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I opened up a bottle of champagne that was left in a wine cooler for approx. 8 years. To my astonishment there were small crystals at the underside of the cork.

The crystals are colorless, odorless and have no specific taste at all. What is going on here?

Image:

Colorless crystals at the bottom of a champagne cork

Close up:

Close up

tschoppi
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1 Answers1

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They are potassium bitartrate crystals (source).

The crystals form because the potassium bitartrate is not very soluble.

Since solubility is a function of temperature, when wine is chilled the solution can become saturated, causing the precipitate to form in the bottle. Stabilizers are often added to prevent this.

In your case, the precipitate is on the cork, and so what I think has happened is that some wine soaked into the cork, perhaps while the bottle was moved. Then later, the bottle was stood upright or on its side where the wine could not reach the cork. As the wine evaporated back into the bottle, the local concentration of potassium bitartrate increased until it reached the saturation point, at which point the crystals grew. Since the crystals take a very long time to dissolve once formed, they will persist even if wine reaches the cork again.

The good news is, they are harmless (especially good news given that you tasted them - please don't make tasting strange crystals a habit!). They are made of the same substance that cream of tartar, which is a common baking ingredient, is made of.

thomij
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  • Ah! I was thinking of acetic acid, until I read "colorless, odorless and have no specific taste at all". – Jori Aug 10 '14 at 11:04
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    Actually, the cork got soaked during the whole storage time. Maybe the cork just provided a good surface for nucleation?

    And I want to add: I took a small sample of the crystals and tasted that because it occurred in a place where I could have consumed it anyway. As such I deemed it harmless enough to warrant this risk.

     – tschoppi Aug 10 '14 at 11:13
  • Yes, you are probably right in that case, that the cork was providing a nucleation site and the wine was super-saturated. This is especially likely if there were any stabilizers in the wine, since they would not be as effective when not in solution.

    And I'm glad you were being careful in tasting them - my comment was meant as a joke, but it is true that as a general rule people shouldn't taste unidentified chemicals.

     – thomij Aug 10 '14 at 11:24
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    The crystals are very common in wine (especially wine made from some varieties of grapes like Shiraz). They also have a major part in the chemical understanding of optical activity as one of Pasteur's achievements was to separate crystals of a tartaric acid derivative into pure optically-distinct crystals. – matt_black Aug 10 '14 at 17:40
  • Given that this is champagne, I'd wonder if the pH drops as the CO2 increases after bottling the uncarbonated wine, thus protonating one of the carboxylic acid groups. pka1 = 2.89, pka2 = 4.40 – MaxW Sep 20 '16 at 08:24