What Happens When You Caramelize an Onion?
It seems like magic, but it's simple science.
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.
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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.