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.
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’!
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.
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.