In 1994, at least 593 confirmed cases of Salmonella enteritidis infection in America were blamed on contaminated Schwan’s ice cream. Mind you, these were only the confirmed cases from people who sought medical treatment - epidemiologists estimate that 224,000 people in total fell sick. That’s over two hundred thousand people that may have been sickened by one batch of bad ice cream that year.

What happened?

When investigators went to the Schwan’s manufacturing plant in Marshall, Minnesota, that was implicated in this outbreak, they didn’t find any problems with the equipment in the plant. There were no Salmonella to be found in the plant grounds, not even in any of the flavorings used to make the ice cream.

But investigators did discover something interesting - a new contract between Schwan’s and the trucking company that carried ice cream ingredients for them. This arrangement increased the amount of liquid eggs (one of the ingredients in the ice cream) the trucking company was carrying.
The trucks would typically carry ice-cream premix (also used in making the ice cream) in between loads of liquid eggs. Neither the eggs or the ice-cream premix were pasteurized before being used in making ice cream.

Aha!

Investigators, checking the trucks, found problems like cracks, soiled outlet valve gaskets and egg residues. Turns out that the trucks were often not cleaned between loads. Samples of the liquid egg obtained from the supplier tested positive for Salmonella.

The investigators concluded that Salmonella got into the ice cream premix that was transported in trucks that also carried contaminated liquid eggs.

That’s all it took, folks - people too lazy to clean out their trucks properly caused hundreds, maybe thousands of people to get Salmonella. Of course, if the ice cream had been pasteurized before freezing, that might have prevented this whole mess from happening.

Of course, this was an isolated incident - ice cream is typically safe to eat. But this case shows how widespread a food microbiology problem can be if just one step in a food manufacturing process is neglected.

Reference:

Hennessy TW et al. “A National Outbreak of Salmonella enteritidis Infections from Ice Cream.” New England Journal of Medicine 334:1281-1286. 1996.

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