P20 | Use of <i>Lactobacillus curvatus</i> in controlling <i>Listeria monocytogenes</i> in a meat product

Purpose. Listeria monocytogenes is a ubiquitous bacterium capable of growing at refrigeration temperatures and poses a risk to Ready-To-Eat (RTE) foods, for which increasingly longer shelf lives are required. The objective of this study was to evaluate the ability of Lactobacillus curvatus, frozen or freeze-dried, alone or in combination with buffered vinegar, to counteract the growth of L. monocytogenes in a RTE meat product (sliced chicken breast packaged in modified atmosphere), by determining the growth potential of L. monocytogenes, according to the EURL Lm Guidelines (rev. 4, 2021) and in accordance with ISO 20976-1:2019. Methods. A preliminary single-batch Challenge Test (CT) was conducted, testing 5 formulations containing: buffered vinegar (A); freeze-dried L. curvatus (B); buffered vinegar and freeze-dried L. curvatus (C); frozen L. curvatus (D); buffered vinegar and frozen L. curvatus (E). The products were supplied by the company already mixed with buffered vinegar and/or L. curvatus (107 CFU/g). Test units inoculated with a mixture of 4 strains of L. monocytogenes (2-3 log10 CFU/g) pre-adapted at 10°C (inoculum 1% w/v, "double septum" technique), control units inoculated with the same technique with 1% w/v physiological saline solution, and uninoculated units were prepared; All samples were stored at 7°C for 9 days and at 10°C until the end of their shelf life of 13 and 17 days. At time zero (T0), and at T3, T6, T10, T13, and T17 (days), 3 test units were analyzed for L. monocytogenes enumeration, 3 control units, and 3 uninoculated units for the determination of lactic acid bacteria (LAB), pH, and water activity (aw). For each formulation, the growth potential of L. monocytogenes was calculated as the difference, expressed in log10 cfu/g, between the average maximum concentration reached during the test and the average initial concentration (if <0.5 log10 cfu/g, the product does not support growth). Based on the results obtained during the preliminary CT, the formulation to be used for the final CT, performed on 3 batches, was selected according to the same operating procedures. Results. During the preliminary CT, the growth potential of L. monocytogenes (log10 cfu/g) was: 5.36 (A); 2.53 (B); 1.07 (C); 0.32 (D); 0.06 (E). In formulations containing L. curvatus, LAB increased by an average of 1.98 log10 CFU/g from T0 to T17; Specifically at T3, after the adaptation phase, the increase in LAB (log10 CFU/g) was: 1.17 (B), 0.46 (C), 1.16 (D), and 0.90 (E). In the final CT, performed using formulation E, the growth potentials of L. monocytogenes (log10 cfu/g) in the 3 batches tested were -0.08, 0.36, and -0.16, respectively. Conclusions. In the preliminary CT, the combination of buffered vinegar and frozen L. curvatus proved to be the most effective strategy in controlling L. monocytogenes. However, in the final CT, the growth potential was comparable to that obtained in the preliminary test with frozen L. curvatus. This suggests that the frozen strain of L. curvatus adapts more quickly and begins to replicate sooner than the freeze-dried form, showing a significantly greater increase between T0 and T3 in the presence of buffered vinegar (E vs C). These data support the use of frozen L. curvatus as an effective strategy for controlling L. monocytogenes in RTE meat products.