The first genetically engineered product for human consumption, an enzyme expected to be of wide use in making cheese, was approved yesterday by the Food and Drug Administration (FDA).

In the historic decision, which came after 28 months of review, the agency said that a bioengineered form of the enzyme rennin -- which has traditionally been extracted from calves' stomachs as part of a mixture called rennet and used by cheese-makers to curdle milk -- presents no safety hazard and may be used in a variety of dairy products.

Officials of the dairy industry, which spends about $100 million on rennet every year, welcomed the new product as an alternative to the natural form of the enzyme. Rennet is of uncertain purity and has soared in price in recent years. The approval also seems to set an encouraging precedent for the number of companies seeking FDA approval for other enyzmes made by genetically altered bacteria.

But federal officials and some biotechnology experts said the time the agency took to approve the application and the unusually detailed explanation the FDA gave for the approval indicate that genetic engineering of foods remains a sensitive issue and that the agency intends to scrutinize applications for more exotic bioengineered products. "The enzyme issues are easy compared to what is confronting the FDA in the future in areas like genetically altered plants," said Edward Korwek, a leading biotechnology attorney. "Some of the other products present a slew of issues that are so difficult they'll make this look like Pollyanna."

The new kind of rennin is produced by biotechnology's workhorse bacteria, a species called Escherichia coli. The bacteria were altered by implanting in them the cow gene carrying the blueprints for rennin. Since all cells read the same genetic code, the bacteria, now carrying so-called recombinant DNA, make rennet identical to the rennin calves make.

The product's manufacturer -- the New York-based Pfizer, Inc. -- grows the recombinant bacteria in large fermentation tanks in the same way as many pharmaceuticals and chemicals are now produced.

The product, also known as chymosin, is then purified and used in the first stages of cheese-making, where as little as an ounce can cause 50 gallons of milk to thicken into curds and whey.

In the unusually lengthy four-page explanation of its action in the Federal Register, the FDA stated that its investigation had found that "the principal active ingredient in the chymosin preparation is the same as that in rennet, and that . . . the source organism and manufacturing process will not introduce impurities into the preparation that would provide a basis for concern."

In comparison to traditional rennet, recombinant chymosin is also purer, since the process of extracting rennet from the stomachs of veal calves leaves the preparation with a significant percentage of other proteins that can't be filtered out.

"Rennet is a crude extract," said Susan Harlander, a professor of food science at the University of Minnesota. "There is all kind of protein that isn't rennin. I'm much more comfortable with the recombinant form."

Pfizer officials said they had agreed to sell the product to a number of major U.S. and foreign cheese makers, promising a lower price than the natural version, which has risen from $50 to $85 a gallon in the past year.

At least five other recombinant enzyme petitions are before the FDA -- three for different forms of rennin and two for an enzyme used as a thickening agent in food preparation -- and a number of companies are known to be working on similar bioengineered versions of natural flavors that are currently expensive and difficult to make in large quantities.

But while yesterday's decision appears to clear the way for those to be approved, it leaves unanswered how the government will react to later generations of genetically engineered food products, which are far more than simple copies of natural substances. For example, several companies have proposed introducing foreign genes into fruits and vegetables, either to improve the flavor or protect the product against pests.