Marcelle was making a big batch of feijoada and started the black beans soaking the night before we were going to have it for dinner. As she was preparing to pour off the blue/purple water they had soaked in I asked her to save some for me.
The dinner was delicious. And I decided that I'd see what colors I could come up with after dinner. In the picture below you can see the spectrum of colors obtained from my black bean juice with vinegar and baking soda.
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| Black bean indicator: acidic on the left, neutral in the middle and basic on the right. |
If you want to get some indicator in 10 minutes instead of 10 hours you can soak the beans in 70% isopropyl alcohol. The alcohol will make a solution much faster.
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| Acid, neutral, and basic forms of anthocyanins. |
The primary color changing molecules in black beans are anthocyanins. As the pH of the solution changes the structure of the molecule changes. This changes its absorption spectrum thus changing the color of the solution. These reactions are in equilibrium making them great natural indicators.
5 comments:
I found that grape juice soaked into a cloth made a pretty good indicator. It turned bright blue with base and grape juice purple with acid.
Grape juice is a good indicator because it too has lots of anthocyanins.
I have noticed grape juice as an indicator, but never black beans. What about beets?
The major coloring agent in beets is called betalain and it is a pH sensitive molecule. I have not yet played with it to see what range of colors and pHs it has.
You can see its structure here
http://en.wikipedia.org/wiki/Betalain#Chemistry
A much more saturated indicator solution can be made by mixing 20 grams of beans with 50 mL of water and microwaving on high for 1 minute. Dilute the liquid 1:1 with cold water. Use 1 mL of the dilute indicator solution per 0.5-2 mL of testing liquid.
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