LISTEN YOU crook, you have 48 hours in which to pick up this stuff and replace it."

The ultimatum is delivered in the tough, New York-accented, gravel voice of someone who not only means what she says, but gets results.

If the "crook," a vendor who has tried to palm something off on the New York school food system, doesn't do as he is told, he receives a note. The note is written by the administrator of the city's $100 million a year school food program, Elizabeth Cagan. It thanks the offending vendor for his "gift to the children of the city of New York." After that, few people are likely to try to put one over again on the silver-haired, chain-smoking, imposing former school teacher.

Cagan, who is 60, came up through the ranks of the school system. She became the chief administrator and director of school food service only two years ago. According to admirers, her first-hand knowledge of how the system functions and where the bodies are buried have helped her turn around what Institutions Magazine once described as "the worst school lunch program in the country."

Cagan says she gets things done because "I have a big mouth. I was always considered a maverick."

She has made some dramatic changes by bringing critics of the school lunch program into the department and letting them implement their ideas, by demanding fiscal responsibility, and by issuing edicts governing the quality and type of food allowed in the program.

Those who had to provide the products kept on insisting her demands couldn't be met.

Occasionally they couldn't.

"I'll never have the impact I had in the first year," Cagan said over dinner in a dimly lit Manhattan restaurant where her voice was able to drown out the piano player directly behind her. "I'm not a food person in this regard."

She didn't know, for instance that it is technically impossible to make ice cream without a certain percentage of sugar or that cookies will not hold their shape if they don't contain enough sugar. In her ignorance she had decreed that no foods which contain more than 11 percent sugar could be served.

"The cookie people came in with these outrageously shaped cookies because they wouldn't hold together. The first year we got the cookies down to 30 per cent sugar; now they're down to 20 per cent. If I hadn't insisted on 11 per cent, we would never have done what we did. The ice cream is now 24 per cent sugar."

Cagan was also told it would be impossible to reduce the amount of sugar in the milkshakes.

She insisted.

"Wouldn't you know, they got it down to 11 per cent," she smiled and pounded her fist on the table.

"If you want to come in and change specification," she explained, "you can't know it can't be done. We spend $65 million a year on food. That's too much money to give up without trying to get what we want." She also has been able to get government-donated commodities to conform to their specifications: All flour is either unbleached, enriched white or whole wheat. Fruits are canned either in light syrup or their own juice. Rice is brown instead of white.

"Liz made mistakes," says one of her admirers, Dr. Joan Gussow, who is chairman of Columbia University's program in nutrition education. "But if she made them it was on the side of God. Her intentions and directions are all in the right direction."

Not that all of the changes have filtered all the way down. Some of the schools still have "dreadful food," according to those who have to eat it.

In her first year Cagan got the Chancellor of the New York schools to put out an edict banning the sale of junk food in vending machines. It was the third ban on junk foods put out by the chancellor's office. This time Cagan met with high school students to explain her actions and let them vent their displeasure.

Now snack foods can be sold -- in a separate location from the lunch line -- but only those that meet the restrictions which have been implemented on sugar, salt and fat. Some foods, such as candy and soda pop, cannot be sold at all.

Not all of the resistance to changes came from the students: parents and school administration personnel opposed a ban, too. Cagan believes "the worst junk food has the highest profit," but the higher the profit, the more money the school has to spend on football uniforms and special trips. Cagan cannot decree how parents raise money for school activities, so she is "encouraging them" to find alternatives to cake and candy sales . . . with some success.

When Cagan decreed there would be no more sugar-coated cereals in the school breakfast program, her workers kicked up a fuss. "They said the kids wouldn't eat breakfast. So at first they were allowed to have sugar at the counter, but not on the table. Now there is no sugar."

When she banned "non-beneficial additives," such as monosodium glutamate, artificial colors and flavors, the Glutamate Association of the United States and Canada sued her. Eventually they dropped the suit.

She doesn't win all her battles. When she merely mentioned that she was thinking about banning nitrites, 'the U.S. Meat Packers descended on my office, with people from the state and federal government, mind you, and said I myself would start a recession across this country."

However, about 150 schools no longer serve products which contain nitrites and Cagan says that even in schools that use nitrites, "I limit their use."

Richard Reed, chief of school food management and nutrition for the state of New York, expresses only admiration for the way Cagan has tackled these problems. "We've all been waiting for assistance from USDA, waiting for them to come out with new meal patterns, new prohibitions about so-called non nutritional foods -- low sodium, low sugar, no additives. But not Liz. She's not waiting. She said 'it's been proven these things aren't good. Let's go.'"

USDA acknowledges, with admiration, that "she's been way ahead of us."

Right now Cagan is trying to get rid of all frozen meal-pack lunches, known outside school as TV dinners. There were 400 schools with meal packs when she came in. Now there are about 100. By next year she expects to have no more than 50 schools with meal packs, and those will be made in other schools that have kitchens, not by an outside firm. Not only, says Cagan, are the meal packs the most expensive form of school lunch, "they're the one kids discard most of."

Because she had none of the degrees most food service professionals have, Cagan hired experts to advise her. At least one of her choices raised almost as many eyebrows as her own hiring. Barbara Friedlander, who once worked for CAN, Consumer Action Now, as an advocate to improve the city's school lunch, was hired as the nutrition education coordinator for the city. She has no degrees in nutrition either, but has studied extensively in the field. Among food people she may be best known as the author of the 1972 book "Earth, Water, Fire, Air," subtitled Cookbook for the New Age.

While it was not the first vegetarian cookbook on the market, it was certainly among the first to question convenience foods, and seek a return to whole fresh foods, simply prepared. This is the bias that Friedlander brings to the foods served in schools. She has had an enormous influence on Cagan.

Nonetheless, one successful component of the New York City school lunch program is a concept they inherited: The Energy Factory. Neither Cagan nor Friedlander thought much of it at first, because it relies on the fast food concept.

Members of the union for food service personnel, worried because fewer and fewer children were eating school lunch, latched on to the idea conceived in Las Vegas, Nev. There the traditional Type A meals were supplanted by various combinations of fast foods, beefed up with vitamins but high in fat, sugar and salt. That became New York's energy Factory, which was introduced into two schools in the fall of 1977.

Cagan didn't like the Las Vegas concept and says, "it is a perfect example of where the government is being defrauded. The kids are given a Coke instead of milk on the Type A lunch."

Cagan insists that even in those schools which have the Energy Factory concept, fast foods with reduced salt, sugar and fat content, an alternative meal must be offered. Only two schools have the Energy Factory, complete with fast food service lines, but Cagan says at least 50 percent of the high schools have all the items in the Energy Factory without the gimmickry. In addition, all of the high schools have salad bars.

It's difficult to find Cagan's critics, even though she looks astonished when asked if she has any. "Constantly. It depends on the day." But most of them are burrowed within the city government. She has the greatest trouble with the school cafeteria personnel and with school principals, many of whom consider school lunch an annoyance that should be dispensed with as quickly as possible. Some high schools don't even serve lunch, the day's session is over before lunch period. Cagan is having difficulty convincing some of these principals that they ought to reinstitute a lunch program.

Reed says that without someone like Cagan, "it couldn't have been done. She is the driving force behind this whole thing. Her predecessor had knowledge and ability, but he didn't have what Liz Cagan has . . . that tremendous capacity to do. I think it's already had an impact on other jurisdictions in the state," he said.

It's already had an impact on Fairfax County, Va., where parents have told the county's food service director to find out why they can't get rid of artificial colors and flavors in the food, as New York City did.

Cagan says they can. "All you have to do is set specs. If you can't get them you have to get advocacy pressure. Then parental pressure. If you can't get anywhere with the school food service director, go above them. Then go for help . . . to groups like us."