Issues update
Salmonella in high fat/reduced water activity foods - a hazard not to be overlooked
Updated 24 April 2007
Latest outbreak (August 2006)
Contamination of peanut butter with Salmonella Tennessee has been linked to a salmonellosis outbreak affecting over 400 people in the United States (1). This outbreak has again demonstrated the ability of Salmonella spp. to be a food safety problem in products that food manufacturers and consumers traditionally perceive as low risk. Other high fat/reduced water activity products that have been implicated in Salmonella outbreaks include chocolate and cheese.
Why is Salmonella an issue in these products?
- High fat and reduced water activity protect against heat inactivation during processing
- High fat contents protect against acidic conditions of the human digestive system
- Salmonella cells can survive in these products for extended periods
- A microbial "kill step" (eg. heat) may not be used in the manufacturing of some products
- These products are "ready-to-eat" and are frequently consumed without any cooking
Enhanced heat resistance
Salmonella can survive heat treatments used in manufacturing of high fat/reduced water activity products (eg. chocolate conching). One study showed that even at 90°C it took over an hour to inactivate 90% of Salmonella Typhimurium in molten milk chocolate (2). Another study found that Salmonella inoculated into peanut butter only decreased by 1000 cells after 50 minutes of heating at 90°C (3).
Low levels still cause illness
The most recent outbreak associated with chocolate and Salmonella occurred in the UK between February and June 2006. The manufacturer did not immediately instigate a recall when Salmonella was detected in some products (during in-house testing) as it thought the levels found were too low to cause illness. Thirty-seven people did in fact become ill with salmonellosis as a result of consumption of chocolate products manufactured with contaminated chocolate crumb mix. The level of Salmonella present in these implicated products has, to date, not been reported.
As few as 1 cell per gram of Salmonella Oranienberg was found in chocolate implicated in a 2001–2002 European salmonellosis outbreak that affected 439 people (4). Low levels of contamination were also identified in Australia's 1996 peanut butter associated salmonellosis outbreak. Less than 3 cells per gram of the implicated serotype (Salmonella Mbandaka) were found in batches of product associated with consumer illness (5). Cheddar cheese was implicated in an outbreak of Salmonella Typhimurium that affected more than 1500 Canadians in 1984 (6). Implicated cheese products were found to contain only 0.4 – 9.3 most probable number (MPN) Salmonella per 100g. The presence of such low levels of cells meant that not all samples in the same batch tested positive for Salmonella contamination, with only 7 to 50% of 30 sub-samples from the same batch testing positive.
Survival during long shelf life
Long term analysis of the Canadian cheese samples described above, showed that Salmonella could survive in the cheese throughout 8 months of refrigerated storage (6). After 12 months storage at room temperature, Salmonella was still detectable in chocolate bars implicated in a 1982 UK Salmonella Napoli outbreak (4).
Take appropriate action
Detection of low levels of Salmonella in high fat/reduced water activity products should be taken seriously, as was demonstrated by the 2006 UK chocolate outbreak. A precautionary recall of twenty-five confectionary products was instigated by a Canadian manufacturer when Salmonella was detected in a chocolate ingredient (soy lecithin) in November 2006 (7). In March 2007 an Australian cheese manufacturer instigated a voluntary recall after Salmonella contamination was detected in one batch of product (9). The Food Standards Australia New Zealand (FSANZ) Guidelines for the Microbiological Examination of Ready-To-Eat Foods (8) recommend that ready-to-eat products should be free from Salmonella. Manufacturers of high fat/reduced water activity products such as those discussed here and similar products (eg. chocolate nut spreads), should take precautionary action if Salmonella is detected in ingredients or finished products.
Food Safety Programs reduce risk of product contamination
Detection of microbial contamination may be hindered as salmonellae may be present in such low numbers that distribution is uneven throughout product batches. Therefore emphasis should be placed on preventing contamination occurring in finished products, rather than relying on detection by routine product testing. Vigilance is required to ensure post heat treatment contamination does not occur because the primary ingredients of peanut butter, chocolate and cheese should have received heat treatments (roasting peanuts, roasting cocoa beans and heat treating milk) that effectively eliminate or reduce levels of vegetative bacteria. Inadequate factory design (eg. waste water pipes located above open ingredient storage vats), poor pest control practices and cross contamination of heat treated ingredients by non-heated ingredients are all ways Salmonella may find its way into finished products.
For further information and resources to assist with development of Food Safety Programs refer to:
FSANZ Food Standards Code 3.2.1 Food Safety Programs
The Food Safety Toolkit™
Australian Confectionary Manufacturers of Australasia Ltd
Dairy Industry Association of Australia
References
1. http://www.cdc.gov/ncidod/dbmd/diseaseinfo/salmonellosis_2007/outbreak_notice.htm
2. Goepfert J. M. and Biggie R. A. 1968. Heat resistance of Salmonella Typhimurium and Salmonella Senftenberg 775w in milk chocolate. Applied Microbiology 16: 1939-1940.
3. Shachar D. and Yaron S. 2006 Heat tolerance of Salmonella enterica serovars Agona, Enteritidis and Typhimurium in peanut butter. Journal of Food Protection 69 (11): 2687-2691.
4. Werber D., Dreesman J., Feil F., van Treeck U., Fell G., Ethelberg S., Hauri A.M., Roggentin P., Prager R., Fisher I.S., Behnke S.C., Bartelt E., Weise E., Ellis A., Siitonen A., Andersson Y., Tschape H., Kramer M.H. and Ammon A. 2005 International outbreak of Salmonella Oranienburg due to German chocolate. BMC Infectious Diseases 5 (7). Available from: http://www.biomedcentral.com/1471-2334/5/7
5. Scheil W., Cameron S., Dalton C., Murray C., Wilson D. 1998. A South Australian Salmonella Mbandaka outbreak investigation using a database to select controls. Australian and New Zealand Journal of Public Health 22 (5): 536-539.
6. 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 (12): 1062-1066.
7. http://www.foodqualitynews.com/news/ng.asp?n=72322-hershey-s-lecithin-salmonella
8. Food Standards Australia New Zealand (FSANZ). 2001. Guidelines for the Microbiological Examination of Ready-To-Eat Foods
9. http://www.foodstandards.gov.au/foodmatters/foodrecalls/currentconsumerlevelrecalls/cheesemicrobialsalmo3497.cfm