PHOTO BY JOSIE DALTON ESPLIN

Margarine wouldn't exist without butter, of course. From its inception, margarine was an attempt to mimic the taste and texture of butter. In the 1860s, Emperor Napoleon III offered a prize to the person who could produce an edible fat substitute for butter. Hippolyte Mege-Mouries, a French chemist, created oleomargarine, a combination of clarified beef fat, water, and a bit of tributyrin--a milk fat--to give it a buttery taste.

Mege-Mouries called it oleomargarine after the fatty acid then called margaric acid. It turns out that margaric acid (named after the Greek word for pearl--margarites--for its pearl-like luster) is actually a combination of stearic and palmitic acids, fatty acids often found in animal fats.

It would be hard to find beef fat or other solid animal fats in today's margarines. Soybean oil is the most common base in U.S. margarines; manufacturers also use sunflower, corn, palm, safflower, cottonseed, peanut, and canola oils. But they can do so only because about a century ago, French and German chemists figured out how to convert liquid vegetable oils into a semisolid product.

Butter owes its semisolid state to the saturation of its fatty acids. Fatty acids are the building blocks of fats and oils. A fatty acid molecule has a long, unsubstituted carbon chain with a carboxylic acid group at one end. Many fatty acid chains in butter (palmitic, stearic, and myristic, for example) are saturated with hydrogens; they contain no double bonds. These unvarying chains pack together nicely, and it takes more heat than is available at room temperature to get them to loosen to a liquid.

Fatty acids in oils, on the other hand, are mostly monounsaturated or polyunsaturated. Oleic and linoleic acids, for example, have one or more double (olefin) bonds along the fatty acid chain. Because natural double bonds usually assume the cis configuration, where the olefin hydrogens are located on the same side of the chain, these fatty acids contain a bend of about 30°. Fatty acids with cis double bonds don't pack well and are liquid at room temperature.

French and German chemists learned how to solidify liquid oil through hydrogenation. They heated the oil in the presence of a metal catalyst and pressurized hydrogen gas, causing hydrogen atoms to add randomly to the double bonds. Complete hydrogenation saturates all double bonds. Partial hydrogenation saturates some and converts others from cis to trans. In a trans double bond, the hydrogens are kitty-corner, which allows for compact packing of the fatty acid chains. The more complete the hydrogenation, the firmer the oil becomes--and the longer the shelf life. Oils turn rancid as double bonds oxidize in air.

Over the past century, margarine makers have favored partially hydrogenated oils--ones that remain soft enough for spreading on toast. To that semisolid fat base, they add a butter flavoring (buttermilk, whey, or a nondairy substitute), salt, emulsifiers (such as soy lecithin), a preservative like sodium benzoate or citric acid, some vitamin A to give it the nutritional value of butter, and a dash of yellow color (often ß-carotene).

In margarine's early days, its main selling point was its low cost. A strong dairy lobby, however, supported the Margarine Act passed by Congress in 1886, which added a two-cent tax to margarine and required expensive licenses to make or sell it. Most states also passed laws forbidding margarine manufacturers to add yellow coloring to the naturally pale spread. Not until 1967 did the last state, Wisconsin, repeal its coloring ban. By law, regular stick margarine must be 80% fat and 20% water or milk (with added vitamin A). Reduced-fat margarines and spreads, however, can range from 60% fat all the way to fat-free.

Margarine got a boost in the 1970s when studies came out indicating that dietary cholesterol and saturated fat raise LDL (low-density lipoprotein) cholesterol levels in the blood and increase the incidence of heart disease. Made with vegetable oils, margarine has no cholesterol and less saturated fat than butter. Naturally, margarine advertisers began to trumpet the health benefits of eating margarine.

However, further research on the trans fatty acids found in partially hydrogenated oils took many by surprise (C&EN, Sept. 22, 2003, page 33). Trans fatty acids, though unsaturated, were found to also increase LDL ("bad" cholesterol) levels in the blood.

In the U.S., the Food & Drug Administration will require labeling of trans fats on all products by 2006. In anticipation of the U.S. regulation, many margarine makers have found alternative fats and oils or alternative forms of hydrogenation that minimize the creation of trans fatty acids.

Grocery store shelves now hold a dizzying variety of margarines: Soft or hard. Made from safflower or soybean oil. Partially hydrogenated, saturated, or polyunsaturated. Made with yogurt. Made with olive oil. Some are even mixtures of butter and margarine.

Today, nutritionists say to go easy on both butter (high in saturated fat) and margarine (high in trans fatty acids). The American Heart Association still recommends margarine over butter but also advises consumers to choose the soft liquid or tub varieties. Look at the ingredients and nutrition facts, AHA says, and buy the variety with no more than 2 g of saturated fat per tablespoon and with liquid vegetable oil (not hydrogenated) as the first ingredient.