Where's the beef? In a meaty peptide.

Meat-and-potatoes eaters may one day make tofu more appetizing for themselves by sprinkling it with the taste of steak.

As part of a long-term effort to learn why cooked meat soon develops a disappointing "warned-over" flavor, physiologists have pinpointed a short peptide as the chemical responsible for beef's taste.

The peptide exists as a string of eight amino acids, and its probably forms as meat ages and enzymes within the meat break down specific proteins, U.S. Department of Agiruculture researchers reported this week at the spring meeting of the American Chemical Society in San Francisco.

This meaty peptide might one day serve as a subtitute for monosodium glutamate or make less expensive meat taste like prime cuts, says Arthur M. Spanier of the USDA's Southern Regional Research Center in New Orleans. It could also improve the flavor of precooked beef used in institutional and convenience foods, which account for 35 percent of the beef sold in the United States.

For several years Spanier, Peter B. Johnstein and their USDA colleagues have sought to get rid of the two processes that cause beef to develop its warmed-over flavor.

In one process, iron in the meat stimulates the breakdown during storage of fatty substances called lipids. The lipids combine with oxygen and become rancid, says Johnsen. Another process causes a decrease in the meaty, brothy taste of beef.

Two years ago, the USDA group succeeded in eliminating the rancid flavor by adding a derivative of a natural polymer called chitin to meat. The chitin binds to iron and slows the oxidation of fat (SN: 9/16/89, p.189). Over time, however, "there's still a decline in the desirable flavor," says Johnsen.

The USDA researchers now know that enzymes remain active as beef makes its way from slaughterhouse to oven to refrigerator and finally to the plate as leftovers. They isolated the beef-flavor peptide by analyzing the contents of pieces of top-round steak at these various stages.

Japanese researchers first discovered the peptide in 1978 among the residue of peptides and amino acids produced by "digesting" meat with a vegetable enzyme. But the USDA group is the first to prove that this peptide forms naturally in beef, Johnsen says.

"This is very significant," says Chi-Tang Ho, a food chemist at Rutgers University in New Brunswick, N.J.

By demonstrating that the peptide occurs naturally in substances generally eaten by people, the USDA researchers could make it easier for this substance to gain U.S. Food and Drug Administration approval as a food additive. However, Ho and Johnsen say it remains unclear how fast -- or even whether -- the new flavoring will make it to the dinner table.

The peptide seems to contribute to "umami," the little-known fifth taste sensation that some scientists think is perceived -- along with salt, sweet, bitter and sour -- by specific receptors in the mouth. The beef-taste peptide may evoke its savoriness by interacting with several types of these receptors. Japanese scientists have shown that small sections of the peptide do stimulate the four other tastes. In addition, enzymes may break up the peptide during storage so that its fragments cause sour or bitter sensations rather than beefy ones.

The USDA group plans to look for similar compounds in pork and poultry and to try to identify the enzymes and proteins involved in the formation and breakdown of the beef peptide.