California public health officials have refused to clear two California food-processing plants linked to the spinach E. coli outbreak, despite the company's insistence that investigators have found no evidence of contamination in its factories.

Charles Sweat, chief operating officer of Natural Selection Foods LLC, said Thursday that test results showed that two of the company's processing plants have been found to be clean of the deadly bacteria. Sweat told reporters that the tests were conducted by the U.S. Food and Drug Administration, the California Department of Health and an independent laboratory.

However, Dr. Kevin Reilly, deputy director of the Health Department's prevention services branch, would not confirm Sweat's assertions.

"As of today, the California Department of Health Services has not cleared the processing facilities in the investigation," he told a conference call with reporters on Thursday.

Investigators earlier traced the E. coli outbreak, which has killed one person and sickened 187 others including an Ottawa woman, to Natural Selection Foods. The company supplies spinach to more than a dozen companies. Officials are still unsure as to how the bacteria came into contact with the food.

Farming practices under investigation

Reilly said authorities are continuing to investigate farming practices in the Salinas Valley in an attempt to pinpoint how the outbreak began. From the company's records, inspectors think the tainted greens came from at least one of nine farms in Monterey, San Benito and Santa Clara counties.

He also said public health officials are working on developing more preventative measures to ensure the quality and safety of the produce.

"Good agricultural practices are pretty well understood. The key is going to be consistency and doing that 100 per cent of the time in 100 per cent of the farms," Reilly said. "One breakdown of that process can create the next outbreak."

New safety procedures planned

Meanwhile, Sweat said that his company planned to introduce new safety procedures, testing a sample from each lot of greens it packages.

"While we had food safety practices in place, we need to move to a new level of food safety," he said.

Nine bags of spinach found in seven U.S. states are currently being tested in labs across the country, said the Centers for Disease Control.

"All packages were marketed as baby spinach and labelled with the same brand name. The 'DNA fingerprints' of all nine of these E. coli match that of the outbreak strain," the CDC said in a release.

Health officials are also investigating two more deaths, possibly related to the case.

The Canadian Food Inspection Agency is continuing to warn consumers not to eat fresh spinach imported from the U.S.

Officials have confirmed that the E. coli strain O157:H7 was present in the contaminated spinach. The O157:H7 strain, a deadly form of the bacteria, contaminated water in Walkerton, Ont., in 2000, killing seven people.

E. coli stands for Escherichia coli, a bacteria commonly found in the intestines of animals
and humans.

With files from the Associated Press