a gooey, transparent liquid, and Americans are learning the hard way that today's low-fat margarines cannot be used as a substitute for butter in cooking.

"They are virtually unusable,'' said Donna Martino, an Altoona, Pennsylvania, homemaker who has ruined several meals. "I just tried to brown meat in it.

There is so much water in there, it makes the meat soggy.'' Nabisco has received about 900 complaints since January 1993 from angry bakers whose time-tested family recipes failed because of low-fat versions of Fleischmann's margarine, spokeswoman Ann Smith said.

The increase in complaints began soon after Nabisco introduced several low-fat margarines. Many consumers may not realise the differences: Nabisco alone produces margarines with seven fat percentages.

"Fat is what makes the world go round,'' said Arun Kilara, a food science professor at Penn State University. "Any food that you really enjoy has fat.

It is a flavour carrier.'' Some researchers say margarine, long considered a healthy alternative to butter, may actually double the risk of a heart attack because of a artificial fats created in hydrogenation.

For that reason, and because of taste, Kilara uses real butter.

Most people who cook often know not to use low-fat margarine in baking, but dabblers don't know the difference.

"People who don't normally bake, bake during the holiday season,'' Smith said. "When you're dealing with a family recipe that has been passed down ... when it doesn't come out right, the consumer gets angry.'' Though Fleischmann's low-fat products are marked with a small disclaimer reading "Not for use when frying or baking'', most consumers don't notice the fine print, Smith said.

Promise Extra, which contains just six grams of a fat per serving -- compared to 11 grams of fat for butter and full margarine -- has a small disclaimer on the back of the package. The letters are about one-sixteenth of an inch high.

The low-fat spreads should be labelled better or placed in a separate area from the full margarine and butter, said another Altoona homemaker, Candie Anderson.

"I never really looked at the container,'' she said. "I assumed that if it came in sticks, it would work.'' It didn't. Anderson ruined the eggs she was frying.

"The margarine just evaporated and everything stuck,'' she said. "The eggs were quite ripped up, so I ate the damaged ones and used butter to fry my husband's.'' Margarine that is at least 80 percent oil -- the type that dominated the market for years -- is fine for cooking.

"Twenty years ago, if you substituted margarine for butter in recipe, you wouldn't have seen much difference,'' Kilara said.

"To make these margarines more attractive to consumers, fat is reduced and water is added,'' Kilara said. "These spreads taste fine on muffins or toast, but if you try to put some in a pan to fry an egg, the water evaporates quickly and there is very little fat left to fry anything.'' What's left is a sticky liquid made mostly of chemicals used to stabilise the water, Kilara said.

Using a margarine that is less than 68 percent oil will create dramatic differences in frying and baking, Smith said. When Nabisco baked cookies using full margarine and low-fat margarine for a publicity campaign, she said, the low-fat cookies came out flat.

Not surprising, Kilara said.

"The very shape of some baked goods exists because of the structure of fat,'' Kilara said.

Martino has sworn off low-fat margarines and prefers to exercise moderation with butter.

"I'd rather do without than to use those spreads,'' she said. "They're a strange substance. They don't even melt when you put them on a hot piece of toast.''