Of microbes and milk; probing America's worst salmonella outbreak

FDA Consumer, Feb, 1986 by Chris Lecos

The task force went to extraordinary lengths to find the cause of the outbreak during its five-month probe.

The group met in a "war room" in the plant and methodically set out to prove or disprove a wide array of theories that might account for the outbreak.

Company records were meticulously checked so that the task force could reconstruct, as closely as possible, the operating procedures of the plant. The plant's equipment was taken apart almost piece by piece. Its storage tanks for unpasteurized and pasteurized milk, pasteurizing equipment, pipelines milke fillers and other major pieces of equipment were examined for physical defects. In addition, thousands of samples were collected for laboratory analysis. Both physical and laboratory examinations were made of valves, pipes, gaskets, packaging materials, and even the ink on the packages. The plant's compressed air and water systems, conveyors, floors, sewers, and even its roof were examined. In effect, the entire plant environment was exposed to the scientific and engineering scrutiny of the investigators.

In addition, all of the plant's milk sources--dairy farms in Wisconsin, Illinois and michigan--were checked to see if the Salmonella typhimurium strain could be found. It wasn't. Tanker trucks--which are tagged to show when they were last cleaned and sanitized--were selectively checked. Some were not tagged properly, and the task force noted that it could not exclude all the trucks, or some of the products shipped in them, as a possible means of bringing the organism into the plant.

Elaborate tests were done to uncover possible defects in equipment.

For example, all the milk storage tanks were tested with colored dyes and checked for hairline cracks and pinholes that could serve as bacterial breeding grounds. The tanks are 15 to 55 feet high. The tests required the erection of scaffolding in the tank interiors. During the examination of the tank that held pasteurized skim milk, several suspicious areas were found about 20 feet up from the bottom. Four-inch holes were cut into the tank and foam insulation for any evidence of milk residue or moisture leakage. There was none.

The task force also ruled out any possibility that the pasteurization system was overwhelmed by huge doses of Salmonella-contaminated milk. Only an enormous dose of Salmonella--estimated at 1 trillion organisms per milliliter--could theoretically enable a few organisms to survive the plant's pasteurization process. "In practice," the report said, "this level cannot be achieved. . ."

The investigators' concern that the strain may have been resistant to heat also was ruled out after tests by FDA and other microbiological experts. An employee could have been a chronic carrier of the Salmonella organism but, the task force concluded after testing the employees, this also was unlikely.

The investigation of the sabotage theory, which had received considerable publicity, also involved Illinois state police, who interviewed 129 plant employees and former employees. Many were given lie detector tests. The report noted that a person or persons with a culture of Salmonella typhimurium would need access to, as well as a thorough knowledge of, the plant and its operation and then be able to "commit a series of intentional acts"--undetected--in order to contaminate the milk on an intermittent basis on at least three production dates in 1985 as well as during the year before. No significant leads or evidence of sabotage were found.