## 3 Results and Discussion
### Antimicrobial Activity of PLA-(Co-cit) Films against L. monocytogenes on Food Samples  
The results of the three experiments investigating the effect of the films on the growth of _L. monocytogenes_ in foods are presented in Figures 3-5. The storage temperature of 8 \({}^{\circ}\)C was chosen to represent a poorly-functioning but not untypical domestic refrigerator [31]. Presumptive _Listeria_ spp. colonies were not detected in the uninoculated control samples in any of the three experiments, indicating that none of the foods were naturally contaminated. The background aerobic plate count (Figure 3A) in the first experiment with turkey slices was around 6 log CFU/cm\({}^{2}\). No consistent changes were observed throughout the 10-day storage period. The _L. monocytogenes_ count (Figure 3B) also stayed constant in the samples with the membranes (with and without Co-cit). In the control containing no film, the _L. monocytogenes_ count began to increase on day 7 but then declined to a level below the detection limit by day 10. By that point, the samples had shown evidence of desiccation (in this experiment, they were not placed in a sealed jar with wet cotton wool). This decrease in water activity, together with the associated increase in salt and nitrite concentration, is the likely cause of the loss of viability of _L. monocytogenes_. The minimum water activity (a\({}_{\mathrm{w}}\)) for the growth of _L. monocytogenes_ is 0.90, while the optimum value is greater than or equal to 0.97 [32]. Heat-treated cold meals have a water activity of about 0.96-0.99, making them a suitable substrate for the growth of _L. monocytogenes_[33]. This observation showed that the PLA films offer some protection from desiccation, despite their high water permeability. In the second experiment, also using turkey but with a much lower background flora (<100 CFU/cm\({}^{2}\)), the counts on PCA (Figure 4A) and Oxford agar (Figure 4B) were very similar. As the aerobic plate count of the uninoculated samples was below the limit of detection for the entire storage period, it is likely that the counts on PCA are mostly the inoculated _L. monocytogenes_. Significant differences were observed between the three inoculated sample types. The membranes had an inhibitory effect on the growth of _L. monocytogenes_. Although this effect was slightly greater in the membranes containing Co-cit, the difference between membranes with and without Co-cit was fairly small compared to the difference between samples with and without membranes. The total increase in count over the 10-day incubation period was around 1.4 log CFU/cm\({}^{2}\) in the control, 0.6 log CFU/cm\({}^{2}\) in the samples with membranes with no Co-cit, and 0.2 log CFU/cm\({}^{2}\) in samples with Co-cit. Figure 3: _Cont._  
In the experiments with smoked salmon, no significant differences were observed between membranes with and without Co-cit. On day 4, the _L. monocytogenes_ count (Figure 5B) in the samples without a membrane was about 1 log CFU/cm\({}^{2}\) higher than in those with membranes, whereas no differences were observed at the previous time points. The _L. monocytogenes_ count (Figure 5B) in the control remained stable for the first two days and increased by around 1 log CFU/mL on day 3. The salmon used in these experiments was a smoked product and may have contained inhibitory smoke components such as phenolic and carbonyl compounds. This was not determined, but the fact that _L. monocytogenes_ grew on the control samples indicates that any such compounds' inhibitory effect present was mild or absent. In these three experiments carried out on food surfaces, the PLA membrane itself contributed the most to the bacteriostatic action of the system, whereas the presence of Co-cit was a secondary contributory factor, despite the high concentration used. Food is a more complex and concentrated medium than laboratory media, and higher concentrations of antimicrobials are often needed for similar inhibitory effects to be observed due to interference by food components.