A bulletin for the Australian Food Industry August 1998
Contents: Protozoan parasites in water and food | Infective dose of Salmonella in ice cream | Microbiological status of beef in Australia | Hygiene monitoring by ATP luminometry | Botulism associated with home preserved mushrooms | Quality system guidelines | Spoilage of low salt margarine
Infective dose of Salmonella in ice cream
The recent food poisoning incident in South Australia associated with consumption of gelati has once again drawn attention to ice cream and related products as vehicles for food infections. Over eighty cases of infection caused by Salmonella oranienberg were reported in South Australia during March. Most of the victims were treated by their own doctors but several required hospital treatment. Investigations into the source of the S. oranienberg are continuing.
Laboratory examination of the contaminated gelati indicates that the infective dose of S. oranienberg may have been very low. The organism could not be detected in the standard 25g samples but could be detected when 100g samples were tested.
This is consistent with results obtained during the investigation of the 1994 food poisoning outbreak in the United States associated with contamination of ice cream by Salmonella enteritidis. The total number of people affected in this outbreak was over 200,000 nationally with the tally in Minnesota, the State in which the ice cream was manufactured, exceeding 30,000.
In a report published not long after the incident (Journal of the Association of Food and Drugs Officials 53 (3) 1995 48-68), William L. Oemichen, then Deputy Commissioner of the Minnesota Department of Agriculture, described the impressive detective work carried out by his Department in conjunction with the Minnesota Department of Health and the US Food and Drug Administration (FDA). In an echo of Australia's fragmented food legislative system, it was the Department of Agriculture which had primary regulatory responsibility under Minnesota law for food safety within the State. The FDA became involved because product crossed State boundaries.
Oemichen and his colleagues concluded that the most likely source of the S. enteritidis in the ice cream was from the contaminated residue left in a truck trailer, which had been used to transport raw eggs, prior to the hauling of pasteurised ice cream mix manufactured off-site. A number of changes have now been instituted at the ice cream plant which was permitted to recommence manufacture about five weeks after being shut down.
Another conclusion reached by the investigating team was that the level of S. enteritidis cells in the ice cream was much lower than one per gram and the number of organisms in a single serve may have been as low as seven. Oemichen qualified this conclusion by noting that given the time which had elapsed before suspect product was examined, this result may not have been a true indication of the number of these organisms present in the ice cream when the outbreak occurred.
We have noted previously (Food Safety & Hygiene, September 1996) that in certain circumstances the infective dose of Salmonella spp may be very low. Confirmation of the initial work in the Minnesota incident has now been published by Vought and Tatini (Journal of Food Protection 61 (1) 1998 5-10). These workers from the Minnesota Department of Agriculture and the University of Minnesota respectively were able to study samples of the implicated ice cream in storage at -20°C for up to 12 months.
They found no evidence of decrease in number of S. enteritidis during 16 weeks storage at -20°C. The implicated ice cream was approximately six weeks old at the height of the outbreak in 1994.
These authors concluded that their data indicated that 25 to 50 cells of S. enteritidis caused mild to severe illness and that children were at higher risk from these infective doses than adults. Some young children may have become ill after consuming as few as 10 cells.
Vought and Tatini cite this outbreak as another example where high fat and/or sugar in food protect salmonellae from natural gastric barriers and allow the bacteria to reach the small intestine where they cause symptomatic illness. The infective dose in these products is thus substantially lower than that generally reported.
Both sets of authors note that the incident could have been avoided if a more robust HACCP program had been in place and if final product examination had involved direct testing for salmonella in 50g product rather than relying on testing for indicator organisms.
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