The Pantry Staple Jacques Pépin Uses To Thicken Watery Soup

Thick soups typically start off with a roux — a mixture of equal parts fat and flour that are whisked together before liquid ingredients are added. Generally, you won't need to thicken a soup any further when it starts off with a roux, but oftentimes it can still end up with a watery consistency. The most common way to fix this is to add a flour or starch slurry or to simply let it reduce. But Jacques Pépin has a different approach.

In an episode of his cooking show, "More Fast Food My Way," the celebrity chef revealed that he skips the initial roux step altogether. Instead, as Pépin demonstrated while making a cream of leek soup, he waits until the end to thicken the soup and uses instant mashed potatoes to do so. No slurry is necessary when adding instant mashed potatoes; Pépin simply stirs it directly into the broth and it becomes visibly thicker. Simple as that! 

Instant mashed potatoes are made with russet potatoes, one of the starchiest varieties of potatoes. This high starch content is precisely what makes it an effective soup thickener. Thickening happens when the amylose molecule in the starch comes in contact with water. When heat is added to the equation, the amylose absorbs the water, causing it to gelatinize. As a result, the mixture thickens.

Thickening soup with other starches like flour and cornstarch work similarly, but they're prone to clumping. This happens when the starch cooks before it has the chance to disperse throughout the soup, which is why it's best to turn it into a slurry before adding it in. This simple trick — which is one of the many alternative uses for instant mashed potatoes — avoids this problem as the potatoes are already cooked prior to being mashed and dehydrated. Since there's no risk of the starch cooking prematurely and ultimately clumping, instant mashed potatoes are a more foolproof thickening agent for soups.

Like any other thickener, you don't necessarily need to measure out a precise amount of instant mashed potatoes when adding it to your soup. However, it's best to stir in a little at a time, especially since starch has to be activated by heat in order to actually thicken. In the recipe for his leek soup, Jacques Pépin uses about 1 to 1½ cups of instant mashed potatoes per quart of chicken broth.

If your soup is still on the watery side, you can always add more instant potatoes than Pépin suggests, but keep in mind that the ingredient does have a distinct flavor. The more you add, the more potato-y your soup will taste. Too much can also cause your soup to turn cloudy, but ultimately the ideal amount of instant potatoes depends on how watery your soup starts out and how thick you want it to be.