Magic color changing blueberry residue
August 8, 2009 6:18 PM   Subscribe

You buy a bag of delicious frozen blueberries and empty its contents into a plastic tub which you keep in the freezer. When the last of the berries have been consumed, a light pinkish-red residue remains in the tub. You proceed to rinse out the container, and after a few seconds under running water the color suddenly and dramatically changes to a deep dark blue. Why?

The wikipedia article on anthocyanins mentions that the difference in chemical structure that occurs in response to changes in pH is the reason why anthocyanins are often used as pH indicator, as they change from red in acids to blue in bases. Does simply diluting the blueberry-goo with water really alter the pH that significantly? Or is something else at work here?
posted by Rhomboid to Food & Drink (13 answers total) 8 users marked this as a favorite
Best answer: Likely it was already in a changed state due to low pH. When you added lots of water you diluted the acid substantially, so the indicator changed color.
posted by Chocolate Pickle at 6:20 PM on August 8, 2009 [2 favorites]

Best answer: I can't think of another explanation. Add some vinegar and see if it turns back.
posted by Fiery Jack at 6:36 PM on August 8, 2009

Best answer: Does simply diluting the blueberry-goo with water really alter the pH that significantly?

Consider that the blueberry goo including the acidic juices from the blueberries themselves, as well as additional added sugars or sugar syrups, and possibly some food enzymes or vitamin C to act as a preservative, which may increase the acidity. Then the goo may have thawed and refroze several times, which may have allowed it to partially ferment, again increasing the acidity.

The consider that you may have really hard, alkaline water. Voila. Dramatic change in color.
posted by Cool Papa Bell at 6:44 PM on August 8, 2009

Not an answer, but I consider that a great question. I have wondered that myself, but never thought of finding out!
posted by fifilaru at 6:52 PM on August 8, 2009

I'm inclined to think that Cool Papa Bell is on the right track with the hard water thing.

A couple of days ago I was dismayed to find blueberry juice leaking all over a white cotton skirt of mine. Result: deep reddish-purple stains all across the front, like a tie-dye session gone horribly wrong. Dousing the skirt with water did nothing to change the color. However, the vigorous application of a slimy bar of soap dramatically changed them to deep navy blue. It was like magic. I probably would've been more fascinated at the time if I wasn't so worried about my skirt being utterly ruined. After scrubbing at the skirt for about half an hour, I retreated to the Internet to find out what caused the chromatic witchcraft. I learned that soap was basic and blueberry stains are a bear to get out. Sad, but fascinating little lessons I learned that day.

*The skirt was saved in the end.
posted by Diagonalize at 7:06 PM on August 8, 2009 [1 favorite]

Does simply diluting the blueberry-goo with water really alter the pH that significantly?

Yeah, sure. pH is a function of the amount of hydronium running around. Within certain limits, a greater concentration of acid will result in a lower pH. For any given acid there's a minimum pH value it's possible to attain, but it can certainly be higher than that if the concentration is lower.

One teaspoon of sulfuric acid mixed into a swimming pool isn't going to yield something that can burn your skin off. Likely it would be imperceptible, and the pH would remain really near to 7.0.

By the same token, dilution of your blueberry juice will reduce the acidity of the solution, and it's completely believable that it could raise it high enough to make the indicator change.
posted by Chocolate Pickle at 7:28 PM on August 8, 2009

Diagonalize how did you save your skirt??
posted by txvtchick at 7:41 PM on August 8, 2009 [2 favorites]

Response by poster: I did as Fiery Jack suggested and tried adding a bit of vinegar and it indeed turned pink again quite quickly, so I guess that's really all that's going on. In the name of science I took some pics, but the only digital camera I have is terribly crappy so you can't really see the blue all that well:

Initial blueberry goo
After a quick rinse
After a slosh of vinegar
posted by Rhomboid at 7:55 PM on August 8, 2009 [9 favorites]

I needed to be cheered up, and this question made me happy. (:
posted by aniola at 9:32 PM on August 8, 2009

Also, though it may not be too relevant here though the container was frozen, pH is also temperature dependent. A fancy lab pH meter will also tell the temperature of the solution being measured.
posted by sevenless at 12:05 AM on August 9, 2009

Oh man, I can't believe I never realized that this cool blueberry effect is due to pH! I've wondered about this for a long time!
posted by Cygnet at 5:31 AM on August 9, 2009

By far the easiest such vegetable indicator to get is from red cabbage. You boil it and the pigment comes out in the water. Continue to boil the water in order to concentrate it. You can make indicator strips by soaking filter paper (e.g. coffee filters) in it. Turns red with acid and blue with base.
posted by Chocolate Pickle at 9:51 AM on August 9, 2009

Oh, gosh, didn't realize people were curious about the skirt. It just took a fair bit of vigorous scrubbing with soap and a little vinegar. I'd caught the stains while they were still wet, so it wasn't too difficult, just tedious.
posted by Diagonalize at 8:59 PM on August 12, 2009

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