Shares of Spherix Inc. gained as much as 12 percent yesterday after the company said Kellogg Co. had been issued a patent enabling it to include Spherix's low-calorie sugar substitute in foods such as ready-to-eat cereal.

The nation's No. 1 cereal maker wouldn't say yesterday whether it had specific plans to add tagatose to its products, nor would Kellogg say whether it had ordered significant amounts of the sweetener. And Spherix Chief Executive Officer Gilbert V. Levin cautioned that the patent doesn't guarantee that Kellogg will be adding tagatose to anything.

Still, Kellogg is one of the first major food companies to publicly show interest in tagatose, which looks and tastes similar to sugar but has 40 percent of the calories. Pepsi-Cola Co. also has applied for tagatose-related patents.

"The advantage of having the process patent is that it will provide us the opportunity to either improve existing brands or explore new innovations with this sweetener product," said Celeste Clark, Kellogg vice president for corporate and scientific affairs.

The fact that Kellogg is interested in even exploring uses for tagatose is an encouraging sign for Spherix's long-patient investors, said H.G. Wellington & Co. analyst Lincoln A. Werden.

"It means these big consumer companies are giving serious consideration to using tagatose in some of their products," said Werden, whose firm holds hundreds of thousands of shares of Spherix's stock and who personally owns hundreds of shares.

Spherix's stock rose as high as $8.70 yesterday before easing to close at $8.35, up 60 cents, on the Nasdaq stock market.

Unlike many low-calorie sugar substitutes, tagatose has sufficient bulk to be used in baking a cake. It's been cleared for sale in the United States since last year, but it has yet to hit the market. The reason: Despite Levin's decades-long push to develop a low-calorie sweetener, large quantities of tagatose have yet to be manufactured. Spherix licensed the sweetener to Swedish dairy giant Arla Foods for all uses in food. Arla has a joint venture with German sugar producer Nordzucker under which tagatose is to be produced in a plant near Hanover, Germany.

The joint venture is expected to sell tagatose to food and beverage companies at about $2 a pound, Werden said. He expects Spherix to get a 5 percent to 10 percent royalty on all such sales.

Spherix has held a U.S. patent since 1988 on the use of tagatose as a low-calorie sweetener and bulking agent. Its proprietary tagatose manufacturing process also is covered by patents.

But Levin said yesterday that food and beverage companies such as Kellogg can apply for tagatose "improvement" patents, which cover use of the product in their proprietary recipes and beverage formulations. Clark said the patent issued to Kellogg, however, covers the process used to apply it.