Why is food that is browned tastier than the same food cooked to the same temperature via steaming, boiling or microwaving?
Thank the Maillard reaction (which occurs when the surface temperature of food is more than 300 degrees Fahrenheit) and caramelization (which kicks in at about 320 F). These chemical reactions not only bring about a color change, they also produce hundreds of flavor compounds that create the rich, savory notes and appetizing aromas we associate with roasted, grilled and seared dishes – notes and aromas that are noticeably absent from foods cooked by wet methods like steaming, boiling and poaching.
For a food to reach such high surface temperatures, it must be fairly dry on the outside. When in direct contact with a hot skillet or surrounded by the scorching air of a 400 F oven, the water molecules on the surface of foods quickly vaporize, resulting in the familiar sizzling sound of searing or roasting foods. Once all the moisture has evaporated, the dehydrated exterior of the food becomes hot enough to trigger the Maillard reaction in just a few minutes.
By contrast, foods cooked by wet methods are only able to reach 212 F, the boiling point of water. (The Maillard reaction can take place at lower temperatures, but it takes much, much longer, such as when meat broth develops a dark color and rich flavors after hours of simmering.)
So steak grilled to medium rare (135 F) will taste, and look, quite different from a steak poached to the same internal temperature, since the exterior of the former will have reached a much higher temperature.
So what’s the difference between the Maillard reaction and caramelization?
The Maillard reaction takes place only when both protein and carbohydrates or sugars are present. Cooked at high heat, protein breaks down into its building blocks, amino acids, which then react with a group of sugars known as simple sugars. (Ribose, which is found in beef, pork, salmon, chicken and many types of mushrooms, is one example of a simple sugar.) The reacting amino acids and sugars rearrange themselves to form ring-type structures that reflect light in such a way that the food takes on a brown hue. These transformations also produce a plethora of volatile flavor and aroma compounds, which combine with one another to produce even more nuanced tastes and smells. Ranging from malty, grassy and chocolaty to oniony, meaty and earthy, they depend on the exact composition of particular foods.
Unlike the Maillard reaction, caramelization is exclusively the breakdown of sugar molecules under high heat. Sugars such as glucose and sucrose unravel, and a wealth of sweet, bitter and nutty flavor molecules are formed.
The ratio of the Maillard reaction to caramelization that transpires on a piece of food depends on its protein and carbohydrate content. Carrots, for example, have lots of carbohydrates, so roasting these root vegetables will result in considerably more caramelization than Maillard compounds. Seared steak, on the other hand, will have been browned and enriched mostly from the Maillard reaction.
Not all dry cooking methods produce browned and ultra-flavorful foods. Microwaves generate electromagnetic waves, causing water molecules within foods to vibrate, which in turn produces enough heat to cook muscle or vegetable tissue. Unlike the air in conventional ovens, the air in microwaves is not heated, so it can’t brown the surface of most foods. (Sliced bacon and very thin slices of vegetables and fruit are exceptions: Since they dehydrate easily while cooking, they are able to reach the high temperatures needed for the Maillard reaction and even caramelization.)