If you've read a cookbook or recipes online recently, chances are high that you've come across a recipe that includes caramelized onions. It seems they're everywhere—on sandwiches, in sauces or piled on steaks. Unfortunately, if you've read those recipes, chances are better than even that you've found contradictory and confusing information on how to caramelize them.
After mushrooms, there's probably more confusion about cooking onions than any other vegetable. You'll see caramelized onion recipes that call for sugar, salt or baking soda (or none of the above); heat levels that vary from low to high; and methods that claim to take anywhere from 20 minutes to an hour. You'll often read that you should choose sweet onions to maximize the sugar available for caramelizing. What's the real story?
Types of Onions
First, a short side trip into onion cultivation. (It will help clarify what comes later.) You might think that the important difference among onions is the color—yellow, white or red. Not true. Although there are minor differences in the way these onions taste, the big difference in onions of any color is between spring onions and storage onions.
Spring onions, as you might imagine, are harvested in the spring, before they are fully mature. They’re relatively mild because they contain fewer of the sulfur compounds (compared with storage onions) that give onions their sting. The so-called "sweet" onions are yellow spring onions cultivated in soil that is especially low in sulfur to make them even milder. That's the reason that almost all the branded sweet onions are named after the places where they’re grown—for instance, Vidalias (Georgia), Walla Walla (Washington) or Maui (Hawaii). Without the sulfur-poor soil in those areas, the onions wouldn’t be as mild. Contrary to popular belief, these onions do not contain more sugar than storage onions; it's the lack of sulfur that makes them seem sweeter.
Storage onions are grown to maturity and harvested in the fall. They’re sturdier than spring onions, with thick, brittle layers of skin to protect them. They're the ones that really make your eyes water as you cut into them because they contain more sulfur compounds. However, lots of sulfur is exactly what you want when you're cooking onions.
Caramelization and the Maillard Reaction
Now that you're an onion cultivation expert, you're probably wondering what that has to do with cooking them. Which onions are best for caramelizing? And how, exactly, do you caramelize onions?
The short answer is you don't. Strictly speaking, caramelization is what happens to sugars when exposed to relatively high heat. When you're browning onions, no matter how you do it, you rarely reach the temperatures necessary for caramelization. The browning you see is, instead, caused primarily by the Maillard reaction, which is the reaction between sugars or other carbohydrates and amino acids. Maillard flavors are more complex and "meaty" than caramelized flavors.
Recipes that call for adding sugar to the onions and cooking at higher temperatures may result in a little true caramelization, but it's negligible compared to the Maillard reaction. And it should be clear that since sweet onions have no more sugar than storage onions, they're not going to caramelize any more than their storage cousins.
It actually turns out that their lack of sulfur compounds is a definite handicap when it comes to browning them, especially if you're cooking them for a long time. The sulfur compounds in storage onions, while harsh and irritating when they're raw, undergo changes under heat that are responsible for much of the complexity in the flavor of browned onions. Without them, you'll end up with onions that are mildly sweet but otherwise pretty bland.
How to Brown Onions
The problem with using the term "caramelized" for browned onions is more than just inaccuracy. What causes confusion is that the term is used for two very different methods and results. The first method, which involves very slow cooking, results in onions whose cells have broken down so far that they almost form a paste. They brown slowly and evenly, almost from the inside out.
The second method cooks the onions more quickly over higher heat so that they brown before they have a chance to break down. You end up with browned onions that retain their shape and some texture. They also retain much more of their volume.
So which method is better? The answer, of course, is it depends. Sometimes you want the silky texture and mellow but complex flavor of slow-browned onions, as in this roasted red pepper soup. Sometimes, for instance on a patty melt sandwich, the more assertive flavor and integrity of the onion pieces you get from the quick-browned method is preferable. For French onion soup, you could use both.
Both methods are pretty easy. You can make big batches of either type and keep them on hand for all kinds of recipes.