Let's uncork some chianti, order up a mushroom pizza and talk a bit about that mozzarella gambit over at the Department of Agriculture.

Perhaps motivated by the big dairy surplus, the department has the idea that frozen pizzas ought to have more real cheese, not the ersatz mozzarella that adorns much of the frozen pizza sold in retail stores.

So last summer the USDA's Food Safety and Inspection Service proposed requiring that every frozen pizza with meat have at least 12 percent cheese topping, with no less than half of that being real cheese. Now only 10 percent of the cheese topping has to be real cheese.

The USDA also wants pizza labels to tell consumers more clearly when they're buying imitation cheese. But the department can only get its regulatory hands on pizzas with meat products; the Food and Drug Administration is responsible for meatless pizzas and requires only that products with cheese substitutes be clearly labeled.

All of the hubbub surprised a lot of people. Food processors, for instance, thought that Reaganomics meant less regulation.

Consumer advocates were equally taken aback by the proposed regulation on cheese in meat pizzas. "It was unprecedented, a complete shock to everyone," said Tom Smith of Public Voice for Food and Health Policy. "This administration has never gone to bat for consumers on labeling issues." Smith noted that the administration previously threw out rules that would have required that meat that had been mechanically deboned be labeled to that effect. "They are not consistent."

Whatever, the USDA asked for comment on its proposal and, paisano, did the comments come back. The department is learning that tampering with pizza pie is about as serious as mucking around with the flag and motherhood.

The department has been inundated with such reaction--more than 2,000 pro and con statements--that it has extended the comment period to April. Although the issue has kicked around the USDA for years, the frozen pizza industry said it needs more time to work up economic-impact data.

"This is an unusual response to one of our proposals," said Robert F. Hibbert, director of standards and labeling. "But there have been a lot of form letters, organized mail. These things are not elections, though; it's too easy to stuff the ballot box."

Pizza makers and food brokers have bombarded the USDA with letters protesting the proposal, claiming it will cost them millions of dollars and cause unemployment in the imitation cheese industry. Cheese makers and dairy-farmer cooperatives applaud the proposal, seeing it as a way to sell more milk. Consumers seem to be divided.

A few sample comments:

* Dan Emery of Apopka, Fla.: "Don't let special interests push us any further into the poorhouse than we already are. Personally, I won't pay more for pizza so some thumbsucker in the dairy industry can buy another Mercedes."

* Rose Mark of West Sedom, Ariz.: "I am so glad the government is going to do something about pizza."

* A Mrs. Grochowski from Milwaukee: "I hope they leave the pizzas just the way they are and not add any real cheese. My 2 1/2-year-old son has asthma and allergies. He can't have any milk products. The pizza with fake cheese is a real treat for him."

* Guy G. Smith, Wellesley, Mass., applauding bigger, clearer labels: "Old people like myself do not always have perfect vision in spite of all that opthalmologists can do."

The USDA's foray into the pizza kitchen goes back to 1973 when the department proposed a rule that would have allowed the use of uncooked meat in pizza products and set cheese content levels at 12 percent.

Reaction was small, but most of it was negative, so the department decided to leave well enough alone. But then in 1978 the Wisconsin Cheese Makers Association, alarmed at the increasing use of cheaper imitation cheese in frozen pizzas, asked the USDA to order clear label identification of substitutes.

A year later, after Jeno F. Palucci, one of the kingpins of the $1 billion-a-year frozen pizza business, asked the USDA to modify its standards on meat content, the department again sought general comment on the ideas. The result is the pending proposal, which also requires that all meat on a pizza be cooked before the pizza is frozen.

Meanwhile, however, the nation's dairy surplus began mounting. And the USDA, by this year spending nearly $3 billion to buy excess milk, butter and cheese, came under increasing pressure from the dairy industry to help ease its overproduction problem.

The pizza proposal came just as public criticism of the federal dairy program was hitting a new crescendo. Hibbert denies that there was a connection. "There are people here at the department concerned about these dairy issues," he said, "but our consideration of the pizza cheese options preceded the current crunch. From our perspective, it's always been a labeling issue."

Now, would someone please deal with the anchovies?