How cool it must have been to be present—a fly on the cave wall—at the precise moment humans first applied fire to food and the realization that the transformations due to heat made their hunted-and-gathered fare taste better. Millennia of food science and cooking technology sought to understand and refine those efforts, but it wasn’t until Louis-Camille Maillard and his discovery just a century ago that we fully began to understand the delicious mechanism behind the color and flavor of cooked food.
In order to understand Maillard reactions, it’s best to compare its effects with that of a commonly used (though often incorrect) term to describe browning in cooked foods. Though we usually refer to browning and flavor as the result of caramelization, that term is actually quite narrowly defined as the pyrolysis, or decomposition, of sugar. (Lucky for us, the amber color and flavor of decomposing sugar look and taste good! At temperatures exceeding 150°C/300°F sucrose undergoes an irreversible, simultaneous change of chemical composition and physical phase. Caramelization remains an active area of study as it is a complex and poorly understood process that produces hundreds of chemical products.)
In contrast, Maillard reactions produce browning and chemical compounds under a different set of circumstances. Much of what we refer to as “caramelized” is actually “Maillard-ized”—browned onions, the browning of seared meats, or the golden crust of a freshly baked loaf of bread. Maillard reactions also play important roles in the production of chocolate, coffee, maple syrup, and the malting process used in beer.
The reaction was named after French chemist Louis-Camille Maillard (1878–1936) following research he conducted in 1912 and published in the Comptes Rendus, in an attempt to figure out how amino acids linked up to form proteins. He discovered that when he heated sugars and amino acids together, the mixture slowly turned brown. But it was not until the 1940s that people noticed a connection between the browning reaction and flavor. World War II soldiers were complaining about their powdered eggs turning brown and developing unappealing flavors. After many studies done in laboratories, scientists figured out that the unappetizing tastes were coming from the browning reaction. Even though the eggs were stored at room temperature, the concentration of amino acids and sugars in the dehydrated mix was high enough to produce a reaction.
Most of the research done in the 1940s and 1950s centered on preventing this reaction within the context of the growing processed food industry. Eventually, however, scientists discovered the role the Maillard reaction plays in creating flavors and aromas. A landmark paper in 1953 by John E. Hodge—a chemist working at the USDA field office in Peoria, Illinois—further fleshed out Maillard’s earlier work and led to the elegant “Hodge Scheme.” His “Chemistry of Browning Reactions in Model Systems” stands as the most cited article in the Journal of Agricultural and Food Chemistry to this day.(Unfortunately, Hodge’s paper is not readily available in full; here is a recollection: Hodge, Citation Classic 1979.] There remains much to learn, and the study of Maillard reactions has broad interest in fields far beyond cooking.)
The Maillard reaction is, to put it simply, a series of reactions between an amino acid (the building block of protein) and a sugar. This phenomenon is responsible for brown colors and countless aromas and flavors in cooked foods. The basic necessities for Maillard reactions are:
— Reducing sugar (such as glucose, fructose, lactose, maltose, or ribose)
— Increase of temperature
— Removal of water
— And though not necessary, an increase of pH also promotes Maillard reactions
A great example of the similarities and differences between caramelization and Maillard can be seen by comparing a conventional caramel sauce with dulce de leche. With the former, one cooks sucrose beyond the threshold of caramelization and finishes with dairy, such as cream and butter. The latter can easily be made by boiling a can of condensed milk for a few hours. The presence of milk proteins from the beginning of the process and the lower temperature of the dulce de leche demonstrate the difference, as does the flavor of each when tasted side-by-side. Similarly, a typical soft caramel could be prepared using both approaches—either caramelizing the sucrose first, or by combining the sucrose with dairy at the outset—in both cases, color and flavor from Maillard reactions will begin to occur just before the typical final cooking temperatures, in the neighborhood of 118°C/245°F.
It is true that browning can be the result of both caramelization and Maillard. It is also important to note that sucrose is not a reducing sugar, and thus will not directly promote Maillard reactions. However, in special cases where some of the sucrose is inverted during the cooking process (cleaving its glucose and fructose molecules), Maillard reactions can occur without the addition of other reducing sugars.
With protein, sugar, and heat (and the removal of water due to evaporation), we pass a miraculous threshold of flavor. Removing water is necessary to allow us to exceed temperatures above 100°C/212°F. The slow rate of Maillard reactions below 100°C explains why we are compelled to pre-sear (or post-sear) meats when they are cooked sous vide.] If we are patient, however, we may get Maillard products at far lower temperatures—even in aged cheeses or Champagne, which can develop interesting “toasty” flavor profiles. We already know that the fermentation of cocoa beans greatly affects the flavor of chocolate—an interesting possibility is that fermentation could increase the types of amine-containing molecules available to participate in Maillard reactions.
￼￼￼THE FOLLOWING FIVE EXAMPLES OF MAILLARD REACTIONS ARE OFTEN MISTAKEN FOR CARAMELIZATION:
“Caramelized” Onions: While onions do contain some naturally occurring sucrose that contributes to the golden color and sweet flavor of browned onions, the browning is due primarily to the heating of proteins and simple reducing sugars like glucose and fructose (remember, sucrose does not promote Maillard reactions). As the sliced onions are heated, moisture at the surface begins to evaporate. As temperatures climb well above the boiling point and more water evaporates, brown colors develop and hundreds of different flavor compounds are created. These compounds, in turn, break down to form yet more new flavor compounds, and so on. Each type of food has a very distinctive set of flavor compounds determined by the quantity and proportion of different amino acids. In the case of the onions, the browning reactions can also be promoted by the addition of a small amount of baking soda, which will raise the pH.
Seared Meat: While it may seem obvious that meat contains protein, it may not seem so obvious that sugars are present in meat as well. Ribose, for instance, is a simple sugar found in beef, pork, salmon, and chicken (as well as some mushrooms). When a piece of meat is set into a hot pan, the immediate sizzle we hear is the sound of water leaving the surface of the meat and evaporating on the surface of the hot pan. The combination of protein, sugar, heat, and removal of water all contribute to the flavorful brown crust that results. If the same piece of meat was dropped into boiling water, it would not achieve the same browning and complex flavor, because the temperature of the boiling water (100°C/212°F) is not hot enough to trigger the Maillard reactions, not to mention that there would be no loss of water.
Browned Crust of Bread: As a loaf of bread bakes in a hot oven, complex reactions are taking place; the gas produced by the yeast expands and the proteins and starches set and gelatinize to form the final structure of the loaf. On the exterior surface of the loaf, proteins and sugars (during the fermentation process some the flour’s starch converts to maltose) react with the intense heat of the oven (usually well above 204°C/400°F); and with rapid evaporation of water, the crust develops and browns. This browning and complex flavor does not occur within the center of the loaf because the interior temperature may only reach 93°C/200°—too low for Maillard reactions—and because desirable moisture is trapped within the loaf.
Brown Butter: As discussed below, brown butter is prepared by heating butter and gently cooking until the solids begin to brown. This browning occurs after all of the water (16 percent of the butter) has evaporated and the temperature increases; and of course because the remaining nonfat milk solids are made up of proteins and lactose. This immediately demonstrates to the pastry chef how dairy products added to almost any preparation may, under the right circumstances, produce complex Maillard browning and flavor reactions in baked goods (like the classic financier) and confectionery items (soft caramels and more). The dulce de leche described above is another example of how milk solids (which comprise roughy 23 percent of sweetened condensed milk) help create flavor.
Pretzels: The characteristic flavor and brown color of a pretzel is similar to that of the loaf of bread described above, but with an additional treatment with a highly alkaline solution. Before baking, pretzels are dipped into, brushed, or sprayed with a mixture of water and an alkaline—either weak, like baking soda (sodium bicarbonate), or very strong, like lye (sodium hydroxide). This rise in pH greatly exaggerates the browning reactions.
It was my foray into dairy and ice cream science that helped me to connect the Maillard dots in my everyday cooking. Another significant a-ha moment came soon after with the discovery of what a few chefs were doing with white chocolate—counter-intuitively “roasting” it to add much-needed character. Such thinking shows how a better understanding of composition and function lead us in creative directions. My frustration in perfecting a brown butter ice cream a decade ago was another exercise with inventive results.
The original formula I had been using for the brown butter ice cream, nicked from a Michelin-starred chef, was delicious, but fickle due to its high fat content—often resulting in a coarse, grainy texture. A few ill-informed attempts to adjust the recipe still didn’t yield favorable results. However, once armed with the data and the knowledge of how to balance the water, solids, and fat of an ice cream, it seemed logical to focus on the browned butter solids that make the flavor of the ice cream so special.
Good quality butter will have a fat content of at least 82 percent by weight; water accounts for most of the remainder, about 16 percent of the total weight of the butter. Nearly all of that water cooks off or evaporates in the process of browning butter. So really, the most important constituent of butter is what’s left: the 2 percent made up of nonfat milk solids. When clarifying butter, these proteins and sugars are the foamy scum that we carefully skim off; for brown butter, we allow those solids to brown. As the water cooks off, we’re left with just the fat and the solids. Once that water has evaporated, the fat can finally exceed that temperature barrier of water’s boiling point where the Maillard magic happens.
We can make our own butter by over-whipping cream to the point where the fat molecules jam themselves together, squeezing out a good deal of water. And in that water—the original buttermilk—remains the majority of its milk solids. So if we were to compare equal measurements of butter and cream, we’d discover that the cream contains 6 percent milk solids by weight, or three times more milk solids than butter. It’s the same process as for the butter, but as one would imagine, it just takes longer. As more and more water evaporates, the natural emulsion of the cream breaks up, causing the non-fat solids to separate from the fat. Whereas the tiny brown particles from butter are difficult to extract, these non-fat solids tend to clump together. When carefully strained we have a sort of brown butter “powder” that can be added to just about anything. There remains some amount of fat among the solids, but little enough to easily adjust for in a recipe. There is, however, a very simple alternative solution to increase the non-fat solids, one that I initially overlooked: one can simply add non-fat milk powder. This eliminates the necessity to stand over a pot of reducing cream or butter, and also greatly boosts the yield of the non-fat solids.
Here’s a great paper detailing Maillard reactions in dairy products: Dairy Maillard, Food & Function 2012
To see how Maillard works in its most simplistic form, we can try an experiment found in one of my favorite books, Harold McGee’s The Curious Cook. By heating some corn syrup (glucose) and amino acid supplements (such as lysine, cysteine, or glutamine) we should be able to see the Maillard browning and perhaps even isolate some of the specific aromas that might result from different amino acids. And since we mentioned caramelization, you can also read Harold’s excellent post on the subject.
Below, a few of my favorite applications highlighting Maillard reactions:
Big thanks to NYU Chemistry Professor Kent Kirshenbaum, my partner in two Maillard-related presentations for the World Science Festival
The Curious Cook: More Kitchen Science and Lore, by Harold McGee 1992
Catching Fire: How Cooking Made Us Human, by Richard Wrangham 2009