A spokeswoman for the ministry of agriculture said the investigation was intended to ensure that the company was observing food safety regulations after traces of dioxin, a powerful carcinogen, were found in potato peelings sold for cattle feed.
The contamination was traced to marl clay from Germany used in the manufacturing process.
McCain claims to produce one third of the french fries consumed around the world.
Two dairy farms were closed after milk was found to exceed the allowed limits for dioxin.
The agriculture ministry said 88 farms were able to open on Friday, but a further 103 continued to be quarantined pending the result of tests.