An Introduction to the Maillard Reaction: The Science of Browning, Aroma, and Flavor

[Photograph: J. Kenji López-Alt]

If you're a regular reader of Serious Eats, you've definitely seen us refer to the Maillard reaction time and again. That's because the Maillard reaction is responsible for the browned, complex flavors that make bread taste toasty and malty, burgers taste charred, and coffee taste dark and robust. If you plan on cooking tonight, chances are you'll be using the Maillard reaction to transform your raw ingredients into a better sensory experience.

But the Maillard reaction doesn't just make food taste delicious. Understanding the reaction, even on a surface level (that's a Maillard pun, and you'll totally get it soon), is a gateway to understanding the chemical and physical processes of cooking. Grasping the variables involved and learning how to manipulate them is one of the best ways to become a more confident cook—it's the difference between being a slave to a recipe and being free to make a recipe work for you.

The good news is that the Maillard reaction is everywhere, which means plenty of chances to practice and learn. We use it so often that it's easy to forget it's there, but when it's missing, you'll certainly notice. Imagine a steak that tastes boiled instead of roasted, or a stir-fry that tastes more like a stir-steam. Each one represents a missed opportunity to exploit the Maillard reaction's potential.

So, What Is the Maillard Reaction?

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[Photograph: J. Kenji López-Alt]

The Maillard reaction is complex. So complex, in fact, that it's only in the last few years that scientists have begun to figure out what it actually is. While they still don't entirely understand it, they do know the basics: The Maillard reaction is many small, simultaneous chemical reactions that occur when proteins and sugars in and on your food are transformed by heat, producing new flavors, aromas, and colors.

Practically speaking, the Maillard reaction makes food more enticing to us humans, encouraging us to dig into a steak, drink a coffee, or chug a beer.* Unlike all the other omnivores prowling this earth, we no longer tend to find a hunk of raw cow shoulder particularly appetizing. But if that same muscle is ground up, formed into patties, and seared on a flattop, we'll eagerly line up around the block. In large part, that's because we have evolved to respond to two important signals when encountering food. The first is a "nutrition" signal that tells us the food will deliver a hefty dose of easily digestible calories, vitamins, and minerals. The second is a "general harmlessness" signal that tells us the food won't kill us. The Maillard reaction is evolution's way of combining these two signals into one super-signal, specific to the roasty or browned flavors of cooked food.

* Yes, even beer undergoes the Maillard reaction—when the grains are roasted prior to brewing.

All that cooking we've come to seek out is, at its heart, the process of applying heat to food over a period of time. If all goes well, it also makes you hungry. A burger, to extend our example, is composed of a basic set of building blocks: proteins, sugars, and water. The Maillard reaction is what can happen to those proteins and sugars when heat and time are added to the equation.

Long story short: With the right amount of heat, moisture, and time, those specific sugars and proteins will act like a couple of lust-drunk lovers making out in the back of a Chevy, rapidly becoming a tangled, hot mess, until, nine months later, a whole new creation emerges. Except that with the proteins and sugars, it takes minutes, not months, and instead of a child, the result is an increasingly complex array of flavor and aroma molecules, along with a darker color courtesy of newly formed edible pigment molecules called melanoidins.

Heat, Moisture, and Time

[Photograph: J. Kenji López-Alt]

The first thing you need for the Maillard reaction to take place is heat. A steak left to sit on the counter for a week at room temperature will certainly undergo some chemical changes, but the Maillard won't be one of them.

That steak doesn't just need heat, though—it needs a relatively high level of it if you want surface browning to kick in. Boiling water, which tops out at 212°F (100°C) at sea level, isn't hot enough. That's why a boiled steak turns gray instead of dark brown, exciting the palate of exactly no one.

The Maillard can work at lower temperatures, and with a lot more water. If you cook a chicken or beef or vegetable stock at a bare simmer for eight or 12 hours, the result is still a brown, fragrant liquid—a dead giveaway that the Maillard has occurred.

But most of us aren't cooking stocks for that many hours, and none of us are boiling a steak for anywhere close to that period of time. Instead, we're roasting, frying, and grilling. These cooking processes happen relatively fast, in minutes rather than hours, and for the Maillard to happen quickly, we need to drive off enough moisture to break free of that 212° cap.

By cooking a steak in a ripping-hot skillet, you can dehydrate its surface thoroughly enough that the temperatures on that surface will begin to climb, to upwards of 300°F (149°C). At that point, the Maillard reaction will kick into full gear, creating new flavors, aromas, and the characteristic brown colors that give the reaction its more commonplace name, the "browning reaction."

This is why it can be a smart move to pat your meat dry with towels or let it dry in the fridge for several hours before you cook it. It's also why you should salt your meat either more than 45 minutes in advance of cooking (allowing enough time for the salt to draw out moisture through osmosis from the meat, which then reabsorbs that salty brine, turning the meat more tender and moist) or immediately before (allowing you to avoid significant moisture loss through osmosis altogether). Ideally, you'll have enough time to combine the two using a technique called dry-brining: salting the meat generously, then letting it air-dry in the fridge at least overnight and up to a few days before cooking. You'll end up with meat that's deeply seasoned while also sporting a nicely dried surface, perfectly primed for maximum Maillard once roasted or seared.

Proteins and Sugars

[Photograph: Vicky Wasik]

Heat, moisture, and time may be key to getting the Maillard reaction going, but without proteins and sugars to work with, it simply won't happen. Proteins are long chains of amino acids, crumpled up like wads of paper. Some of them are Maillard-susceptible, meaning they really love to bond with sugars. But not just any sugar will do. Molecules of complex sugars, like starches or table sugars, are too big to react with Maillard proteins. Instead, these proteins require "reducing sugars," which are essentially simple sugars that attract amino acids at certain moisture and temperature levels.

That's a critical point: The Maillard reaction starts with a somewhat limited set of proteins and sugar molecules, and, as these bond and mix over time, more and more new molecules are added to the equation. It's kind of an incestuous molecular orgy, when you stop to think about it. (Gross! And also...yum!) These promiscuous molecules mix and match over and over, billions and trillions of times per second, on the surface of a food, forming a growing, recursive, recombinatory aroma and flavor engine.

This engine is influenced by temperature, time, and pH—all things that home cooks can control. If you want to make lots of flavor and aroma compounds, just raise the pH a little with baking soda (as Kenji does to make quick-caramelized onions for his Pressure Cooker French Onion Soup). Looking for a crisp, browned crust? Just lower the pH with a little acid, or increase the temperature. Want a little of both? Frying in fats gives you the best of both worlds.

But...Why Do We Like It?

[Photograph: J. Kenji López-Alt]

Let's think about the humble potato for a moment. A raw potato, most of us would agree, is pretty unappealing. Sure, you can eat a raw potato, and it won't hurt you—after all, it's just a large lump of concentrated starch, and starch is energy that's essential to our survival. But, thanks to the twists and turns of our evolution, we humans can no longer efficiently digest that raw spud. Our digestive system would struggle to break down a potato's complex starches into simpler ones, and it would fail to extract many of the nutrients hidden inside. Cooking breaks down the starches and unlocks those nutrients, improving their ability to be absorbed into our bodies.

When we cut up a potato and then roast it, a sequence of events takes place. First, the water on the exposed surfaces mostly boils off, bursting the starches open into a fluffy mass and breaking them down into simpler sugars. As the heat on those surfaces increases due to the loss of water, the proteins and broken-down sugars begin to break down even more, then recombine. A familiar faint-brown color emerges on the surface of each potato chunk. Some of the various protein-sugar molecules created on the surface of the now-cooked potato will lift off into the hot air above the pan, wafting toward your nose. That smell of roasted potatoes tells your body that it's in the presence of a food that can provide it with nutrients it not only needs but can readily use. Take a bite, and your mouth confirms—it's delicious.

Now, I can see some of you in the back saying, "Wait a minute—mashed potatoes are my fave, and they aren't Maillard-ed at all!" You make an important point: Boiled and steamed potatoes, because of the high volume of water present during those relatively short cooking processes, do not undergo the Maillard reaction, yet can still produce delicious results. I'd argue, though, that these potatoes only really become delicious once they're mixed with some other source of flavor and aroma, like butter. Butter's main flavor molecule is called butyric acid, and butyrates, it just so happens, are also the primary aroma molecules produced by the Maillard reaction when meats are roasted. Almost always, the path leads back to the Maillard reaction.

There's More to Maillard Than Just Maillard

[Photograph: Vicky Wasik]

Here's the next thing you need to know: The Maillard reaction isn't the only reaction that can happen to those building blocks of protein, sugar, and water—and, depending on the ratios of those building blocks, you can get different effects out of the Maillard reaction itself.

Cookie dough, for example, is made up of the same building blocks as a steak. What differentiates the two is the proportions: A steak is obviously much higher in protein, while cookies have a lot more sugar. This has a profound effect not only on the way in which the Maillard reaction occurs, but also on the degree to which these foods experience other, related reactions, like caramelization.

Caramelization is what occurs when sugars are heated and begin to react with water in a process known as hydrolysis, breaking down and reforming into a complex, sweet, nutty, and slightly bitter substance known as...caramel. I like to think of caramelization as a first cousin to the Maillard reaction. When protein levels are low, sugar levels are high, and the temperature is north of 350°F (177°C)—such as in a batch of cookies baking in the oven—caramelization becomes a much more prominent factor. Like the Maillard reaction, caramelization also produces a darker color and more complex flavor, which is one of the reasons the two are often mistaken for each other.

Keep in mind that, though different, these reactions are not mutually exclusive. Both the Maillard reaction and caramelization can and do take place in both a steak and a cookie, but they produce markedly different, often complementary, flavors and aromas in each.

A steak is made of muscle, which is mostly protein and water and comparatively little sugar; the high concentration of protein leads to a Maillard reaction that yields more flavor molecules and fewer aromatic ones. Cookies, on the other hand, are the opposite: With a high volume of sugar and relatively little protein, the Maillard reaction produces more aromatic compounds and fewer flavor molecules.

On the other hand, because cookies have more sugar, they also undergo more robust caramelization, which contributes flavors that the Maillard reaction didn't. The steak, meanwhile, is short on Maillard-produced aromas, but thankfully, the scent of its lightly singed fat does the trick, contributing the aroma it might otherwise have lacked. The ability to leverage both of these processes can help us create more delicious food.

My grandfather used to say that cooking by recipes alone would have never given us the magic that is chicken and waffles. Sure, a recipe can show you how to make each one separately, but it's experimentation that taught us that putting both together tastes better. Knowledge of the Maillard reaction reveals my grandfather was even more right than he knew. Chicken and waffles go so well together because they are the perfect combination of different kinds of Maillard reactions. In the waffles, it's a sugar-heavy Maillard that's high on aroma and low on flavor; in the protein-heavy chicken, it's the opposite. Together, slathered in maple syrup (hello, caramelization!), you have an ideal science-driven meal just begging to be consumed.

It's another reminder that cooking is just edible science—the Maillard reaction is our geeky foundation, recipes our experiments, and you, our scientist, whose sustenance, satisfaction, and, ultimately, survival depend on the results.