Why Do Lentils Foam When Cooking? And Does It Matter If They Do?

Extreme close-up of green lentils.

I was cooking dinner the other night, boiling up a big panful of lentils to make a delicious lentil minestrone soup, when I noticed a slightly grey foam settling on the top of the pan. I’d seen it before, as it happens pretty much every time I cook lentils or beans, but I’d never really given it much thought before. I decided to do some research to figure out what causes this strange, soapy foam to appear – and to find out whether it was something I needed to bother skimming off! Here’s what I discovered.

The foam on top of a pan of lentils or beans is created by ‘saponins’ within the beans, which form a lather in water. The foam is not dangerous, and it’s totally safe to consume. You can skim it off the top of your lentils if you like, but it’s totally fine to leave it too.

Thank goodness! I wasn’t going to poison my family by serving them foamy lentils. But why do so many people recommend skimming this foam away, if it’s totally harmless? Let’s look in more detail!

Green lentils cooking in a pan of boiling water.

Where does the foam on top of a pan of lentils or beans come from?

The grey foam that forms on top of a pan of lentils is caused by a substance in the beans called saponins. This substance lathers up in water, just like soap – in fact, the word ‘saponins’ actually comes from the Latin word for soap, ‘sapo’!

Saponins are found in all sorts of foods. Legumes are some of the richest sources of saponins – chickpeas, kidney beans, soybeans and lentils in particular – as well as quinoa, oats, and a wide variety of vegetables. And thanks to the grape skins, there are even saponins in red wine! So they’re pretty hard to avoid.

These various plants have evolved to produce saponins as a natural method of pest control. The saponins form a waxy, protective layer around the skin of the plant, and give it a bitter taste, making them less attractive to insects, birds and animals. As we all know from our high school biology classes, natural selection means that the plants that produced highest levels of saponins were the least likely to be eaten, so those are the plants that went on to reproduce.

Just like with regular soap, you need to add water to a food containing saponins, and then agitate it, to create the bubbles. This explains why the foam is only released from your lentils after you’ve added plenty of water and given them a good stir!

Red lentils cooking in a pan of boiling water.

Is the foam on top of a pan of lentils harmful?

Nope! It’s all totally harmless stuff – just created by normal, totally edible, substances that are naturally found in many different foods.

Despite the fact that plants evolved to contain saponins to deter animals from eating them, the level of saponins in our food is so tiny, it’s not harmful to humans. Larger quantities might make us think twice, but if you’re just eating a normal amount of food, it’s nothing to worry about.

In fact, clinical research has shown that saponins may even have major health benefits. For example, a study in the Journal Of Medicinal Food commented that saponins can lower cholesterol, lower the blood glucose response, and even lower the risk of cancer. I won’t delve too much into the health benefits of saponins here, since I’m not a biologist or dietitian, but suffice to say, unless you have a pre-existing condition that means you have trouble digesting legumes, there’s nothing harmful about the tiny levels of saponins in beans and lentils.

Should you skim the foam off when cooking lentils and beans?

Really, it’s up to you. There’s no super important reason that you have to skim this foam away. As I said, it’s certainly not going to do you any harm.

The only health-related argument for skimming away the foam from a pot of lentils is that some people claim it makes them less… flatulence-inducing, shall we say? Some people do seem to be more sensitive to beans and lentils than others, reporting that they cause gas, stomach irritation, bloating, and other digestive issues.

So if that’s the case for you, it might be worth seeing whether removing the foam makes any difference to your symptoms.

Extreme close-up of brown lentils.

So why do people skim away the foam from lentils?

A lot of people like to skim the foam off the top of a pan of lentils primarily because it’s just a bit ugly. It’s usually a light grey colour, and in all honesty, it doesn’t look particularly attractive. It’s almost enough to put you off lentils for life.

If there’s a lot of foam, it can also make it harder to see what’s going on underneath – you ideally want to be able to see your lentils, to keep an eye on how they’re cooking!

If you’re just cooking lentils on their own, you’ll probably end up draining the liquid away once they’re cooked, so the majority of the foam will be drained away then anyway – in which case, this entire conversation is moot.

However, if you’re not planning on draining the lentils (for example, you want to add extra veggies to make a tasty lentil soup), you probably don’t want to leave the foam floating on top. It looks a bit scummy, and the slightly slimy texture probably isn’t one you were hoping to feature in your soup! So in this case, I’d skim away the foam before adding any additional ingredients.

The easiest way to remove the foam from your lentils is just to use a large metal spoon, like the one I’ve linked above. Slowly skim it across the pan, gently scooping up the foam from the top of the water. Then just discard the foam – I chuck it straight in the sink.

To be honest, my main issue with foamy lentils is that if the pan boils over, it’s takes a lot more elbow grease to clean up burnt-on lentil foam than it does to clean up plain lentil water. But perhaps I should just get better at not boiling over my pans.

How can you avoid getting foam on top of your lentils?

If you’re not a fan of the foam that always appears on top of your lentils, and you can’t be bothered to skim it off each time, there are a few things you can try in an attempt to reduce the amount of foaming.

First, try rinsing your lentils before cooking them. This will help to rinse away any excess starch from the lentils, as well as leaching out some of the saponins. This way, when you then add fresh water to cook the lentils, there will be less chance of ending up with a pan that’s 50% foam.

You can also try adding a spoonful of oil to your lentils as they cook, which forms a barrier around the lentils and reduces the amount of foam that forms on top.

Whether you leave the foam on your lentils or not, whether you try to prevent it or just let it do its thing, you’ll always end up with a delicious, nutritious end result – so stop worrying and go and eat your lentils!

Extreme close-up of red lentils.

Becca Heyes

Becca Heyes is a full-time writer, recipe developer, and bean eater. She spends most of her time writing for her vegetarian food blog, Easy Cheesy Vegetarian, and lives in the UK with her husband, two tiny children, and even tinier dog. She thoroughly enjoys eating excessive amounts of lentils as research for this website.

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