OSAKA -- Osaka Prefectural Police conducted a series of searches Wednesday after the discovery that the Mister Donut chain in Japan had been using an unauthorized antioxidant in its dumplings.

The searches were carried amid allegations that the firm, operated by Duskin Co., violated food sanitation laws.

Police inspected a food processing firm in the city's Nishi-Yodogawa Ward that was in charge of the manufacture of the dumplings, and a food and beverage wholesaler in Kanazawa, Ishikawa Prefecture. They also searched a public health center in Suita, Osaka Prefecture, where Duskin, whose main business is renting cleaning equipment, is based.

According to Duskin officials, the antioxidant t-butyl hydroquinone was mixed in the vegetable fat used to make the outer layer of the dumplings. The health ministry has not authorized the use of the substance in food, although its use is legal in other countries, including China and the United States. The dumplings in question were produced in China.

Duskin President Takeshi Ueda has acknowledged that the firm was aware that the move might be a violation of food sanitation laws.

Mister Donut sold more than 13 million of the dumplings between October and December 2000, the officials said. And after learning that t-butyl hydroquinone was being used in its products, it went on to sell a further 3 million dumplings.

The firm said it stopped using the antioxidant after a company with which it had dealings pointed out the problem. However, it continued to sell the dumplings because inspections of its inventory found no trace of the substance.

Also on Wednesday, the Suita health center instructed Duskin to submit by Friday a report on the import and distribution of the meat dumplings in question.