Contamination of dairy products by the food pathogen Listeria monocytogenes.

Contamination of dairy products by the food pathogen Listeria monocytogenes is always a concern. Some manufacturers, however, are of the opinion that the normal background microbial population in the factory should be able to inhibit the growth and survival of the pathogen. The question is, is this belief really true? A study by Dr Schirmert and colleagues in Norwegian small-scale cheese plants attempted to find out. Their results were published in the Journal of Dairy Science, Volume 96 of 2013, pages 6161 to 6171, the title of the paper being: Microbial background flora in small-scale cheese production facilities does not inhibit growth and surface attachment of Listeria monocytogenes. In another study on the topic of the pathogen, the question was addressed whether replacement of sodium chloride (salt) by potassium chloride in the manufacture of cheese will provide similar protection against Listeria monocytogenes. Worldwide (also in South Africa) there is concern that because of the positive association of high salt intake and hypertension (high blood pressure), salt as preservative and flavour product should be reduced in foods. One way of doing this is to replace salt (NaCl) with potassium chloride (KCl). The study by Dr Hystead and colleagues was published in the same journal on pages 6172 to 6185 under the title: The effect of sodium reduction with and without potassium chloride on the survival of Listeria monocytogenes in Cheddar cheese.

In the study on the effect of the background microflora, samples from the air, food contact surfaces (storage surfaces, cheese molds and brine) and non-contact surfaces (floor, drains and doors) were analysed. A total of 1314 isolates were identified belonging to 55 bacterial genera, one yeast species and 6 mold species. Lactococcus spp obviously were dominant. There were substantial differences in microflora composition between factories and sites of sampling, especially in the drains and the rubber seals of doors, and much less variation on the cheese shelves and salt brines as could be expected. Of the isolates, 62 bacterial isolates and the one yeast were tested for antilisterial activity; none showed significant inhibitory effects. The pathogen was also co-cultured on ceramic tiles with bacteria dominant in the cheese production units. None of the tested isolates altered the survival of the pathogen. Thus, despite the large variation in types of microflora on the surfaces tested and on the other hand the narrow group of specific bacteria dominant in cheese manufacturing, there was no evidence that background microflora in the factory will inhibit the growth or survival of Listeria monocytogenes.

The salt-to-moisture ratio is used to gauge if conditions for producing good-quality cheese have been met. If the NaCl is reduced the salt-to-moisture ratio deviates from the ideal, resulting in changes in culture acidification profiles. Sometimes KCl is used to restore the ratio and, as mentioned above, to act as a substitute in health-associated arguments. In the control of Listeria monocytogenes in cheese making both salt content and pH are vital; therefore, when NaCl is reduced or KCl added one suspects changes in pH leading to alteration in culture acidification and possible less control of the pathogen. The effectiveness of control was therefore studied by Dr Hystead and colleagues with cheese manufacturing treatments of normal NaCl (ideal salt-to-moisture ratio), 25% reduction and 50% reduction, with and without the addition of KCl. KCl was added on a molar equivalent to the NaCl it replaced to maintain an equivalent salt-to-moisture ratio. Listeria monocytogenes was inoculated at different stages post-processing to simulate what is normally experienced in practice. Control of the pathogen was influenced by time of exposure, culture cocktail, NaCl reduction and KCl. With 50% reduction in NaCl, control was less effective than with 25% reduction, whether or not KCL was present. Addition of KCl to restore the salt-to-moisture ratio was effective depending on time post-inoculation and temperature. However, when contamination was induced was crucial. With post-processing contamination during the later stages of ripening, Listeria monocytogenes was capable of survival in the cheese regardless of which culture was used, whether or not NaCl had been reduced, or if KCl had been added to maintain the effective salt-to-moisture ratio. Thus, conditions over time of ripening appear to become favourable for the growth of Listeria monocytogenes irrespective of the controlling media.