Food Safety Focus (94th Issue, May 2014) – Incident in Focus
Listeria monocytogenes in Cheese Products
Reported by Ms. Janny MA, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety
On 18 April 2014, the US Centers for Disease Control and Prevention (CDC) announced its final investigation on a listeriosis outbreak which involved eight persons, including two mother-newborn pairs and a newborn, with one death in the US. Results of the investigation, with food (fresh cheese curd) and environmental samples tested positive for Listeria monocytogenes, indicated that cheese products made by Roos Foods were the likely source of the outbreak. This article discusses the risk of L. monocytogenes in cheese products.
Cheeses can generally be obtained by coagulating the protein of milk and by partially draining the whey resulting from the coagulation. Nowadays, over 500 types of cheeses are available worldwide, with variations deriving from different cheese manufacturing processes, e.g. type of milk, coagulation method, starting culture, addition of salt and ripening etc.
In fact, there are various ways to categorise cheeses. Traditionally, cheeses have been classified principally by their moisture content-
Soft cheese - Has a higher moisture content, e.g. Feta, Brie, Camembert
Semi-hard cheese - Moisture content sits between soft and hard cheeses, e.g. Edam, Gouda
Hard cheese - Has a lower moisture content, e.g. Cheddar, Emmental
Extra hard cheese - Dry, slightly brittle, suitable for grating, e.g. Parmesan
Cheeses may also be grouped according to their principal ripening -
Unripened/ Fresh cheese - Ready for consumption soon after manufacture, e.g. Cottage cheese, Ricotta
Ripened cheese - Not ready for consumption shortly after manufacture; must be held for such time, temperature and other conditions that results in the necessary biochemical and physical changes characterising the cheese, including -
- Mould ripened cheese - ripening has been accomplished primarily by the development of characteristic mould growth
- Internal mould ripened: c haracterised by the growth of Penicillium roquefortii resulting a network of blue and green veins throughout the cheese (blue cheese), e.g. Danish blue, Roquefort, Stilton
- Surface mould ripened: characterised by the growth of Penicillium camemberti on the cheese surface, e.g. Brie, Camembert
- Cheese in brine - has no actual rind and preserved in brine e.g. Feta
Listeria monocytogenes in Cheeses
Cheeses, particularly soft cheeses, have been implicated in listeriosis outbreaks worldwide. Foodborne listeriosis is a relatively uncommon but serious disease caused by L. monocytogenes, a pathogen that can be killed under normal cooking temperature but is able to grow slowly at refrigerated temperature as low as 0°C. Asymptomatic infection of listeriosis probably occurs in most healthy people, but it can pose serious health risks for the susceptible population including pregnant women, elderly and immunocompromised individuals such as patients with AIDS and diabetes mellitus.
The presence of L. monocytogenes in cheeses may be originated from the ingredients particularly raw milk or can come from the processing plant environment, including the equipment, personnel or cross-contamination between finished products and raw materials. If the temperature as well as other conditions especially acidity and water content permit, L. monocytogenes can grow to high levels upon prolonged storage.
Cheeses of Higher or Lower Risk
Since pasteurisation, by heating milk to a specific temperature for a set period of time, kills L. monocytogenes effectively, cheeses made with pasteurised milk are generally considered of lower risk unless post-process contamination occurs.
For cheeses made with unpasteurised milk, their safety relies on a range of factors that influence the presence, growth, survival and inactivation of pathogenic microorganisms including L. monocytogenes.
In general, soft cheeses made with unpasteurised milk are of much higher L. monocytogenes risk than hard/ extra hard cheeses made with unpasteurised milk as the formers are likely to be less acidic and contain more moisture, which provide a favourable environment for the growth of L. monocytogenes, than the latter. A recent risk assessment study conducted by Food Standards Australia New Zealand also pointed out that the estimated L. monocytogenes risk from the consumption of certain raw milk soft cheeses i.e. feta and camembert is low in the general population but is high in the susceptible population. However, the L. monocytogenes risk upon the consumption of raw milk cheddar cheese (a type of hard cheese) and extra hard cheese in the general and susceptible populations is negligible and low/ very low respectively.
Key Points to Note:
- Cheeses, particularly soft cheeses, have been implicated in outbreaks of listeriosis worldwide.
- Cheeses made with pasteurised milk are generally considered of lower risk.
- Soft cheeses made from unpasteurised milk are the most risky.
Advice to susceptible populations
- Read food labels and choose cheeses carefully before consumption.
- Hard and extra hard cheeses are generally safe.
- Avoid soft cheeses (e.g. Feta, Brie, Camembert) and blue cheeses (e.g. Danish blue, Gorgonzola and Roquefort).
- For other types of cheeses, choose only those made from pasteurised milk.
- Do not eat if in doubt.
- Store cheese products strictly in accordance with the instructions on the labels.
Advice to the trade
- Maintain good food and personal hygiene and avoid cross-contamination.
- Provide sufficient information on food label for the consumers to make informed food choices.
- Properly label whether the cheese products are made from raw/ unpasteurised or pasteurised milk.
- Consider providing more information e.g. description on firmness of the cheese products.