Kitchen mysteries

The following three excerpts come from Kitchen Mysteries: Revealing the Science of Cooking by Hervé This. Hervé This is a physical chemist from Suresnes, France, who works at the Institut National de la Recherche Agronomique. He is the cofounder of molecular gastronomy, the application of science to the art of cooking.Question: Why does bread crust have more flavor than the crumb?Why does bread crust have more flavor than the crumb? Why must meats be seared in butter when preparing a stock for a sauce espagnole, for example? Why must a leg of lamb be rubbed with oil before it is put in the oven? Why is some beer golden? Why do roasted coffee and chocolate smell so good?There are countless questions of this kind in cooking, but the answer to many would be, in short, "Maillard reactions." Indeed, it is these chemical reactions that create brown, odorant, and sapid compounds in cooking.Universal as these famous, oft-mentioned Maillard reactions are, they are still not well known. Nevertheless, the principle is a simple one: as soon as molecules containing a chemical amine group NH₂ are heated in the presence of some specific but common sugar such as glucose, a water molecule is eliminated and the two reagents are bonded in what is called a Schiff base. Let us not linger over this compound, since it is more or less rapidly replaced by an Amadori product, which will react with other compounds to form cyclical "aromatic" molecules.

As their name indicates, these aromatic rings give odorant properties to the compounds that contain them; some of them also have a strong color.Whole books of scientific articles on Maillard reactions are published with some regularity, and in 1990 a famous chemistry journal devoted a comprehensive article more than twenty pages long to this topic, describing many of the odors created. Nevertheless, the products of Maillard reactions are countless and still not sufficiently well known. The golden brown that cooks look for when sautéing in fats is produced by many reactions, but Maillard reactions are preeminent. The reactions take place at the high temperatures attained by the fat, whereas they do not occur when foods are boiled, since the temperature is limited to the boiling point of water. How to improve our cooking now that we know the power of Maillard reactions? By using them! As we cook, let us watch for sugar-protein combinations. Let us think of Peking duck. Let us heat foods quickly at first, so that Maillard reactions can take place, then more cautiously, in such a way that the actual cooking takes place without eliminating volatile compounds. Do we want to cook meat in a microwave? Let us not forget to sear the meat before cooking it, and let us oil or butter the surfaces so that the heat is transmitted to them efficiently.Question: How do we avoid the discoloration of green vegetables when cooking them?The intense green that vegetables acquire after cooking for a few seconds in boiling water results from the release of gases trapped in the spaces between the vegetable cells.

Generally, these pockets of air act as magnifying glasses that highlight the color of the chloroplasts.Vegetables, however, are usually cooked longer than a few seconds, thus destroying the atmosphere that shows these vegetables in their best light. Spinach cooked too long turns brown, sorrel as well; leeks lose their greenness, and so on. How to retain that appetizing color?The cooks of antiquity were the first to make advances toward explaining this phenomenon. They observed that green vegetables remained very green when saltpeter or ashes were added to cooking water. Why?When a green vegetable is heated, some of its cells burst, releasing various organic acids. The hydrogen ions of these acids react with chlorophyll molecules because these molecules contain a large square chemical pattern, the porphyrin group, at the center of which is a magnesium atom. Now, the hydrogen ions have a bad habit of taking the place of the magnesium ion in this porphyrin group and of thus transforming the various kinds of chlorophylls into pheophytins, which absorb different components of light. Instead of retaining all the light rays except those of the color green, pheophytins reflect a mixture of wavelengths that produce the perception of a horrible brown.But from this analysis emerges a solution: not heating the vegetables for too long, so that the magnesium will remain in its chlorophyll cage.A few corollaries are equally essential. To retain the color of green vegetables, avoid lidded earthenware pots and opt for steaming, because if they are not immersed in water, the vegetables are not in contact with the hydrogen ions. If vegetables are cooked in water, large quantities of water should be used. Finally, adding vinegar to the cooking water for green vegetables should be absolutely avoided, as it will enhance the bad effects you wish to avoid. Be aware, too, that many juices from fruits are very acidic (and that the acidity one perceives can be hidden by sugars).Naturally, inventive cooks have thought of cooking green vegetables in the presence of salts, which provide ions that can occupy the positions hydrogen ions would otherwise take. That is why green vegetables were cooked in copper pans, called "regreening pans," and why, later in history, copper salts were used; with these methods, the green remained intense . . . but the vegetables became toxic. Indeed, a law prohibited the practice of adding copper salts in 1902. More recently, processes using zinc ions have been patented.Adding a base to the cooking water in order to neutralize the acids as they form has also been considered. This solution was already familiar to the Romans. Apicius, famous for his gastronomical extravagances, wrote, "Omne holus smarugdinum fit, si cum nitro coquantur" (All vegetables will be the color of emerald if they are cooked with niter). The same effect occurs with ashes, where potash is present. Alas, niter, or saltpeter, and potash ruin the taste.Question: Why does old flour makes good bread?Bakers know that flour that has been stored for a month or two makes better bread than fresh flour. Why?We have just seen how kneading unwinds and aligns the proteins and how the proteins remain linked by hydrogen bonds and disulfide bridges that ensure the formation of intramolecular loops that serve as springs and give the dough its elasticity.Disulfide bridges are bonds that are established just as easily between sulfur atoms in the same protein as between sulfur atoms in two neighboring proteins. Their reestablishment, after an extension, is compromised by the presence of thiol groups on the neighboring proteins: the bond is established with the neighboring protein and not within the intramolecular groups; the loops do not reform after being stretched.

When a disulfide bridge is stretched with a thiol group in the proximity, there is a danger of a hydrogen atom passing over to one of the initially bonded proteins. The dough is more fluid than elastic. Aging the flour, which is accompanied by an oxidation of the thiol groups, gives dough better elasticity because the disulfide bridges reform better.Let us note however that water can also give up hydrogen atoms to sulfur atoms when the dough is kneaded too much. But this danger is only a problem with mechanical kneading equipment. Bread makers usually get worn out kneading by hand well before this threat surfaces. And the actual practice of kneading? Begin by placing the dough on the far side of the kneading surface; unstick the dough from the surface and form a ball that you then bring toward yourself by folding it, trapping air in the process. Pound the folded dough firmly and repeat, flouring from time to time.Excertped from Kitchen Mysteries: Revealing the Science of Cooking by Hervé This and translated by Jody Gladding. Translation Copyright © 2007 Columbia University Press; Copyright © 1993 Editions Belin. Used by arrangement with Columbia University Press. All rights reserved.Links within this article:H. This, Kitchen Mysteries: Revealing the Science of Cooking, Columbia University Press, 2007. http://www.columbia.edu/K. Steinriede, "Food, with a side of science," The Scientist, April 2007. http://www.the-scientist.com/news/display/53169/Institut National de la Recherche Agronomique http://www.inra.fr/K. Thomas, "From chemist to chef," The Scientist, Dec. 2006. http://www.the-scientist.com/article/display/36661/