Food Safety and Cheese
Institute of Food Science and Technology Position Statement
Contaminated cheese has been responsible for outbreaks of food poisoning by several types of bacteria and sporadic cases of illness associated with contaminated cheeses have also been reported. Experience shows that there have probably been many others that were undetected or unreported. Some of these bacteria can cause severe illness with long-term consequences and death. For example, Listeria monocytogenes can cause meningitis and septicemia with up to 30% mortality. Gastroenteritis due to Salmonella spp. can lead to long-term illness such as reactive arthritis. Infection with Salmonella paratyphi B may lead to septicemia (enteric fever). Brucella melitensis causes undulant fever, a severe disease that can be long-lasting and incapacitating. Verocytotoxigenic Escherichia coli O157: H7 causes enteritis but may also cause hemolytic uremic syndrome (HUS) and kidney damage, particularly in young children and others with a weaken immune system. The role of this bacterium in foodborne disease was only recognized in the early 1980s, but improvements in methods for the detection and isolation of this organism have led to a progressive increase in the number of outbreaks and sporadic cases of infection detected.
Several types of cheese have caused outbreaks of food-poisoning. In thirteen out of sixteen outbreaks in the cheeses were known to have been prepared using unpasteurized milk. It is true that there have also been outbreaks of food-poisoning due to cheese made with pasteurized milk; in these incidents there was evidence of faulty processing such as inadequate heat treatment during pasteurization, mixing with raw milk after pasteurization, or contamination during further processing or distribution of the product. For example, in the production of the Mexican-style cheese involved in the outbreak of Listeriosis pasteurized milk was used but it was reported that (a) it was possible to by-pass the pasteurizer, (b) on several occasions 10% more raw milk was delivered than could be pasteurized given the capacity of the pasteurizer, and (c) tests of cheese on the final day of production showed that more than 10% of samples had an "excessive level of phosphates... “Consistent with insufficient pasteurization or the introduction of raw milk into the milk after the initial pasteurization" (Linnan et al. 1988). Although unusual, a recent outbreak of botulism associated with cheese indicates that this organism can multiply in some cheeses.
In mold-ripened cheeses the pH falls during the fermentation by lactic acid bacteria then rises due to the activity of the mold; in soft mold-ripened cheeses this rise can allow the multiplication of bacteria, including pathogens, to very high numbers.
Raw milk cannot be guaranteed to be free from pathogenic bacteria (Rampling 1996). Most of the types of bacteria shown in are liable to be present in some samples of raw milk. A survey of raw milk from farm bulk milk tanks in England and Wales in 1992-3 for Salmonella and Listeria spp. showed that of 1673 samples, 0.36% contained Salmonella spp. and 5.08% contained Listeria Monocytogenes (O'Donnell, 1995). The presence of these bacteria may result from direct excretion from the udder or as a result of fecal contamination. Accurately controlled pasteurization kills these bacteria. In general, if the milk has not been pasteurized it is difficult to ensure the safety of the final cheese no matter how good the control of hygiene during production. Following the major outbreaks of Listeriosis due to contaminated cheese (Linnan et al. 1988; Bille, 1990) many producers of cheese have changed from the use of unpasteurised to pasteurized milk and great improvements have been made in the conditions of hygiene in major cheese factories (Lund, 1990).
In order to ensure the production of safe cheese the following measures are important:
(2) the raw milk should be refrigerated to minimize multiplication of bacteria
(3) the milk should undergo a full pasteurization
(4) after pasteurization good conditions of hygiene should be maintained during production of the cheese to prevent contamination.
In view of all these considerations the Institute of Food Science and Technology considers that it is important to draw attention to the real hazards to human health due to pathogenic bacteria in raw milk cheeses, particularly of the soft and semi-soft type, and to encourage the use of pasteurized milk in the production of cheeses.