listeria cheese

listeria cheese

Listeriosis is a severe food-borne illness, caused by the pathogenic micro-organism Listeria monocytogenes. This micro-organism poses a risk especially to young children, immunosuppressed and elderly people, and the unborn child. The fatality rate of listeriosis is high (20-30%), and the incidence in The Netherlands is 3.9 cases per million. The Food Safety Objective for Listeria monocytogenes has been set to 100 cfu/g in ready-to-eat foods able to support the growth of Listeria monocytogenes placed on the market during their shelf-life and absence in 25 g before the food has left the immediate control of the food business operator, and absence in 25 g for ready-to-eat food intended for infants and ready-to-eat foods for special medical purposes that is placed on the market during their shelf-life (EC No. 2073/2005). Listeria monocytogenes is ubiquitous in the environment, the incubation time can vary a lot (2-90 days), Listeria monocytogenes is capable of forming biofilms and can grow at a low pH, low water activity and low temperatures (FDA/CFSAN 2001).
According to model simulations Listeria monocytogenes is able to grow in semi-hard cheeses following contamination with this pathogen. However, in practice and according to a study by Northolt et al. (1989) this risk seems to be minimal in traditional semi-hard cheeses. However, in recent years a number of trends in the manufacturing of semi-hard cheeses have occurred, including foil-ripening of cheese, production of low-fat, low-salt cheese, and increased consumption of pre-packed and grated cheese. These changes could affect the survival and outgrowth of Listeria monocytogenes in semi-hard cheeses.

Aim
This project aims to determine critical points in the chain that determine contamination and/or growth of Listeria monocytogenes in different semi-hard cheeses. Relevant growth data of Listeria for different product and storage conditions will be collected and generated. The overall aim is to improve predictive models for Listeria in semi-hard cheeses.

Research
The steps in the chain that determine the risk of contamination or growth of Listeria in different cheese will be determined. Risk profiling models in the dairy chain will be used to generate risk profiles for the presence of Listeria in cheese. This information is used to identify knowledge gaps.
Subsequently, relevant growth data will be collected for Listeria for different products and storage conditions, using relevant strains (preferably from the processing environment). First, strains will be selected, e.g. persistent strains from the factory environment. These strains will be used to determine why growth in cheese is not the same as growth predicted using model calculations. Factors that could play a role are pH, organic acids, packaging and processing conditions (foil-ripened cheese, sliced cheese) and local conditions in the cheese. Growth will be monitored in medium and model substrates (micro- and mini-cheeses).
The next aspect of the work is to evaluate whether the growth data of ‘laboratory’ strains differ from growth data of relevant strains in relevant matrices. For a number of isolates growth and survival in e.g. milk, cheese, and brine will be determined. This can also include Listeria species other than Listeria monocytogenes (like Listeria innocua), which may be used as indicators for the presence and survival of Listeria monocytogenes.

Commission Regulation (EC) No. 2073/2005 on microbiological criteria for foodstuffs

FDA/CFSAN 2001 Draft Assessment of the Relative Risk to Public Health from Foodborne Listeria monocytogenes Among Selected Categories of Ready-to-Eat Foods

Northolt MD, Beckers HJ, Vecht U et al. Listeria monocytogenes: heat resistance and behaviour during storage of milk and whey and making of Dutch types of cheese. Netherland Milk & Dairy Journal 1989; 42: 207-219.