YOU MIGHT HAVE seen the television commercials recently that show people singing and dancing in the streets over a fast-food breakthrough -- a new kind of hamburger. In one of them, a man wearing what appears to be a lab coat has just announced that one of the more intractable problems of our times has been solved -- the problem of how the lettuce and tomato are to be kept cold while the burger is kept hot.

The solution -- so simple that a million Americans probably slapped their foreheads and said, "But of course!" -- is a two-part container in which the hot and cold components are maintained separately until they are put together by the consumer. It's good to know someone has been working on it all this time. He should be nominated for the Nobel Fries.

Not quite everyone has gotten into the joyous spirit of the thing, however. Off to the side there this week, with an "Eat your carrots" scowl on its face, was the Center for Science in the Public Interest, which announced that while some people were dancing and singing outside a hamburger stand, others had been engaged in the dreary business of testing the fat in which the leading fast-food burger merchants did their deep frying. What they found was that most of them use "beef tallow and other saturated fats that promote heart disease."

The center said customers should be warned about this, and should be told of other ingredients in the food they are served at fast-food restaurants. It has petitioned the Food and Drug Administration to require such restaurants to list the ingredients on the wrappers of the food they sell. Among other things, this would serve as a warning to people who are allergic to certain substances, the center said.

A spokeswoman for the National Restaurant Association told us that this would be an intolerable hardship for fast-food purveyors, who would, for instance, have to use different cups for each type of soft drink, and that the end result of it would be to drive up costs to the consumer. Still, the association's suggested procedure for those who fear allergic reactions seems a bit cumbersome: call the headquarters of a hamburger chain and inquire about the ingredients before you go there to eat. We have hopes of another breakthrough. Any industry that can exercise such ingenuity in quality control over what goes on our hamburgers can surely find a way, someday, somehow, to let us know what goes into them.