Appearing in the May/June 1997 issue of Public Health Reports
Health officials at the Minneapolis Department of Health report on an outbreak of food poisoning from contaminated machines that dispense hot chocolate. Seven of 39 vending machines tested in the city were contaminated, of these, two had levels of B. Cereus sufficient to cause illness.
In August 1994, three employees at a Minneapolis manufacturing plant complained of nausea and vomiting soon after drinking a hot chocolate drink from a vending machine in the plant cafeteria. The machine was flushed, the hoses were changed and the containers were refilled with new powdered chocolate mix. New cases of illness occurred when workers drank the chocolate the next day.
The machine, made of stainless steel and food grade plastic had a holding tank where water is heated and stored. It also dispensed coffee, chicken broth, and tea. All of the mixes were stored as powders and each was mixed in its own vessel in the machine. There should have been no cross contamination between the different drink mixes. A sample hot drink from the machine grew B. cereus in large quantities--170,000 colonies per gram. The symptoms described by the workers were typical for food poisoning with B. cereus. Neither the water reservoir nor the powder was contaminated.
Further investigation of the hot chocolate machine revealed B. cereus contamination in the machine's mixing cup, on a silicone gasket where the shaft of the mixing blade entered the cup, and on the fluting of the cup itself. When additional machines were tested in the city, the total of contaminated machines rose to 7 in 39 tested. The contaminated machines had three things in common:
- All were manufactured prior to 1980.
- None was cleaned by the operator more than once a week. (Cleaning typically meant 'flushing the hoses' with hot water from the holding tank.)
- Affected machines either had no systems for automatic 'flushing' with water from the holding tank at regular intervals or the existing mechanism had been disconnected.
"Typically, B. cereus contamination is found in cooked foods that are left for extended periods at room temperatures and then served. Heating stimulates the germination of spores commonly present in the food, causing the illness-inducing toxin to develop. In the present case, spores may have been present in the dry chocolate mix at acceptably low levels but germinated and multiplied rapidly when exposed to warm water for extended periods of time in places where beverage pooled or where it remained on various machine components. No other likely source of contamination was identified."
The authors conclude, "Periodic testing of vending machines is thus warranted, particularly when they are used by vulnerable populations such as the aged or immunosuppressed, for who illness may have severe consequences. City sanitarians should be aware that vending machines may be a potential reservoir of B. cereus contamination."