The Maillard reaction (pronounced "my-YARD") is a culinary phenomenon that occurs when proteins in food are heated to temperatures of 310 F or higher, causing them to turn brown.
Named for the French chemist Louis-Camille Maillard who discovered the process at the start of the 20th century, the Maillard reaction is similar to the process of caramelization, where sugars turn brown when heated.
But color is not the only thing changing. Both the Maillard reaction and caramelization produce new flavors and aromas as well, which is the main reason we care about them in a culinary sense.
But cooking is not the only thing that causes foods to turn brown.
Enzymatic browning is a process that causes some foods to turn brown all by themselves (or more accurately, when exposed to oxygen). Examples are when bananas develop brown spots on their skins or when grapes turn brown when they become raisins.
And you can taste the difference. A ripe banana isn't just sweeter than an unripe one, it has a deeper, more complex flavor as well. And a raisin has a deep, rich, smoky flavor that wasn't present in the grape.
The enzymes that cause this browning are naturally present in the foods, so it's literally a case of the food turning itself brown. It's a biological process.
And remember, it's the presence of oxygen that triggers the process. Which is why apples turn brown soon after you cut them.
These enzymes can be deactivated by acids, which is why tossing cut up apples in lemon juice helps prevent them from browning. Heat can also denature the enzymes, which is why briefly blanching an avocado helps prevent it from turning brown when you cut it.
Non-enzymatic browning is another way foods can turn brown. But instead of enzymes in the food, it's an outside force, namely heat, causing it to turn brown.
Heat causes chemical reactions in food, triggering the creation of new chemical compounds, many of which have distinct flavors and aromas. And there isn't just one reaction taking place, but hundreds, each of them producing new compounds.
And these new compounds are themselves heated, thereby producing new chemical reactions, new flavor compounds, and so on, in a cascade of flavor and aroma creation. All these flavors and aromas combine to produce a highly complex experience for your nose and taste buds, none of which would have been possible in the raw form of the food.
Proteins Vs. Carbohydrates
So the Maillard reaction takes place when the amino acids in proteins are heated. Caramelization happens when carbohydrates (i.e. sugars) are heated. And even though the two are separate processes, they do sometimes produce the same flavor compounds, and the same color changes.
Foods like carrots, mushrooms and onions contain very little protein, but they do turn brown when cooked. Which means that mostly they're undergoing caramelization, not the Maillard reaction.
Meat, on the other hand, only has protein, plus fat and water, but no carbs, so it only undergoes the Maillard reaction, not caramelization.
And foods like wheat contain quite a bit of protein, as well as carbs, so when bread is toasted, it's undergoing both the Maillard reaction and caramelization.
Potatoes are low in protein, but french fries turn nice and brown when they're cooked because the Maillard reaction increases as the temperature increases, and deep frying is an incredibly effective method for transferring very high temperatures to the surface of a piece of food.
For Best Results, Pat Your Meat Dry
Another effect of cooking meat is that its surface dehydrates and develops a crispy texture. This, combined with the browning effect of the Maillard reaction, is what produces the characteristic brown "crust" on a seared steak. But that crusty texture itself is not a function of the Maillard reaction, but simply the result of the hot pan or oven drying out the surface of the meat through evaporation.
However, the two effects go hand in hand. Because as we said earlier, the higher the temperature, the more browning takes place. If a piece of meat is wet, it's going to produce steam when it hits the pan. And steam maxes out at 212 F, which is too low to trigger the Maillard reaction.
Moreover, the steam will have the effect of cooling off the pan, so that a pan which might otherwise be heated to 400 F is subjected to a gust of 212 F steam, which effectively cools it to, say, 300 F. This significantly hampers the Maillard reaction, at least until the steam is gone and the pan has a chance to heat back up.
All of which is to say that to maximize the Maillard reaction, it's a good idea to thoroughly pat your meat dry before cooking it.
What to Do When Braising
Note, too, that the Maillard reaction will only take place using dry heat cooking methods like grilling, sauteeing and roasting. Braising is a wonderful way to cook certain cuts of meat, but it won't brown the meat, since it's a moist heat method that works by immersing the meat in a liquid that's heated to, ideally, around 205 F. This is the temperature at which connective tissue starts to break down, but still low enough so that it doesn't boil the meat, which would cause it to toughen.
Obviously, 205 F is not hot enough to trigger the Maillard reaction. But because the Maillard reaction is so essential to producing the rich, complex "cooked meat" flavors we're so accustomed to, it's common practice to sear a piece of meat in a skillet before braising it, so those complex flavors have a chance to develop.
Some cooks choose to sear the meat after braising it, but this prevents those complex flavors from infusing the liquid during the braising process.