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I opened a recently purchased wine (a Grenache 2012), and the part of the cork that was in contact with the wine looked like the picture

residue in cork

Part of the residue looked like small crystals, some whitish and others reddish.

What is that residue, and how did it get there?

2 Answers 2

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Tartrate crystals, they are formed from tartic acid which occurs naturally in grapes. From Wine of the Month:

First, let's deal with tartrate crystals. They are formed when a method called cold stabilization is performed on a wine. Cold stabilization is often done in white wines to remove excess potassium bitartrate, a natural substance found in grapes also known as "cream of tartar." White grapes contain fairly large amounts of potassium bitartrate. If most of it is not removed, the wine will form crystals when placed in the refrigerator. These tartrate crystals will either cling to the underside of the cork, if the bottle is on its side, or fall to the bottom of the bottle, if it's standing up, where they appear to be ground glass to the uneducated eye

Also from the same article:

it's totally harmless

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  • Whoops! You beat me by a moment. +1.
    – hoc_age
    Sep 4, 2015 at 17:36
  • Note that their density is more similar to that of the wine than the sediment in red wine, which means they are more difficult to remove by decanting. The 'fining' process (which often involves strange things like egg white and china clay) is designed to remove them, so wines using organic or vegan methods is more likely to have more tartrate.
    – abligh
    Sep 4, 2015 at 19:25
  • Thanks. I assumed they were harmless and drank the wine anyway :)
    – user1906
    Sep 4, 2015 at 22:02
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You have a common, harmless situation: tartaric acid or other tartarates. They are naturally occurring in grapes (and other plants). They tend to collect on the cork (especially in red wines) and at the bottom (especially in white wines).

They're harmless and tasteless, though they do have an unpleasant gritty texture. Be careful to leave them in the bottle when pouring the last bit out of the bottle if you see some sediment, or just leave the last sip in the glass.

Several other articles have more detail about this, including from...

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