TORONTO - The manufacturer of meat slicing equipment cited by Maple Leaf Foods (TSX:MFI) as the possible cause of its deadly listeriosis outbreak insists there has never been a serious food safety issue with its machines.

A spokesman for Illinois-based Formax, Inc., says there are nearly 300 Formax S-180 meat slicing machines installed at processing plants around the world, and an estimated 2.3 billion kilograms of sliced meat have been produced over the last 13 years without any safety concerns.

But company spokesman Brian Sandberg says technical experts are reviewing the machines and manuals as a precaution, to confirm that "all instructions are clear and effective for the operation, maintenance and sanitation of these machines."

At a news conference on Friday, Maple Leaf Foods CEO Michael McCain said the Listeria bacterium may have accumulated deep inside two of its Formax S-180 machines, spawning the listeriosis outbreak that has caused 13 deaths.

He said the discovery of the possible accumulation wasn't made until the machines were fully dismantled, which went beyond the equipment's normal cleaning procedures.

McCain said the company followed the machine's sanitization instructions to the letter, which included cleaning the equipment on a daily, weekly and monthly basis.

The large machines, which measure roughly four metres in length and three metres in height, will now have to be disassembled regularly to ensure bacteria doesn't accumulate inside, McCain said.

"We will build that disassembly process into the normal protocol, which is really not that practical on an ongoing basis," McCain said, as he acknowledged the difficulty in taking apart the cumbersome machines.

"Long term, if that can't be sustained then we'll replace the equipment," he said.

But McCain also said results of the investigation are not definitive, since the pervasiveness of Listeria "makes an absolute determination (of the outbreak) impossible."

Other contributing factors may include the location of a service elevator, floor drain and bins - although the product likely did not come into contact with those surfaces.

The plant has remained closed since Aug. 20 and will not open until a comprehensive investigation has concluded, a "deep sanitization" of the plant has been completed, and the Canadian Food Inspection Agency is satisfied no safety issues are outstanding, the company said.

In addition to the 13 deaths connected to the outbreak, another six deaths are under investigation.

In all, 38 cases of listeriosis have been confirmed and 22 more are suspected.

Prime Minister Stephen Harper has pledged to conduct a "broad" independent investigation into the listeriosis outbreak.

Federal officials have said they believe the worst of the outbreak may be over and few new cases are now expected.