## 3 Results and Discussion  
### Photometric Growth Inhibition Experiments  
The effect of the added preservatives and acids on the growth rate of the four investigated bacteria, with no added Co-cit, was determined (Tables 1-4). With the exception of the two acids, the added preservatives did not cause a change in the pH of the broth of more than 0.4 units compared to the control broth pH of 7.1 (data not shown). Sorbate (10 g/L) and NaCl (50 g/L) caused significant (\(p\) < 0.05) growth inhibition of all four bacteria. Benzoate (2 g/L) caused a significant reduction in growth rate in three of the bacteria and a fairly extensive reduction (to 67% of the control value) in _L. monocytogenes_. However, the latter reduction was not statistically significant due to a large variance in the growth rate. Propionate (20 g/L) significantly affected only _S. aureus_, whereas nitrite (0.15 g/L) reduced the growth rate of _L. monocytogenes_ and _E. coli_. Acidification of nutrient broth to pH 5.5 with hydrochloric acid did not significantly affect (\(p\) > 0.05) the maximum growth rate of any pathogenic microorganism. This behavior was in contrast to that employing nutrient broth with acetic acid pH 5.5, which significantly reduced (\(p\) < 0.05) the growth rate of _L. monocytogenes_ and _E. coli_ and increased their lag phases while significantly (\(p\) < 0.05) increasing both the growth rate and lag phase \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Modification of Nutrient broth**} & \multicolumn{4}{c}{**Ratio of Growth Parameters of _Listeria monocytogenes_ in:**} \\ \cline{2-5}  & **Modified NB with Co-cit/Modified NB without Co-cit** & **Modified NB/Standard NB** \\ \hline  & **Max. Growth Rate** & **Lag Phase** & **Max. Growth Rate** & **Lag Phase** \\ \hline None (standard NB) & 0.61 \({}^{\star}\) & 1.0 & - & - \\ Sorbate (10 g/L) & 0.57 \({}^{\star}\) & 1.0 & 0.69 \({}^{\star}\) & 2.1 \\ Benzoate (2 g/L) & 0.89 \(\dagger\) & 0.8 & 0.67 & 1.8 \\ Propionate (20 g/L) & 0.56 \({}^{\star}\) & 0.9 & 1.03 & 1.0 \\ HCl (pH 5.5) & 0.37 \({}^{\star}\)+ & 2.0 & 0.95 & 1.2 \\ Acetic acid (pH 5.5) & 0.20 \({}^{\star}\)+ & \(>\)2.6 \(\ddagger\) & 0.57 \({}^{\star}\) & 3.6 \\ NaNO\({}_{2}\) (0.15 g/L) & 0.52 \({}^{\star}\) & 1.1 & 0.48 \({}^{\star}\) & 1.2 \\ NaCl (50 g/L) & 0.51 \({}^{\star}\) & 1.8 & 0.41 \({}^{\star}\) & 0.6 \\ \hline \hline \end{tabular}  
* * Significant (\(p\) < 0.05) difference between the mean maximum growth rates in standard or modified broth with and without Co-cit (column 2) or between modified NB and standard NB (column 4). Statistical analysis was not performed on the lag phases. \(\ddagger\) Significant (\(p\) < 0.05) difference between the ratio of maximum growth rates with and without Co-cit in modified broth compared to that in unmodified nutrient broth (0.61). \(\ddagger\) Some or all of the recorded lag phases in broths with Co-cit were \(>\)24 h.  
\end{table} 
Table 1: Relative maximum growth rates and lag phases of _Listeria monocytogenes_ in modified and standard nutrient broth (NB) with and without cobalt citrate [Co-cit (0.5 mM)]. of _P. aeruginosa_. Weak acids, such as acetic acid, are normally more antimicrobial at a given pH (within a certain range) than strong acids because the undissociated, uncharged molecules can enter the cell more easily and then dissociate in the cytoplasm. This occurs when the pH is fairly low relative to the pKa of the acid, meaning that the acid is mostly undissociated [25]. The pH used in the experiments (5.5) is above the pKa of acetic acid (4.76), so the acid would have been mostly in an ionized state and, therefore, less antimicrobial. However, the reduction in growth rates observed in _L. monocytogenes_ and _E. coli_ shows that the acid had some inhibitory properties. The reason for the increase in the growth rate of _P. aeruginosa_ in the presence of acetic acid is unclear. However, it is known that the organism can grow well on acetate [26] and, thus, may have benefitted from the additional carbon source.