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I ran out of organic brown rice today, and I happened to have some lentils, but when I went to rinse them with the spray nozzle on my sink (set to warm) they started to create a bubbly foam. The lentils were in a bowl not a strainer. I'm not used to this, so I'm wondering if it's natural. But I'm also recalling how when I made homemade potato chips and when I rinsed thin slices of potatoes I got a similar foaminess.

What's causing this foam? Is it natural with lentils?

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Yes, it's perfectly natural. Dust being rinsed off the outer surface of the lentils makes the water more viscous and helps to trap air in the form of small bubbles. In a strainer, the dust would be rinsed away, so you don't see the same effect; a spray nozzle especially adds a lot of turbulence, which causes more air to get trapped. You can get similar results when rinsing many processed, dried grains or pulses.

The effect from potatoes is similar, but it's caused by proteins in the potato being rinsed out into the soaking water. Neither has anything to do with the presence of arsenic, so don't worry.

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    I just assumed it was some unseen waxy, phospholipid bilayer-y kind of answer. They're organic, so I was curious as to why they smelled chemically. BTW, I can easily look this kind of stuff up, but I like to see what Internet people know. – user62511 Oct 29 '17 at 4:47
  • I've never seen dust create foam, nor have I seen dust make water viscous. What's the source for this assertion? – RonJohn Nov 20 '19 at 3:51
  • @RonJohn you've never seen mud? That's an extreme case but it's the same principle. Any particulates suspended in the water interfere with smooth flow of the water molecules and create viscosity. It's not an especially pronounced effect but it's enough to trap small bubbles. I'm my own source here, this is based on memories from college chemistry and a few sources I looked up online when I wrote this answer in 2017 but didn't think to record. – logophobe Dec 23 '19 at 15:22
  • I thought of mud when writing "nor have I seen dust make water viscous", but mud is a suspension not a fluid. – RonJohn Dec 23 '19 at 15:59
  • Besides, the correct answer to the question is "saponines". – RonJohn Dec 23 '19 at 16:01
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I've been given a jar of water from drained cooked lentils. Shake it vigorously and it will produce foam. Whisk it and I'm told it will turn stiff like the consistency of whipped cream!

  • This is not related to rinsing lentils. – rumtscho Dec 30 '18 at 22:51
  • Helen, you might want to read up on 'aquafaba'. – Joe Dec 30 '18 at 23:39

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