June 6, 2021

🧅 The Flavor of an Onion

Learn about transforming onion's pungent flavor.

Onions are part of the allium family along with garlic, leeks, and shallots. They have a potent, sulfury flavor that makes them unpleasant when eaten like an apple. But it’s also what keeps away wascally wabbits, and other animals that might eat them. But lucky us, we know how to tame the onion. More on that in a bit.

When you think about the flavor of an onion, there’s a lot of layers to work through. 😉

How so? First off, onions are packing. 🔫 Glutamates that is. What are glutamates? They’re amino acids that give food its savory, umami quality. More commonly, it’s known as the “G” in MSG.

But the real magic of onions is how they can be transformed. They’re a multi-dimensional ingredient. How you treat them changes their flavor:

  • Macerate them in vinegar and they become acidic yet sweet.
  • Lightly cook them in a pan without browning them for about 10 minutes and they’re subtle and savory.
  • Sweat them like this even longer and their flavor gets more complex and concentrated.
  • Slowly brown them, and they become sweet and rich.

However, the best part of onions is how they enhance the food around them. They give a savory quality to a sauce or soup. They give a crunch to your burger. They impart flavor to a risotto without even being noticed. They’re an indispensable ingredient in the kitchen.

How You Cut an Onion Changes Its Flavor

Obviously, onions are potent. But have you ever noticed that onions don’t have an aroma? That is until you cut them. Why? Their intense flavor (and smell) comes from a reaction between an enzyme called alliinase and certain amino acids. And because they’re stored in separate parts of the onion, those two only come in contact when the cell walls are ruptured.

So that means you can change the intensity of the flavor based on how you cut up the onion. The more cell walls you rupture, the stronger the flavor. Dicing an onion ruptures a lot of cell walls. But a food processor gives you even more potent onion flavor.

What about slices of onion? You can amp up or tone down their potency based on how you slice them. Cutting the onion crosswise ruptures more cell walls than cutting lengthwise (aka through the root end). Why? An onion’s cell wall runs from root to tip. And when you cut lengthwise, you cut along the cell walls vs cutting against them.

Onion photo from Buzzfeed

What does this mean for our cooking? Change how you cut an onion based on the flavor you want. A finely diced, raw onion will overpower a salad. But sliced onions won’t. Especially if you treat them. (I swear I’ll get to taming onions in a moment.)

What about building an aromatic base for a dish? If you want the onion flavor to disperse throughout, dice it up before adding it to the pan. Or maybe you don’t have the full onion a recipe asks. Throw the onion half you do have in the food processor. That will really rupture some cell walls and amp up the potency of the smaller amount of onion. Don’t want the onion to overpower a dish? Onion soup is made using pounds of sliced onions so the flavor is more subtle. Otherwise the bad breath we’d get from using that many diced onions would really enforce social distancing. 😉

Taming the Onion

Onions are sharpest when they are raw. But luckily there are multiple techniques to transform or avoid their potent, sulfuric flavor:

  • Apply Heat: Sulfur compounds react and produce new flavors when the onion comes into contact with heat. This creates a compound called MMP. And that’s what gives cooked onions their “meaty” flavor.
  • Add a Touch of Water: The flavor compound MPP is water-soluble. That means adding a little water to sautéed onions can enhance their savoriness.
  • Soak in Vinegar: It only takes about 15 minutes. Soaking raw onions in acid rinses away the harsh flavors and adds pleasant tartness.
  • Soak in Water: Even water washes away those pesky sulfuric flavors. A quick 15-minute soak does the trick.
  • Use a Sharp Knife: A sharp knife will rupture fewer cell walls when you cut the onion. If your knife is blunt, you’ll end up mashing more than slicing.
  • Don’t Let Them Sit: The more time that passes after cutting a raw onion, the more potent the flavor. Why? The more time alliinase has to react with all those amino acids.
Where I learned this: The Science of Good Cooking by Cooks Illustrated, On Food and Cooking by Harold McGhee, Ruhlman’s Twenty by Michael Ruhlman, and this article titled “The Science of Onion Flavor” by Guy Crosby.