# Plant Extracts for the Control of _Listeria monocytogenes_ in Meat Products
## 3 Results
### Growth Control of L. monocytogenes by Plant Extracts in Artificially Contaminated Cooked Ham  
Figures 2 and 3 show the plant extracts effectiveness against _L. monocytogenes_ artificially inoculated in cooked ham slices. The test, performed during the shelf-life at 4 \({}^{\circ}\)C of the product using two different levels of contamination, demonstrated that, unlike sausages, the cooked ham, due to its intrinsic characteristics (pH, aw, etc.) allows for the growth of the pathogen. The samples CLM3 untreated with plant extracts and inoculated with the higher concentration (on average 3.74 log cfu/g of _L. monocytogenes_) showed 5.48 log cfu/g viable counts after 8 days, 7.33 log cfu/g after 15 days and a greater 8.00 log cfu/g at the end of shelf-life (Figure 2). In PELM3 samples treated with plant extracts, the activity of the compound immediately after treatment is highlighted from a decrease of about 1.00 log cfu/g of _L. monocytogenes_ (the counts recorded were between 2.04 and 2.70 log cfu/g). After 8 days of storage, _L. monocytogenes_ viable counts increased up to 4.11 log cfu/g, and they increased up to 6.05 log cfu/g 15 days later, reaching 7.86 log cfu/g at the end of the product shelf-life (Figure 2). The difference between CLM3 and PELM3 was significant during the test (\(p\)-value \(<0.01\)), with an average difference of 1.00 log. On the 5 th and the 12 th day after contamination, the difference between treated and untreated ham slices was of 2.00 log (\(p\)-value \(<0.001\)). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sampling**} & \multicolumn{5}{c}{**Microbial Profile (log cfu/g)**} \\ \cline{2-6}  & **Mesophilic** & **Lactic Acid** & & & _Enterobacteriaceae_ & _Staphylococcus aureus_ \\ \hline
1 & 6.58 \(\pm\) 0.10 & 6.36 \(\pm\) 0.22 & 6.13 \(\pm\) 0.23 & 3.34 \(\pm\) 0.12 & 1.13 \(\pm\) 0.21 \\
3 & 8.73 \(\pm\) 0.18 & 8.69 \(\pm\) 0.31 & 6.88 \(\pm\) 0.11 & 1.78 \(\pm\) 0.16 & 1.70  
In the cooked ham inoculated with the lower microbial concentration (on average 2.05 log cfu/g of _L. monocytogenes_) a similar trend was observed. In samples CLM2 untreated an increase in _Listeria_ viable counts of almost 1.00 log cfu/g was recorded from the 5 th day; this value increased to 4.00 log cfu/g at 12 th day and remains unchanged until to the end of shelf-life (Figure 3). In the slices treated with the plant extracts (PELM2), in the first 5 days after treatment, we observed a reduction of about 1.00 log, with viable counts of _L. monocytogenes_ reaching on average 0.87 log cfu/g. From the 5 th to the 12 th day of experimentation, the greatest differences in the growth of _L. monocytogenes_ were exhibited in the two samples (PELM2 and CLM2), with a difference of 2.00 log (\(p\)-values \(<\) 0.001). After 15 days, in the treated sample (PELM2), the pathogen growth increased up to about 3.00 log cfu/g and then remained constant until the end of the shelf-life (Figure 3). The difference between the two samples decreased from the 15 th day (\(p\)-values \(<\) 0.01) until the end of the test (\(p\)-values \(<\) 0.05) due to the unchanging _L. monocytogenes_ growth in both the treated and untreated ham slices. Figure 3: Trend of _L. monocytogenes_ during shelf-life of ham cooked with (PELM2) and without (CLM2) plant extract. \(p\)-values of * \(<\)0.05, **\(p<\) 0.01, ***\(p<\) 0.001 and ***\(p<\) 0.0001 were considered significant by \(t\)-test and ANOVA. ns means not significant. Figure 2: Trend of _L. monocytogenes_ during shelf-life of ham cooked with (PELM3) and without (CLM3) plant extract. \(p\)-values of * \(<\)0.05, **\(p<\) 0.01, ***\(p<\) 0.001 and ***\(p<\) 0.0001 were considered significant by \(t\)-test and ANOVA. ns means not significant. In summary, the results of the study carried out using two different levels of contamination showed that the plant extracts added in cooked ham slices produced a slow down of the potential growth rate of _L. monocytogenes_ compared to the untreated control.