I noticed that when I cook black lentils and add lemon juice to the water that's left, the water turns from black to pink! What can be the reason for that?
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2 Answers
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In the black lentils used in Indian cooking (Vigna mungo), the natural water-soluble dye that causes the color change has been identified as quercitin-3-O-pentoside:
which belongs to a class of compounds called flavonols. When you pop hydrogen ions on and off some of the -OH groups, you can get a pure benzenoid form (colorless), a quinonoid form (pink), and an extensive quinonoid form (yellow). The color changes are pH dependent, and you get pale pink at pH < 7, yellow at pH > 7, and colorless at pH 7. When you add your lemon juice to the lentil water, you're getting the quinonoid form.
Other species of black lentils probably do something similar.
The pH-dependent color change occurs because when you have $-\mathrm{OH}$ groups on the rings instead of $-\mathrm{O}^-$ groups, the way electrons are confined changes; that changes the spacing between the molecule's energy levels, and changes the colors of light the molecule can absorb. (See my article here for more about how this works).
Many plants and household materials contain molecules that can change color or otherwise indicate changes in pH (See my article here for a list of some of them). Almost any flower, fruit, or plant part that is red, blue, or purple contains a class of chemical compounds called anthocyanins that change color with pH. The color of a flower or fruit depends on which anthocyanins are present, the pH of the pigment-bearing tissues, and the presence of other pigments, like yellow flavones.
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1$\begingroup$ Nice article about household indicators. Should update the ExLax part since phenolphthalein was banned from over-the-counter laxatives in 1999. fda.gov/ohrms/dockets/98fr/012999d.txt $\endgroup$– DavePhDCommented Feb 5, 2015 at 12:55
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$\begingroup$ Thanks, DavePhD---I didn't know that. I'll fix it; that site is long overdue for an update and I'm going to launch a complete makeover this summer. That particular article was written in 1998. $\endgroup$ Commented Feb 5, 2015 at 13:09
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Black lentils contains a natural pH indicator. These dyes change their colour depending on how acidic (or alkaline) the solution is. The dyes are water-soluble, consequently you can observe the colour change in the remaining water too.
The natural dyes, which often belong to the class of anthocyanines, are found in other food too, such as in red cabbage.
answered Feb 5, 2015 at 7:22