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:

  1. the raw milk should be collected and maintained in good hygienic conditions 
  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.

Microbiological tests on finished cheeses have an important place in quality control, but these tests cannot ensure the microbiological safety of the cheese (Desenclos et al. 1996; Rambling, 1996).
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.

* Pasteurized milk was used, but there was evidence that unpasteurised milk was also included. 
** These products are known to have been produced using unpasteurised milk. 
*** Many Brie cheeses are made from unpasteurised milk. NR = not reported. 
# one case of hemolytic uremic syndrome. 
## all cases of hemolytic uramic syndrome.

  • Anon (1994). E.coli O157 : H7 phage Type 28 infections in Grampian. Communicable Diseases and Environmental Health of Scotland 28 (No. 94/46) 1.
  • Anon (1995). Brucellosis-associated with unpasteurised milk products abroad. Communicable Disease Report (32).
  • Bille, J.(1990). Epidemiology of human Listeriosis in Europe with special reference to the Swiss Outbreak. In Foodborne Listeriosis. eds Miller, A.J., Smith, J.L. and Somkuti,G.A. pp 71-74. Amsterdam: Elsevier.
  • Bone, F.J., Bogie, D. and Morgan-Jones, S.C. (1989) Staphylococcal food poisoning from sheep milk cheese. Epidemiology and Infection. 103, 449-458.
  • D'Aoust, J-Y. (1994). Salmonella & international trade. International Journal of Food Microbiology 24,1- 31.
  • D'Aoust, J-Y., Warburton, D.W. and Sewell, A.M. (1985). Salmonella typhimurium phage type 10 from Cheddar cheese implicated in a major Canadian foodborne outbreak. Journal of Food protection 48, 1062-1066.
  • Desenclos, J.C. et al. (1996). Large outbreak of Salmonella enterica serotype paratyphi B infection caused by goats' milk cheese, France: a case finding and epidemiological study. British Medical Journal 312, 91-94.
  • Goulet, V. et al. (1995). Listeriosis from consumption of raw-milk cheese. Lancet 345, 1581-1582.
  • Hedberg, C.W. et al. (1992). A multistate outbreak of Salmonella javiana and Salmonella oranienberg infections due to contaminated cheese. Journal of the American Medical Association 268, 3203- 3207.
  • Linnan et al. (1988). Epidemic Listeriosis associated with Mexican-style cheese. New England Journal of Medicine 319, 823-828.
  • Lund, B.M. (1990). The prevention of foodborne Listeriosis. British Food Journal 92, (4) 13-22.
  • MacDonald, K.L. et al. (1985). A multistate outbreak of gastrointestinal illness caused by enterotoxigenic Escherichia coli in imported semisoft cheese. Journal of Infectious Diseases 151, 716-720.
  • Maguire, H.C.F. et al. (1991). A large outbreak of food poisoning of unknown etiology associated with Stilton cheese. Epidemiology and Infection 106, 497-505.
  • Maguire et al. (1992). An outbreak of Salmonella dublin infection in England and Wales associated with a soft, unpasteurized cow's milk cheese. Epidemiology and Infection 109, 389-396.
  • Nooitgedagt, A.J. and Hartog, B.J. (1988). A survey of the microbiological quality of Brie and Camembert cheese. Netherlands Milk and Dairy Journal 42, 57-72.
  • O'Donnell, E.T. (1995). The incidence of Salmonella and Listeria in raw milk from farm bulk milk tanks in England and Wales. Journal of the Society of Dairy Technology 48, 25-29.
  • Rampling, A. (1996). Raw milk cheeses and salmonella. British Medical Journal 312, 67-68.
  • Sadik, C. et al. (1986). An epidemiological investigation following an infection by Salmonella typhimurium due to the ingestion of cheese made from raw milk. In proceedings of the 2nd World Congress on Foodborne Infections and Intoxication’s 1, Berlin, pp 280-282.
  • Simini, B. et al. (1996). Outbreak of foodborne botulism continues in Italy. Lancet 348, 813.
  • Vaillant, V., Haeghebaert, S., Desenclos, J.C. et al. (1996) Outbreak of Salmonella dublin infection in France, November-December 1995. Eurosurveillance 1, (2) 9-10.
  • The Institute of Food Science & Technology, through its Public Affairs and Technical & Legislative Committees has authorized this Position Statement, dated 15 November 1996, prepared by its Professional Food Microbiology Group.

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