"Is it necessary to sauté aromatics in a dish like a soup or stew that will cook for a long time? Many of my soups, stews, and curries have a base of mirepoix, or onions, and maybe garlic and ginger. I'm wondering in a dish that cooks for 1 hour or more is it necessary to start with the sauté? Does the sauté process add something to the flavor or texture that simmering in liquid wouldn't, or perhaps would I find that the onion would take a very long time to cook while simmering in a soup?"
—Sent by mattskee
Whether it's the classic French mirepoix of onions, celery, and carrots cooked in butter, the New Orleans holy trinity of onions, celery, and bell peppers sweated in oil, or perhaps just some leeks and garlic cooked down in olive oil, most stews and soups start the same way: sautéeing veggies.
If long-stewing is going to soften up your vegetables anyway, why bother softening them in fat to begin with? It's a good question, and one that you can quickly get a practical answer to just by trying it out: Make a batch of an easy chili recipe, like this 30-Minute Chipotle Chicken Chili, but rather than cooking it all in one pot, divide the ingredients in half. Into one pot, dump all of the ingredients, turn on the heat, and let it go. In the second, follow the instructions as written by sweating out the onions in the oil first, followed by adding some of the other aromatics (the garlic and spices), and finally adding the liquid, beans, and meat.
Now taste the two side-by-side. What do you taste? Here's a hint for those of you who didn't actually follow the instructions in the previous paragraph: The chili in which the onions were sautéed will have a mellower, more balanced aroma and a slightly sweeter flavor. It'll taste smoother, more integrated, married better. The one in which the ingredients were simply dumped in will taste off, with a stronger sulfurous aroma and a strange raw pungency. Why the difference?
It has to do with the way aromatic compounds combine with each other in the pot. See, vegetables contain many different aromatic molecules trapped inside their cells. But the aromas you get from raw vegetables are quite different from those you get from cooked vegetables. In fact, the aromas you get from whole vegetables is even different from those that you get from vegetables that have been finely chopped, grated, or have otherwise had their cells ruptured. This is all due to reactions that take place between chemical precursors inside the vegetables cells. When those cells are ruptured, these precursors come into contact with each other and recombine into new compounds. Heat and time can increase the rate of these reactions, and indeed cause brand new ones to occur.
The most famous example of these are in onions. When you first cut open an onion, it has barely any aroma at all. It's only after a few chemicals inside its cells (called lachrymators, from the Latin root for "cry") are combined that its familiar sulfurous, pungent aroma begins to form. Let a cut onion sit in a sealed container over night, and that smell will become quite powerful indeed. Subsequently heating these compounds will cause them to continue to convert to different, less pungent ones, and eventually an onion will soften into a mellow sweetness.
So why does it happen when you cook in oil but not when you simmer them in water? There are two major factors at play: heat and concentration.
Onions and other vegetables cooked in a large pot of water are limited to a maximum temperature of 212°F. This is too low for some of these specific reactions to take place (most famously the Maillard browning reactions). In a pot with oil, on the other hand, you have the ability to heat your vegetables to a higher temperature.
Secondly, concentration plays a large role. When you've got vegetables in a pot with a small amount of oil, the chemicals they are releasing are in an extremely confined area. Some of them may jump right off into the atmosphere to escape (that's why sautéeing onions smell so good!), while others will react with each other due to their close proximity and intense jostling. Throw the same amount of vegetables into a large pot of water, and you've diluted those reactants to the point where they are far less likely to bump into each other—and that's assuming that they even reach a high enough temperature to react in the first place.
Moral of the story: There's no shortcut to good flavor, unfortunately. While certain vegetables can work just fine added directly to simmering soups and stews (say, carrots and celery), other vegetables (onions, garlic, and the like) will almost always need at least a brief sweat in a fat-based liquid before adding the remaining ingredients.
EDIT: This is not to say that with certain recipes that call for raw vegetables to be used that you should sauté them first. Some times that more sulfurous but fresher flavor is what you're going for. Chicken soup or a plain chicken stock, for example. The point is, if a recipe calls for sautéeing, you can't skip the step and hope to end up with the same results!