There are two schools of culinary thought when it comes to the best way to thicken soup, and both of them have equal merit (based on who you ask).

But the closer you look at cornstarch and flour—and the way in which they’re used as thickening agents for soup—the more obvious it becomes that one is actually better than the other. The idea that they’re both the same, is unfortunately, an old wives tale. 

Whether you choose flour or cornstarch to use as a thickening agent, it’s really important to know how to make use of them.

If you just dump a tablespoon of flour or cornstarch into your soup, start stirring and hope for the best, the only thing that’s realistically going to happen… is that you’ll end up ruining i. All the effort that you went to to try and thicken it will have been for nothing.

Fortunately, the way to use either, or both, as a thickening agent for soup is exactly the same: you need to make a slurry first. A slurry is a mixture of flour and water or cornstarch and water and is used to thicken soups, sauces, and gravy.

For a flour slurry, you’ll need to use roughly 1 tablespoon of flour to 1 quart of cold water, and you’ll need to stir it thoroughly to minimize the chances that the flour will start to form clumps when you add the slurry to the soup.

If you’re using cornstarch, the measurements are almost the same, but as it absorbs water far more quickly and efficiently than flour does, it should make a better slurry.

But as flour and cornstarch are both starch-based, they are ideal for adding a little thickness to your soup. 

While there’s a chance that a flour slurry will break down, and that the flour will begin to form clumps in the soup if you don’t keep mixing it in, as soon as you have slowly added and mixed the flour slurry into your soup, you should see and feel it start to get thicker.

Once it reaches your desired consistency, stop adding the flour slurry, but keep stirring your soup.

When your soup starts to cool, the starch content of the flour slurry will start to make its presence felt, making your soup thicker and thicker.

Don’t worry (and don’t start to panic); it’s perfectly normal for this to happen, and all you need to do to return your soup to the desired consistency is reheat it. Once you’ve brought the soup to a simmer, it’ll be ready to serve.

Just remember to watch out for clumps, as they can (and will), spoil your soup’s texture. 

If you use cornstarch slurry, you’ll immediately see and taste the difference. A flour slurry can make a soup look cloudy and muddy, while it won’t affect the taste of the soup, it can definitely make it less appealing. That’s not the case with cornstarch slurry. 

A cornstarch slurry won’t do that and as it more readily and easily absorbs liquid than flour does, it’ll have an almost instantaneous effect on your soup. That is, as long as the soup is at the right temperature when you add the slurry.

The higher the temperature of the soup (as long as it isn’t boiling) you add the cornstarch slurry to, the more effective it will be at thickening it.

As cornstarch won’t affect the color of your soup either, in the cornstarch versus flour as a thickening agent for soup competition, there’s one clear victor. And that’s cornstarch. 

Can You Use Plain Flour To Thicken Sauce?

Can you use plain flour (same as all-purpose flour) to thicken a sauce?

You absolutely can, but if you try stirring plain flour directly into your sauce it won’t work, and you’ll have to throw the sauce away and start from scratch. 

The correct way to use plain flour to thicken a sauce is by making a slurry, which is a combination of plain flour and water.

In a jug or bowl, mix one part water and one part all-purpose flour. Stir the mixture until it’s pasty and consistent, then add it to the sauce and, over medium-low heat, cook it down slowly.

It’s done what it was supposed to, and what you needed it to do, so feel free to discard any leftovers you might have. Now all you have to do is focus on your sauce and keep stirring it until it’s ready to serve. 

How Much Cornstarch Should I Use To Thicken Soup?

The amount of cornstarch that you need to thicken your soup depends entirely on how much soup you’ve made.

For each cup (8 ounces or 237 milliliters) of liquid in your soup, you’ll need to use 1 tablespoon of cornstarch to thicken it.

But don’t whatever you do, add the cornstarch directly to your soup.  If you do, you’ll spoil your soup and you’ll have to go back to the drawing board and start all over again. 

The best way to use cornstarch to thicken soup is by adding the necessary amount of cornstarch to a separate jug and slowly adding cold water to it to create a slurry. 

As you add the water to the cornstarch, keep stirring the mixture until the cornstarch and the cold water form a smooth, liquid paste. This is your cornstarch slurry. 

While your soup is simmering, slowly add the cornstarch slurry to it and keep stirring your soup, which will mix the two together.

Once your soup has reached the thickness you’re looking for, stop adding the cornstarch slurry and just keep stirring until you are ready to plate and eat it.

What Is The Healthiest Way To Thicken Soup?

If you’re following a keto diet—and want to thicken your soup without adding any carbs to it—the best way to do that is by using a keto-friendly thickening agent like mayonnaise.

Be careful while you’re doing it though, as a little mayo goes a long way; too much can spoil the flavor of your soup entirely.

If you’re just looking for a healthy way to add a little thickness to your soup without increasing its calorie content, you could add some beans, stir them in, and blend your soup until it’s smooth.

The beans will add texture and taste to your soup and make it far more nutritious than it would have been without them (100 grams of beans pack 21 grams of protein, as well as a high amount of calcium, iron, magnesium, and vitamin B-6).

Can You Use Self-Raising Flour To Thicken Sauce?

Again, as long as you do not add the self-rising flour directly to the sauce, but make a slurry of it and use the same method as you would with regular flour, there should be no problem using flour to thicken a sauce.

However, some chefs think that self-raising flour is a much more powerful thickening agent than plain flour is, so when you’re adding your self-raising flour slurry to your sauce be incredibly careful.

Adding too much of it to the sauce too quickly could transform your sauce into jello, which is the last thing that you’ll want to happen. Take your time, and it should be absolutely fine. 

In Conclusion

Thickening soup the right way can be a tricky thing to do. But not when you know what to watch out for.

Although you can thicken a soup with both flour and cornstarch, we recommend cornstarch because it does not make the soup cloudy, as some flours can, and it does not bring a weird taste of “doughy rawness.”

What are your tips for thickening soup? Share them below!