The molecules of caramelization finally identified.
SENECA MENTIONED CARAMEL as early as 65 BC, but for more than 2000 years the details of the chemical reactions that give heated sugar its inimitable flavor were unknown. Exploiting recent results in the chemistry of sugars and using modern analysis techniques, Jacques Defaye and José Manuel Garcia Fernandez at the Centre National de la Recherche Scientifique laboratory in Grenoble recently elucidated the structure and mechanisms responsible for the formation of the odorant and taste molecules that make up caramel.
Along with the Maillard reaction, which generates the aroma of roast beef, coffee beans, beers, and bread crust, caramelization is one of the principal methods for the culinary transformation of foods that contain sugars. Whereas the Maillard reaction is a reaction of sugars with amino acids or proteins, caramelization involves only sugars. It is probable that the two reactions jointly play a role in the cooking of most foods containing sugars, the share of each depending on the relative quantities of sugars and proteins.
Although caramelization has influenced the taste and appearance of dishes ever since sugars were first heated, exactly how these transformations take place remains a mystery, and an economically important one at that: In France alone the food processing industry produces 15,000 tons of caramel per year, which are used in the making of milk, cookies, syrups, alcoholic beverages, coffee, and soups.
A Scientific Tradition
The first scientific studies of caramel were done in 1838 by the French chemist Étienne Péligot. For the next twenty years caramel was consigned to purgatory, until M. A. Gélis, Charles Gerhardt, and Gerardus Johannes Mulder proposed in 1858 to divide its nonvolatile component (making up 95% of the caramelized product) into three parts: caramelan, caramelene, and caramelin. Nonetheless, these substances, obtained from successive dissolutions with alcohol and water, were no more clearly defined, chemically speaking, than the famous osmazome that Thenard and Brillat-Savarin claimed to constitute the sapid principle of meats. None of the parts extracted by precipitation is constituted by a single type of molecule.
Investigation resumed in the early twentieth century. Caramel was then believed to contain humic acids, poorly understood reducing compounds whose tanning properties are also found in lignite. The various compounds of the volatile part of caramel were also discovered, including 5-hydroxymethyl-2-furaldehyde and some twenty other compounds that contribute to its penetrating odor (including formaldehyde, acetaldehyde, methanol, ethyl lactate, and maltol).
Subsequently it was observed that caramelan reacts with alcohols. Analysis of the nonvolatile part nonetheless remained a nagging problem until 1989, when modern research methods made it possible to detect the presence of a derivative of glucose.
Water Eliminated
Sucrose is a disaccharide composed of glucose-fructose bonds. Each of these two subunits has a skeleton composed of six carbon atoms. Five of these atoms each carry a hydroxyl (–OH) group. The sixth one bears an oxygen atom attached by a double bond, with a glycosidic bond such as –CH2–O–CH2– binding the two rings. Applying the same methods of analysis they had used in studying the chemistry of sugars, the Grenoble researchers elucidated the main features of the chemical transformations of the nonvolatile part of caramel. Among other things they observed the formation of fructose dianhydrides, in which two fructose rings are connected by two –CH2–O– bonds, which in turn define a third ring lying between them. Several molecules correspond to this description because sugars come in many isomeric forms, which is to say that molecules having the same atoms can differ if the atoms are linked by different bonds.
Finally, the Grenoble chemists showed that during the caramelization of sucrose, for example, the nonvolatile part results from an initial reaction dissociating the sucrose into glucose and fructose. These elementary sugars then recombine, forming oligosaccharides having various numbers of elementary sugars: The glucose may combine with glucose or fructose, the fructose may react with fructose, and so on.
These recent results are commercially important, for they make it possible to consider polydextroses—used to give texture to dishes in which sugar is replaced by intense sweeteners—as naturally occurring compounds. Because polydextroses are naturally present in caramel, they are not subject to the same system of regulation as other synthetic molecules. Moreover, the tendency of various glucides to caramelize can now be investigated more easily.