Why Does Red Lentil Soup Boil Over?

Because red lentils are hulled, they release their starch into the water very quickly. When the water starts to boil, the starch coats the bubbles at the surface and keeps them from popping right away. The bubbles build up into a dense, sticky layer of foam. This layer acts like a lid. Steam builds up underneath it with nowhere to escape, and pressure quietly increases; from the outside, the pot looks calm. That is why a boil-over often happens without warning and catches whoever is watching the pot unprepared. Do not leave the kitchen once the soup starts boiling.

That is why the boil-over happens suddenly. At a certain point, the trapped steam pushes the foam up all at once, and the mixture spills over the edge within seconds. For someone who is not staying by the pot, it can happen during just a few minutes of distraction. The starchy liquid burns onto the stovetop and is hard to clean; on a gas stove, it can put out the flame. So the issue is more than a mess. The same thing can happen when cooking rice or pasta; the starch source changes, but the mechanism is the same.

The first preventive measure is the pot itself. Fill the pot no more than halfway with soup, leaving room for the foam to rise. The second is heat: after the lentils first come to a boil, reduce the heat and keep the water at a gentle simmer. A vigorous boil produces more foam and makes it rise faster. The third measure is the lid: leave it ajar instead of closing it completely. A wooden spoon placed under the lid does the same job by giving steam a way to escape.

The foam itself can also be managed. Skim off and discard the foam that rises during the first few minutes of boiling; this reduces the risk of a boil-over and makes the soup clearer. Adding one 15 ml measure of oil or butter to the pot also noticeably reduces the foam. A wooden spoon laid across the pot is another old method for breaking up the rising foam. Adding oil does not change the soup’s flavor, since an oily sauce is drizzled over it at the end anyway.

Sticking to the bottom also comes from the same starch. Thick soup does not transfer heat upward, so the portion at the bottom burns; scrape the bottom of the pot with a spoon from time to time. If you change the rinsing water several times until it runs clear, both foaming and sticking are reduced. Choose a heavy-bottomed pot; in a thin-bottomed pot, heat concentrates in spots and burning is unavoidable. In soup that will be blended, the burnt flavor from the bottom spreads throughout, so it is important not to let the bottom scorch. If the soup has scorched on the bottom, transfer it to another pot without stirring and leave the burnt portion behind.