I was cooking up a storm this weekend: vegetable stock, cream of mushroom soup, and Japanese Yakiniku-style ginger pork. Each dish involved onions. And as I chopped, boiled, and stir-fried my first, second, third and fourth onion, I started thinking about this wonderful vegetable and its multiple virtues.
I called Jessica over excitedly and fed her a spoonful of the vegetable stock that had been simmering for over 30 minutes. “It’s very good, right? Can you taste the beautiful sweetness? Guess where the sweetness came from?”
“The carrots,” she replied.
“No, the onions! The longer you cook them, the sweeter they taste!” I said and force-fed her another spoonful.
Onions change their flavor by means of cooking and here’s a little science that explains the flavor transformation.
Several complex organic compounds make up a raw onion’s distinctive smell and taste. The primary compounds include (1) thiosulfinates, which is responsible for the characteristic raw onion odor and flavor and (2) (Z)-propanethial-S-oxide, which is primarily responsible for the pungent tear-inducing gas. Interestingly, the two compounds cannot be activated unless the onion’s cell walls are damaged—by cutting, slicing, chopping, or cooking.
Once the onion’s cell walls are damaged, you can then move on to manipulate your desired onion flavor using heat and time. Heat, whether that’s in the form of high-heat frying or low-heat slow simmering, diminishes the raw onion flavor by lowering the quantity of (Z)-propanethial-S-oxide and converting it to thermally processed flavor 3-mercapto-2-methylpentan-1-ol.
Here are three ways to use oil and temperature variations to bring out onions’ flavor.
- Stir-frying sliced onions over high heat for a minute or two. This cooking style keeps the onion texture crisp and removes the pungent raw onion flavor. The quickness of stir-frying helps the onions retain pigments and nutrients.
- Sweating onions over medium heat to the point they have softened and are translucent—this usually takes 10 minutes. This cooking style removes the pungent onion flavor and leaves behind a mild sweetness.
- Caramelizing (until browning) onions over low heat (cooking time ranges from 20 to 45 minutes). This cooking style introduces a richer caramel sweetness that is nutty and buttery. The onions develop and concentrate their flavors. Flavors from seasonings or added vegetables, meats, also blend together better.
Note to readers:
I have the horrible habit of trying to convince Jessica to love the same things that I do. In the case of onions, she resisted: I like them alright. When further pressed, she resisted still: No, I do not not like them (note the double negative), they are fine.
I shall strive to convince her otherwise. In the meantime, enjoy cooking with onions!
The Cooking Science Guy “The Science of Onion Flavor”
McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. Scribner, 2004.