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Why is Garlic Usually Added After the Onion When Sautéing?
So many recipes say to sauté onions first, then add the garlic. Why is that? And why can't I just add them to the pan at the same time?
There aren't many foods that taste good burned, but garlic is something of a special case. Even before it has fully turned into blackened bits of carbon, sautéed garlic can smell and taste acrid and bitter. We sear meats until well browned, we cook onions until rich and caramelized, but, for the most part, we don't seek the flavor of chestnut-colored garlic bits.
To help cooks avoid burning their garlic, a lot of recipes call for sautéing other aromatics first, like onions, carrots, and celery, and then adding the garlic for the last few minutes. But that doesn't mean you can't add garlic at the beginning: you just need to be mindful of a few variables that can spell the difference between success and disaster.
The other thing to keep in mind is that your dish will often taste different depending on when you add the garlic. I'll come back to that in a bit. First, let's see what happens when you sauté garlic all by itself.
One of the reasons garlic is more susceptible to burning than onion is because it has significantly less water content and higher amounts of naturally occurring sugars like fructose.* Also, garlic is often thinly sliced or minced, and those small pieces can cook much faster than larger pieces of diced or sliced onion.
Exactly how quickly garlic cooks, though, depends not only on the heat of the pan, but also the size of the garlic pieces. Let's start with a rip-roaring hot pan.
Garlic Over High Heat
Above we see three cloves of garlic prepared three ways: minced, sliced, and then a crushed whole clove. This photo was taken about 15 seconds after all of the garlic was added to a very hot pan. Note how the minced garlic is already browning around the edges.
At the 30-second mark, all of the garlic is lost, but of the three, the minced garlic is in the worst shape. Because of all the exposed surface area, there's not much left of it that isn't overcooked. The whole crushed clove, meanwhile, is too brown on the surface, but everything below the surface is still fine.
Clearly, adding garlic all by itself to a very hot pan isn't generally a great idea unless you really want that pungent browned-garlic flavor and are ready to add moisture to the pan before things go too far.
Garlic Over Low Heat
Now let's see how garlic does in a pan over low heat.
The photo above shows the same scenario, except the heat is now very low. This shot was taken a full two minutes into the cooking time, and the garlic is just starting to lightly brown.
At about four and a half minutes, the garlic has finally crossed the line into territory that I'd say is undesirable. The takeaway here is that with low enough heat, your risk of burning garlic—even when it's the only thing in the pan—is low, no matter how small the garlic is cut.
What About When You Add Onions to the Mix?
To see what happens when the garlic is cooked with onions from the start, I tried out a couple new tests. Since I've already established that when cooked over low heat, garlic doesn't burn very quickly, let's focus on two high-heat scenarios with onions.* In the first, I cooked sliced onion and minced garlic in a small pan over high heat. In the second, I cooked the exact same quantity of garlic and onion in a large skillet over high heat. In both cases I stirred and tossed the onion and garlic frequently.
* For the ultra-curious out there, I also ran tests of onion and garlic over low heat, and burning is just not an issue.
The reason I tested out pan size is that as the onion cooks, it releases water into the pan, which helps prevent the garlic from burning. I reasoned that in a larger skillet, the onion may not release enough liquid to moisten the entire surface of the pan, allowing some of the garlic to burn. Let's see what happens.
So right above, we have the onion and garlic in the smaller pan almost exactly one minute after I started cooking them. There's already some color developing, but nothing is really burning or getting worryingly brown.
In this next photo, we have the onion and garlic in the larger skillet also at the one-minute mark. So far, this one is looking pretty good too.
Thirty seconds later, at the 1.5 minute mark, and there's a bit more browning in the smaller skillet above. A few tiny pieces of garlic may be edging towards too dark, but nothing terrible has happened here yet.
In the larger skillet, meanwhile, the browning is a little more pronounced, and while it may be hard to see in the photo above, some of the garlic has already crossed over into the over-browned realm.*
*I should note here that while I was working on induction burners that were set to the exact same heat level, I suspect the larger skillet never got quite as hot as the smaller one, either because the skillet is too large for the burner, or because the skillet had a small bulge on the bottom that prevented it from making full contact with the burner's surface. I didn't have an infrared thermometer to measure the surface temperatures in the pans, but from what I observed, the larger pan was a little less hot than the smaller one. All of this is to say that if the larger pan had been as hot as the smaller one, I think we would see an even more drastic difference in the rate of browning in the two skillets, with the contents of the larger skillet already starting to burn.
The one takeaway here is that if you decide to cook onion and garlic together over high heat, make sure to choose the right-sized pan: The larger the pan, the better the chance that your garlic is going to burn.
What About Flavor?
So far, we've established two fairly common-sense rules about cooking onions with garlic without the garlic burning: use low heat and a pan size that won't leave a lot of exposed surface area. We could just stop there, but it leaves one question unanswered: Even if we prevent burning by using low heat and a small pan, is there a significant flavor difference between batches in which one has the garlic cooked in it from the start and the other has the garlic added only for the last couple minutes of cooking?
I cooked up two side-by-side batches of equal quantities of diced onion and minced garlic to find out. In one, the onion and garlic went in the pan at the same time. In the other, I added the garlic about two minutes before the onion was done cooking. Then I asked my colleagues to taste both blind and tell me which they preferred and why.
The two batches don't look significantly different, and there's nothing burned in either of them. And yet the flavor difference was drastic. The onions that were cooked from the start with the garlic had a much mellower flavor—it was hard to detect a strong garlic flavor.
The batch in which the garlic was added at the end, though, had a pronounced garlicky flavor. Most of my colleagues preferred that one, though not all did. If you like stronger garlicky flavors, it's better to hold the garlic back until later, but if you don't, consider adding it earlier on.