CARAMELIZATION

CARAMELIZATION. Caramelization is the familiar browning of sugars through exposure to heat. The most common form of sugar—table sugar or sucrose—is a disaccharide, a combination of two monosaccharides: glucose and fructose. The two sugars can be easily separated using the enzyme invertase, which is essentially what bees do when they make honey from nectar. Fructose caramelizes more readily than glucose, so baked goods made from honey are generally a bit darker than those made with sucrose.

When sugar syrups are heated, they pass through several distinct stages, each having characteristics that are very useful to confectioners. Different sugars reach these stages at varying temperatures. The following table is for sucrose:

Caramelization of sugar begins around 310°F. When it reaches the light caramel stage (at 356°F for sucrose), many complex chemical reactions change simple sugars into a host of different flavoring compounds. Scissions (the breaking of long molecular chains into shorter segments), rearrangements of molecular components, and subsequent reactions between the resulting new compounds all occur in rapid succession. One of the compounds created during caramelization is biacetyl (C₄H₆O₂), which has a warm buttery scent, but there are also traces of as many as one hundred sweet, sour, and bitter compounds. The complexity of the resulting mixture makes the flavor of butterscotch more interesting than the mere sweetness of sugar. Of course, a number of yellow and brown water-soluble polymers are also produced, which accounts for caramel's coloration. These polymers are often used as colorants in commercial food products, from colas to soy sauce, and even in the variety of pumpernickel known as "black bread."

Many cooks assume that all the browning done in the kitchen is the result of caramelization, and it is common to see recipes that describe the "caramelization" of seared meats. However, that browning is actually the result of another set of chemical processes known, collectively, as the Maillard reaction. Maillard reactions are similar to caramelization, except that they involve the interaction of sugars and proteins—specifically, fructose, lactose, and one form of glucose with the amino acid lysine—at higher temperatures than those at which caramelization occurs. More complex carbohydrates, such as the starches found in flour, will also break down when heated into simpler sugars that can interact with the protein. That is one of the reasons that meats are often dusted with flour or cornstarch before searing. Since the Maillard reaction begins with a greater variety of chemical compounds than is required for caramelization, the resulting chemical complexity is greater. These reactions account for the wonderfully savory browning of baked breads, roasted coffee beans, and some cooked meats. If one considers the three differing flavor and aroma profiles of beef when raw, boiled, or roasted, the satisfyingly complex flavor produced by the Maillard reactions in the roasted meat is immediately apparent.

Crème caramel, dulce de leche, and similar desserts owe their flavor and color to both caramelization and the Maillard reaction. It the case of flan, the sauce for the custard is actually a thin coating of hard caramelized sugar used to line the mold before the custard is cooked—the caramel dissolves in water expressed from the cooked custard. In crème brûlée, the caramel topping remains crisp because it is browned à la minute under a broiler or small hand-held torch. Soft "caramel" candies are usually milk-based products that are merely flavored with caramel (but not brittle as true caramel would be).

Caramelization and Maillard reactions require temperatures that cannot be reached when water is present (the boiling point of water limits the cooking temperature to 212°F or less). Caramelization starts around 310°F, Maillard reactions even higher. When the sap of maple trees is boiled to make syrup, caramelization takes place even in the presence of water—because, while the average temperature is below 310°F, the temperature where the liquid is in contact with the hot metal of the evaporating pan is high enough for caramelization to occur. Similarly, the surfaces of roasted meats become dehydrated during cooking, allowing Maillard browning to take place while the interior remains moist.

These reactions (along with similar effects caused by enzymatic processes) can sometimes lead to undesirable browning. For example, when fruit preserves are prepared, the bright color of the ripe fruit must be maintained. Ascorbic or citric acids interfere with enzymatic browning, so they are typically added to low-acid fruits. Similarly, sulfur dioxide prevents the low-temperature Maillard reactions that often occur when carbohydrates and amino acids are present in high concentrations. Sultanas, or golden raisins, are merely raisins in which natural browning reactions have been prevented by sulfur dioxide.

See also Candy and Confections; Carême; Dessert; Processing of Food; Sugar and Sweeteners; Syrups.

BIBLIOGRAPHY

Davidson, Alan. The Oxford Companion to Food. Oxford: Oxford University Press, 1999.

McGee, Harold. On Food and Cooking; The Science and Lore of the Kitchen. New York: Scribners, 1984.

Richardson, Thomas, and John W. Finley, eds. Chemical Changes in Food during Processing. Westport, Conn.: AVI Pub. Co., 1985.

Waller, George R., and Milton S. Feather, eds. The Maillard Reaction in Foods and Nutrition. Washington, D.C.: American Chemical Society, 1983.

Gary Allen