The operator of McDonald's in Japan and Cargill Inc have not discovered any link between a Cargill-owned plant inThailand and a strip of plastic found in a chicken nugget this month that sparked uproar over product safety at the fast-food chain.

McDonald's Holdings Co (Japan) Ltd (2702.T) and Cargill [CARG.UL] said in separate statements on Wednesday that an investigation showed the type of plastic found in the McNugget on Jan. 3 by a customer in Misawa, northern Japan, was not used at the plant in Saraburi, Thailand where the food was produced.

"We are very confident that the plastic film in the nugget occurred outside of our production plant," Chuck Warta, president of Cargill Meats Thailand, said in the statement.

McDonald's Japan said the Misawa store did not use that type of material either, and that the possibility of the plastic entering the nugget during food preparation at the store was low. It was thus unable to determine the origin of the plastic.

News of the find was followed by other complaints of objects discovered in McDonald's Japan food, including a human tooth in a french fry, drawing an apology from the company last week.

The plastic incident was the latest trouble for a fast-food chain hit by sliding sales. On Friday, McDonald's Japan said same-store sales fell 21 percent in December from a year earlier, the 11th straight month of decline, after suffering a shortage of french fries.