One of the most basic kitchen tasks, right up there with seasoning and chopping, is thickening a soup. Nothing against thin soups—sometimes they're exactly what you want. (Think ramen or pho for example, or even a simple chicken noodle soup.) But sometimes what you're after is something a bit more hearty. Not quite a stew, but a thicker, creamier soup.
"You can just reduce it, no?" Yes, you can. However, doing that can cause some of the ingredients in the soup to overcook.
So, what else can you do? You're in luck! In this explainer, we'll show you how you can thicken a soup by way of the soup itself, using starchy ingredients like potatoes and rice, using flour or cornstarch, and even dairy!
Using the Soup Itself
Perhaps the easiest way to thicken a soup is to purée it. Or, put another way, puréed soups are the easiest soups to thicken. This is because the puréeing itself is often all you need to achieve your desired consistency.
Even if you don't want a fully puréed soup, puréeing only some of it can work wonders. Just ladle out a portion of the soup, purée it in a blender, or in a separate container using an immersion blender, and then stir the puréed portion back in. This will work with just about any soup, and requires no additional ingredients or thickeners.
Potatoes, Rice, Pasta, and Beans
If your puréed soup is still too thin, thickening it further is simply a matter of adding a neutral cooked starch like cooked potatoes, cooked rice, stale bread, even mashed potatoes or frozen hash browns. Just simmer briefly to release the starches, then purée.
Don't overdo it, though. Starches absorb liquid as they're heated, so your soup can easily become too thick if you add too much starch, especially if it has time to stand. Start with a little, purée it, return it to the pot and warm it back up, then taste it.
Or don't purée, and leave the potatoes whole so that their natural starches are released into the cooking liquid, giving the soup body and creaminess, and leaving it extra-hearty. You can do this by cutting the potatoes into bite-sized chunks; they'll work their magic in as few as 20 minutes of simmering, at which time you'll be able to easily pierce them with a fork.
"But what kind of potatoes should I use?" Glad you asked! White, red, and Yukon gold potatoes will hold up better than starchy potatoes like russets, which can fall apart when simmered. But if you're planning to purée the soup, russets are great.
You can also use yams and sweet potatoes the same way. Starchy vegetables like carrots, beets, winter squash and pumpkin, parsnips, peas, beans, or corn will also add thickness. Celery root, peeled, diced and simmered, will add a wonderful but not overpowering flavor and aroma of celery to your soup.
Other starchy ingredients like rice, pasta, and beans, as well as grains like oats and barley also naturally add thickness.
For example, a handful of uncooked rice will do the job nicely, with the starches from the rice permeating the cooking liquid as it simmers. When the rice is soft, your soup is done. This method works better than adding cooked rice, although that will work in a pinch, especially if you're puréeing.
Starchy rices like arborio—the kind used for making risotto—are especially powerful thickeners. Uncooked grains like oats and barley will also work the same way, and so will dried pasta and quick-cooking legumes like lentils. Just simmer until they're soft.
Flour, Cornstarch, or Arrowroot
Sometimes ingredients alone are not enough to thicken your soup adequately. In that case, flour, cornstarch, or some other thickening agent like arrowroot can be your friend—especially if you don't have a puréeing tool like a blender.
A great way to thicken a soup using flour is by cooking it with butter or oil to make a roux. This works best at the beginning of cooking, since you simply prepare the roux at the bottom of the pot, then build the soup from there. Since many soup recipes start with sautéing onions and other aromatics in oil or fat, you can simply sprinkle the flour into the cooked aromatics and their fat and stir to form a paste. Then whisk in your hot broth or stock.
But what if you're trying to rescue an already made soup that's too thin? In this case, you can add roux made in a separate pan, or try a beurre manie. A beurre manie is a putty of equal parts (by weight) softened butter plus flour that you knead together and then stir, bit by bit, into your soup. The fat helps join the starch to the liquid so that you don't just end up with clumps of flour in your soup.
You can make a roux or a beurre manie using non-wheat flours like rice flour, as well as your favorite gluten-free flour blends.
Cornstarch is another useful thickening agent. We first combine a small amount of cornstarch with cold water in a bowl and mix until blended. If you add it directly to the hot liquid it will clump and then will not disperse. Then whisk this mixture, called a slurry, into your simmering soup.
Cornstarch alternatives include arrowroot and tapioca starch, and you might use one or the other depending on what type of soup you're making. For instance, cornstarch is less effective in tomato-based soups, while arrowroot can impart an unpleasant texture to dairy-based soups.
Speaking of dairy, the addition of a small amount of cream, sour cream, or yogurt is often enough to thicken a soup without necessarily turning it into a dairy-based soup. Watch out for curdling, however, which happens when the heat of the soup, or acid in the soup, causes the milk proteins (or whey) to separate from the water and fat. In general, the higher the fat, the better the dairy product will combine with a hot soup without curdling. Thus, heavy cream will work better than half-and-half or milk.