URL: http://handle.nal.usda.gov/10113/59480Citation: Tomasula, P.M., Renye Jr, J.A., Van Hekken, D.L., Tunick, M.H., Kwoczak, R., Toht, M.J., Leggett, L.N., Luchansky, J.B., Porto  Fett, A.C., Phillips, J.G. 2014. Effect of high pressure processing on reduction of Listeria monocytogenes in packaged Queso Fresco. Journal of Dairy Science. 97:1281-1295. DOI: 10.3168/JDS.2013-7538.Interpretive Summary: Queso fresco (QF) is a Hispanic - style cheese that is made from pasteurized milk and is well known for its nonmelting and crumbling properties. As a fresh cheese, its moderate salt and high moisture contents make it an ideal medium for the growth of bacteria, yeasts and mold, which can cause cheese spoilage, or the growth of pathogenic bacteria, such as Listeria monocytogenes, which can cause illness in immuno-compromised people. In this study, a technique known as high hydrostatic pressure processing, HPP, which has been shown to kill bacteria on some food products, was used on QF slices that were inoculated with L. monocytogenes and then packaged. HPP was found to injure L. monocytogenes but did not kill it; it regrew slowly during refrigeration. HPP will have to be combined with another method to kill the pathogen. However, HPP does kill much of the spoilage microorganisms in QF, which can extend the shelf life of this cheese.
                                Technical Abstract: The effect of high hydrostatic pressure processing (HPP) on the survival of a five-strain rifampicin-resistant cocktail of Listeria monocytogenes in Queso Fresco (QF) was evaluated as a post-packaging intervention. QF was made using pasteurized, homogenized milk, was starter-free and was not pressed. In Phase 1 of this study, QF slices, 12.7 cm x 7.6 cm x 1 cm with each slice weighing from 52 to 66 g, were surface inoculated with L. monocytogenes (ca. 4.0 log10 cfu/g), individually double vacuum-packaged and stored at 4C.  The slices were then warmed to either 20 or 40C, and treated using HPP at 200, 400, and 600 MPa for hold times of 5, 10, 15 or 20 min. HPP treatment at 600 MPa was effective in reducing L. monocytogenes to below the detection level of 0.91 log10 cfu/g, at all hold times and temperatures. Processing at 40C was effective at 400 MPa and hold time > 15 min but resulted in significant wheying-off and textural changes. In Phase 2, L. monocytogenes was inoculated either on the slices (ca. 4.0 log10 cfu/g) (ON) or into the curds (ca. 7.0 log10 cfu/g) (IN) before the cheese block was formed and sliced. The slices were treated at 20C and 600 MPa at hold times of 3, 10 and 20 min, reducing L. monocytogenes to below 0.91 log 10 CFU/g, and then stored at 4 and 10C for 60 d. For both treatments, L. monocytogenes became less resistant to pressure as the hold time increased with greater percentages of injured cells at 3 and 10 min than at 20 min in which the lethality of the process increased. For the IN treatment, with hold times of 3 and 10 min, growth of L. monocytogenes increased the first week of storage, but was delayed for 1 wk with hold time of 20 min. Longer lag times in growth of L. monocytogenes during storage at 4C were observed for the ON treatment at hold times of 10 and 20 min indicating that the IN treatment may have provided a more protective environment with less injury to the cells. Lag times in growth were not observed during storage of QF at 10C. While 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 which can shorten the shelf life of QF.


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