Assessment of L. Monocytogenes cells’ capacity to recover in cucumber juice after high pressure treatment

Abstract

This study aimed at assessing the ability of the foodborne pathogenic bacterium Listeria monocytogenes to resist high pressure processing when present in cucumber juice and evaluating its capacity to recover during post-processing storage at 8°C for 72 hours. Two L. monocytogenes strains were selected, Scott A and RO15, a reference and a food isolated strains, respectively. The cucumber juice was inoculated with either Scott A or RO15 strain at a concentration of approximately 7 log CFU/mL and treated at 300 and 400 MPa, 8°C, for 8 minutes. The behavior of L. monocytogenes populations was studied by employing both quantitative and qualitative approaches: enumeration of L. monocytogenes viable cells by plate counting on a selective growth medium and measuring the fluorescence intensity emitted by the cells when stained with propidium iodide (PI). The treatment at 300 MPa, 8°C, for 8 minutes determined a reduction rate of less than 1 log CFU/mL for both L. monocytogenes strains, while treatment at 400 MPa led to a significantly higher reduction in cells number, 6 and 5 log CFU/mL for Scott A and RO15 strain, respectively. L. monocytogenes viable cells counting at each time point (0, 24, 48 and 72 hours) revealed no multiplication of survivors during storage at 8°C. However, the fluorescence method suggested a possible cellular membrane recovery process, since fluorescence intensity emitted by the HP treated L. monocytogenes cells decreased continuously over the storage period compared to time 0. The results of this experiment emphasize the necessity of employing other barriers against L. monocytogenes in combination with HPP, to ensure its consumption safety during storage for periods longer than 72 hours.