Assessment of the Growth of Listeria Monocytogenes in Belgian Artisanal Cheeses and Investigations on Factors Governing it

Abstract

Providing safe food to consumers remains a key challenge nowadays, especially considering the trend that favours natural products and food free of additives.

Among foodborne diseases, listeriosis is the fifth most occurring in EU, with more than 2,500 cases identified in 2018. Listeria monocytogenes, the pathogen responsible for this disease, can be carried by various RTE foods, including dairy products. As potential vectors of L. monocytogenes, cheeses have to comply with food safety criteria defined by Regulation (EC) No 2073/2005. By default, cheeses are considered as RTE foods allowing the growth of the pathogen during their shelflife. Therefore, producers have to guarantee that L. monocytogenesis not detected in cheeses placed on the market. Nevertheless, various foreign studies have identified cheese varieties not allowing this growth, and even allowing a decrease in the levels of contamination during storage.

Belgian cheeses, especially artisanal products, are relatively unknown, although this country possesses a rich diversity of cheese varieties and producers. Consequently, not many data are available regarding the behaviour of L. monocytogenes in these products. Belgian cheese varieties are thus considered as allowing the growth of L. monocytogenes during refrigerated shelf-life. Although they are necessary to guarantee consumers’ safety, food safety criteria represent a permanent sword of Damoclès for producers. The detection of L. monocytogenescan indeed result in huge economic losses and important moral consequences.

The main goal of this thesis was thus to assess the growth of L. monocytogenesin diverse Belgian artisanal cheeses, and to understand factors affecting it.

First, a phone survey was performed among 142 Belgian artisanal cheese producers, providing general knowledge on producers, manufacturing processes and varieties. Globally, 16 major types of cheese were identified. One third of varieties were unripened acid-curd cheeses. Another third corresponded to uncooked pressed cheeses, including Saint-Paulin-type and Gouda-type cheeses, mainly found in Wallonia and Flanders, respectively. Soft cheeses corresponded to 18% of observed varieties. Minor varieties were also identified, including half-cooked and cooked pressed cheeses, blue-veined cheeses, Ricotta, Mozzarella, Halloumi and Feta.

From this data, 65 varieties were selected for deeper characterization. Factories were visited and manufacturing process of these cheeses was monitored. Finally, samples were collected for physico-chemical characterization. From these 65 varieties, only two had physico-chemical characteristics naturally inhibiting the growth of L. monocytogenes, i.e. pH ≤ 4.4, or a_w ≤ 0.92, or pH ≤ 5.0 and a_w≤ 0.94. It means that most varieties theoretically allowed its growth, confirming the interest of the present thesis. Collected data did not allow to improve current cheese classification tools.

After that, 32 varieties representative of the diversity of artisanal cheeses were selected in order to assess the growth of L. monocytogenes. It was decided to perform challenge studies for this purpose, with artificial contamination of final cheeses with L. monocytogenes. Briefly, three batches of each variety were studied, except if predictive models showed no growth of the pathogen during storage. For each batch, 12 pieces were collected. Six were artificially contaminated, remaining pieces being control samples.