Listeria monocytogenes Challenge Testing of Ready-to-Eat Uncured Vacuum-Packed Pork Bars with Dried Plasma

Newly developed formulas of ready-to-eat (RTE) products, despite conforming to the latest nutritional trends involving low-processed and high-protein products, may pose a risk of promoting the growth of Listeria monocytogenes during shelf life. Food safety experts recommend challenge tests to assess the growth potential of L. monocytogenes, which will ultimately provide microbiological evidence to determine the food safety limit. The present study evaluated whether RTE uncured vacuum-packed pork bars with dried plasma met the 100 CFU/g safety level at the end of shelf life under certain storage conditions and aimed to develop predictive models for the growth of L. monocytogenes in the above product. The bars were artificially inoculated with a mixture of three strains of L. monocytogenes at two different inoculum densities of 2 log CFU/g and 5 log CFU/g and stored at three different temperatures (2, 4, and 6 °C) and then subjected to microbiological evaluation at specific time intervals up to 21 storage days. The growth potential (Δ-value) for RTE pork bars contaminated with 2 log CFU/g L. monocytogenes inoculum was 0.36, 0.14, and 0.91 log CFU/g at 2, 4, and 6 °C, respectively, while for bars contaminated with 5 log CFU/g inoculum, they were −0.36, −0.40, and 0.64 at 2, 4, and 6 °C, respectively. Statistically higher growth potential (p < 0.05) was detected for RTE bars contaminated with 2 log CFU/g inoculum than with 5 log CFU/g. The results indicate that this type of product must be classified as a food category: “Ready-to-eat foods able to support the growth of L. monocytogenes, other than those intended for infants and for special medical purposes” (Category 1.2. according to EU Regulation 2073/2005). The newly created models can also describe L. monocytogenes growth in an environment where factors, such as temperature, pH, and aw, change with time. The results showed that a higher inoculum density statistically reduced the growth potential values of L. monocytogenes compared to a lower density.