The chemical reaction that occurs when food browns.
Cooking is all about chemistry. Well, actually it’s about biology too. Not to mention geography, economics, culture, sex, climate, politics, and a million other things. But right now let’s concentrate on the chemistry, specifically the chemistry that happens when food browns.
At temperatures between 140 and 165°C (280 to 330°F), the amino acids that are present in just about all foods combine with certain sugars in the food. The resulting compounds reflect light in such a way as to give the food a brown color. And, even more deliciously, the reaction creates the cascade of flavor compounds that we associate with everything from charred meat to roasted vegetables to crusty bread...
This cascade of chemical changes is known as the Maillard reaction, and it’s almost unbelievably complex. Hundreds of different types of molecules can be produced during the reaction, and those molecules can then interact with each other in seemingly infinite ways. Often minute differences in the types and concentrations of the various amino acids and sugars in a food can lead to huge differences in aroma and flavor.
The Maillard reaction happens more efficiently in alkali conditions. You see this most commonly with German-style pretzels...
Pretzels are dipped in an alkaline solution of either sodium bicarbonate (baking soda) or sodium hydroxide (lye) before baking. That alkali environment on the surface of the pretzel produces a more vigorous Maillard reaction than normal, causing the pretzels to obtain a dark brown exterior without burning.
For a phenomenon so fundamental to food and cooking, it’s surprising that the Maillard reaction is named for someone completely unconnected to culinary science. French physician and chemist, Louis Camille Maillard (1878-1936), was on the faculty of the University of Paris, where he studied the physiology of urology and diseases of the urinary tract. His interest in that area led him to investigate the reactions between amino acids and sugars, an area of study directly related to mechanisms of the kidney.
Maillard's research into the mechanisms and biological importance of the reaction between amino acids and certain sugars led to that class of reactions being named for him.
By the way, the Maillard reaction is not the only way foods turn brown. Caramelization also causes that color change, but by a different mechanism. In caramelization, sugars such as fructose and glucose(*) undergo a chemical reaction when heated, changing into a variety of complex polymers known as caramelans, caramelens, and caramelins. The process also releases a plethora of volatile chemicals which give caramelized foods their distinctive tastes and aromas. Various foods undergo different relative rates of Maillard reaction and caramelization when browning. In steak, with its very high protein (and therefore amino acid) concentration, the Maillard reaction predominates. When you caramelize onions to make french onion soup, both processes are going on about equally. When you sprinkle sugar on top of creme brûlée and hit it with a blow torch, that’s pure caramelization...
The browning that happens on sliced apples and bananas is caused by yet another, completely different process, one involving the enzymatic breakdown of phenol compounds into melanin related compounds. It's the same process that causes the dark brown colors in roasted coffee and cocoa beans.
If you just can’t get enough of the Maillard Reaction, you may want to join the International Maillard Reaction Society, an organization of "food scientists dealing with issues related to food browning, nutritionists interested in the physiological role of advanced glycation products and medical doctors performing basic research in the field of diabetes and its complications."
(*)Sucrose, otherwise known as regular white table sugar, breaks down into fructose and glucose when heated.-----------------------------------------