It seems like magic, but it's simple science.

By Gillie Houston
Updated December 13, 2018
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Here's the thing about caramelizing onions: It takes a while. Not 15 minutes. Not 30 minutes. For me, on my stove, it almost always takes at least an hour, and usually closer to 90 minutes. I'm sorry about this. I wish I could fix it. Caramelized onions are delicious, and if I could have them more rapidly, I would make them more often. But if any recipe tells you that you can get that deep brown color and complex sweet-umami flavor from caramelized onions in ten minutes, it is a lie and you should disregard it. The number one thing that goes wrong with caramelized onions is impatience.You can't cook them at a high temperature. This means that the onions will burn before they really caramelize, and you'll have wasted all that work and a couple of onions, too. The way to caramelize onions is low and slow, at a pace that is somewhere between infuriating and meditative. It's possible that you aren't adding enough fat to the pan at first, and that your onions are sticking. That could be a problem, too. If you're way overly cautious and the flame is very, very low, it could also take a lot longer than 90 minutes, though the flavor will probably also be terrific. We want onions to caramelize faster because none of us have time to wait around for that. We want 30-minute dinners, something fast and easy on a Tuesday. There is absolutely nothing wrong with that. There's no wrong way to feed yourself. There is, however, a wrong way to caramelize onions, unfortunately. So save it for when you know you have time, and then use those precious, delicious onions to add flavor to all kinds of things. You can make a killer French onion dip. You can add it to macaroni and cheese. You can make this gorgeous mushroom and chicken penne. You can make French onion pot pie, or good old-fashioned French onion soup. The world is full of possiblities for your hard-won caramelized onions. All you need to do is take your time. 
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While many people have experienced the sheer magic of throwing pungent chunks of raw onion into a pan only to be rewarded with a richly sweet, golden brown end result, the science behind this browning process is a mystery to most of us. Even the most passionate of French Onion Soup enthusiasts probably can’t explain how a little heat transforms one of the nature’s most severe vegetables into a supple and sweet addition to any dish.

And while some might want to keep this culinary magic shrouded in secrecy, the rest can read on to reveal the scientific sorcery behind the onion’s glorious golden transformation.

So, What Exactly Is Caramelization?

In the simplest terms, caramelization is the browning process that occurs when sugar is heated. However, there are numerous different forms of caramelization depending on the types of sugars in the food and the kind of reaction taking place.

The specific kind of browning taking place when you heat an onion is called pyrolysis. This is a non-enzymatic form of browning, meaning it isn’t taking place as a result of contact with oxygen, but rather a chemical process. An easy way to picture the difference between non-enzymatic and enzymatic browning is to imagine the contrasting reactions that a sliced apple and sliced onion have to oxygen. While a sliced apple will begin to turn brown soon after making contact with the oxygen (aka. enzymatic browning), the sliced onion will stay its same color even after prolonged exposure to air.

Therefore, an onion can only be browned when it comes into contact with a heat source, which causes a reaction that not only changes the physical appearance of the onion, but also the chemical makeup. The key to this reaction is the natural sugars that are already contained within raw onions.

While onions—and all living things—contain a variety of natural sugars that are broken down for energy through processes like photosynthesis, our taste buds aren’t programmed to taste the large sugar molecules that naturally occur in raw onion. However, we are able to taste smaller simple sugar molecules and recognize them for their pleasant sweetness. So, for the caramelization of onions to take place, these large sugar molecules have to be transformed into the smaller, sweeter versions of themselves.

WATCH: 8 Ways to Cut an Onion

This chemical process occurs when the onion comes in contact with heat, causing the temperature within the cells of the onion to rise. Once this temperature reaches a certain point, pyrolysis occurs, causing the larger starch and sugar molecules of the vegetable begin to break down into smaller units. This happens when the heat causes the cellular bonds that link the large sugars together to break apart, transforming them into the single molecules that give the onion a sweeter, milder flavor.

During this process the texture and appearance of the vegetable is also altered, as the structural starches in the onion break down as well, seemingly magically transforming the onion from yellow or white, crunchy, and astringent to golden brown, soft, and subtly sweet.

How to Properly Caramelize an Onion

To prep for the caramelization process, start by slicing your onions into thin, even pieces—more even slices will make for more even caramelization overall. Add a tablespoon of oil or butter per pound of onion to a skillet over medium heat until it has become hot, but not smoking.

Add your onions to the oil or butter, coating all of the pieces in the fat and spreading them out evenly over the entire pan to begin the “sweating” process, which will take anywhere from 10-30 minutes depending on the quantity you’re cooking. Stir the onions occasionally, increasing the frequency of your stirring once the onions have begun to brown.

Caramelizing an onion to perfection requires time and patience, as the onion—which is about 89 percent water to begin with—needs to sweat out a significant amount of that liquid. This release of water is key to the caramelization process because this excess liquid allows the onion’s structure to begin to break down, and for the vegetable to start to soften.

Despite the textural change that you’ll see in your onions as a result of the sweating period, the actual pyrolysis reaction that causes caramelization doesn’t occur until the internal temperature of the onions has reached 212 degrees Fahrenheit. Therefore, if you get impatient and remove the onions from the pot as soon as they get soft, your onions will lack the deep flavor and color that results from pyrolysis.

You’ll know the chemical transformation has begun when your onions start to slowly turn a caramel color. At this point, it’s imperative to keep a close eye on your vegetables to make sure they don’t burn, stirring them every 5-10 minutes to prevent sticking to the pan or charring. If your onions appear to be sticking to the pan, feel free to add a couple tablespoons of water, wine, or broth to the vegetables.

Once your onions begin to caramelize, you can optionally turn the heat down slightly to prevent possible burning. You also have the option to add in a pinch of sugar or even a tablespoon or two of balsamic vinegar, which allows them to develop and even sweeter, more caramelized flavor. Overall, the caramelization process should take between 30-60 minutes to unleash the full depth of flavor from the onions.

Once you’ve unleashed this chemical reaction, you can put it on display in recipes like Caramelized Onion and Goat Cheese Pizza, Caramelized Onion Dip and Caramelized Onion, Mushroom, and Chicken Penne—and feel free to dazzle your dinner guests with your newfound scientific insight in the process.