OSAKA — Snow Brand Milk Products Co.’s Osaka plant, where a valve contaminated with bacteria has been linked to a massive outbreak of food poisoning, recycled milk products that were returned from stores to make “new” products, according to the Osaka Municipal Government.

The Hokkaido Prefectural Government has also learned that Snow recalled about 700,000 packages of butter last month after consumers complained that the products smelled strange.

The information from Osaka came to light in an interim report detailing sanitation management at the plant, where staff are suspected of negligence that led to the food poisoning outbreak involving the staphylococcus aureus bacteria in its milk. The plant’s hygiene certification is now likely to be rescinded.

Snow Brand reportedly commissioned workers to recycle the returned products inside a refrigerated warehouse, and the report pointed to the possibility that the recycled milk products may have included items that had passed their expiry date.

It was also learned that the company repeatedly mixed warm low-fat milk in a storage tank not designed for such procedures, the city and police said Tuesday.

They said they learned that workers at the plant sometimes poured warm mixtures of water and skimmed milk powder into the tank as they adjusted the density of the milk, possibly raising the temperature in the tank high enough to nurture bacteria.

Low-fat milk is among Snow Brand’s products apparently tainted with staphylococcus aureus, which is believed to have made more than 13,000 people ill.

The city and police suspect toxins were produced after low-fat milk contaminated with staph was mixed in the tank.

According to investigations, low-fat milk produced at the factory underwent final adjustment in the tank before it was pasteurized.

When the milk was thin, workers manually mixed skimmed milk powder and water outdoors at a temperature of 30 degrees, steamed it to dissolve the powder and then poured the mixture into the tank, which is supposed to be used to preserve cooled milk before pasteurization.

Osaka officials and investigators said they plan to examine whether temperatures inside the tank rose to between 36 degrees and 38 degrees, which would allow the bacteria to spread.

Previous investigations have failed to pinpoint the source of the staphylococcus aureus, but authorities believe inappropriate management of the temperature inside the tank promoted the spread of the bacteria in the milk.

Bacteria need nutrition, water and adequate temperature to multiply, and temperature management is vital in producing and processing milk, according to experts.

A specialist has pointed out that a rise in temperature over several hours can result in food poisoning.

Hokkaido learned that Snow had recalled about 700,000 packages of its butter last month after consumers complained that the products smelled like cheese.

Officials said 63,000 packages of Hokkaido Butter and 638,000 packages of Portion Butter were recalled.

Consumers contacted Snow’s Tokyo headquarters between late May and early June to complain.

The products were manufactured on April 16 and 17 at the company’s Horonobe factory in Hokkaido and had an expiry date of mid-October, Snow Brand said.

The Health and Welfare Ministry held a hearing Tuesday to determine whether to revoke the hygiene certification of Snow’s plant in Osaka’s Miyakojima Ward.

Akihiko Sasajima, vice president of Snow, attended the hearing in place of President Tetsuro Ishikawa, who was hospitalized Sunday, and explained the company’s stance.

Later this week, the ministry plans to withdraw the Hazard Analysis and Critical Control Point authorization it granted the company’s factory in Osaka to produce processed milk and lactic drinks.

Snow is suspected of deliberately excluding a valve and pipe believed to have initiated the food poisoning from a list of items to be examined when it applied for HACCP authorization.

In January 1998, the ministry granted HACCP authorization for five products manufactured at the Osaka plant.

Cancellation of the qualification, which is widely recognized by distributors as a certificate for a plant’s proper hygienic control and safety, is a harsh punishment for a food manufacturer, industry sources said.

The company has already announced plans to close the Osaka plant following the food-poisoning outbreak.