With this in-depth post you’ll learn how to caramelize onions and the science behind the process. You’ll learn the easy way, with detailed step-by-step photos and instructions, for guaranteed perfect caramelized onions.
The Basics of Caramelized Onions
Caramelized onions can be an essential cooking ingredient. Slowly cooking onions creates chemical reactions that bring out a sweet and roasted flavor. The bitter bite is removed, and you’ll have a sweet and flavorful onion base that livens up any recipe.
I always semi-caramelize onions for any dish. This involves cooking for 5–7 minutes at a relatively high heat; stirring often to prevent burning. It’s just enough time to bring out flavor and reduce pungency. If you want to take this further and learn how to fully caramelize onions, this guide will do just that.
Recipes often call for caramelized onions, however don’t always explain how to properly do this. Learning how to caramelize onions is easy, but takes time. The good news is you don’t need to watch them closely at such a low heat. You can start them and work on other things, or go to the next step in your recipe. Just check and stir occasionally.
The Science Behind Caramelized Onions and What Makes Them Sweet
Sweating: There are many chemical reactions at work when caramelizing onions. When cooking onions the internal moisture begins to steam; a process know as sweating. The liquid cells in the onion burst and breaks down the structure of the onion. Adding salt draws out more moisture. This is why onions soften as they cook. Onions reduce by roughly half, and actually start to deteriorate, when cooked longer to a caramelized state. My Steak Bruschetta with Goat Cheese and Tomato Jam recipe uses caramelized onions and roasted cherry tomatoes to create an easy savory jam.
Creation of new molecules: A series of complex chemical reactions take place when onions caramelize. Heating causes the water and sugar molecules to separate. As they cook, these dehydrated sugar molecule form together to create new molecules. Eventually sweet tasting disulfides and trisulfides form. This is the same reactions that happens when you cook sugar to form caramel. The post from the Kitchn explains the bitter to sweet process in more detail.
Maillard reaction creates browning: This is another reaction that happens during slow cooking for long periods of time, or cooking quickly with high heat. Any time you grill meat or toast bread in a toaster, you’re seeing the Maillard reaction in effect. Essentially amino acids (part of protein) react with sugars, like glucose and fructose, to create new compounds. These eventually create new flavors, leading to a rich, roasted taste. This process repeats itself until new compounds know as melanoidin pigments are formed. These molecules are what creates the brown color (or grilled appearance) on produce and meats. Food with high sugar (or glucose) content, like fruits, brown easily as well.
How to Caramelize Onions
Basic knowledge and time are all that’s need to perfectly caramelize onions. First peel and dice onions into roughly ½-inch pieces. You can also cut them into strips if you prefer.
Heat olive oil in a large pan or skillet to medium heat. Add onions and salt. Cook for 5–7 minutes, stirring often, then reduce heat to low.
We start with a higher heat to speed up the process, then cook over low heat (if we only cook on low it can take 40-45 minutes to caramelize). Continue to cook on low for at least 20–25 minutes; stirring occasionally to prevent burning or sticking. You’ll see the onions start to shrink and turn translucent.
They’ll eventually turn golden brown. You may need to add another tablespoon of oil to prevent sticking.
The onions will go from golden brown to a rich, dark brown (but not black). They should have shrunk by about half and may even start to break up.
At this stage the onions are done and caramelized. Remove from heat and use in your recipe or as a topping.
You can cook extras and store in the freezer for later use. This saves you time as some recipes only call for a small amount. Caramelize a large amount of onions at once, then freeze in separate portions in small bags. Defrost what you need in the fridge the night before.
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How to Caramelize Onions
- 2 tablespoons olive oil, plus extra
- 3-4 large white or yellow onions
- ½ teaspoon salt
- Dice onions into roughly ½-inch pieces.
- Heat olive oil in a large pan or skillet to medium heat. Add onions and salt, cook 4–5 minutes, stirring often.
- Reduce heat to low and continue to cook; stirring occasionally. Cook for at least 20–25 minutes. You may need to add more oil to prevent from sticking. Onions are done when they are browned (or caramelized) and softened. See photos above for details and step-by-step instructions.
Some scientific details in this post are from Cooks Illustrated: The Science of Good Cooking. Print.
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Have you made caramelized onions? What types of recipes do you use them for?