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Why Does Cork Stain?

Three used wine corks side by side. The right cork is clean, the middle cork is highly stained with red wine, and the third cork has no staining and has markings from a commerical winery.

Welcome to a new series, “Tom Talks Wine.” Last month we introduced Tom, who has been making wine for 50 years! Figuring all that time has to lead to some useful knowledge, we have wrangled him into a once a month column on wine. He starts with a question about cork stains and corking.

Dear Tom,

Over my life I’ve been lucky enough to befriend many home winemakers. This has resulted, of course, in my receiving numerous bottles at various points. I have begun to wonder about the process from grape to glass. Enough that I’ve started my first batch of red wine myself.

As it’s getting on to bottling time, I have been noticing that some corks in homemade wine look soaked through and some do not. Since I never see it in commercial wine, I’m assuming this is one of those things that homemakers do “wrong” somehow. I’ve attached a photo to show what I mean.

What’s going on to cause these stains, and how can I avoid them when I bottle?


Wondering about Winemaking in Washington

Hi Wondering,

You are correct. Something the winemaker did at home caused this. I have a few ideas of what probably happened, but first a little cork history.

Cork is the spongy bark of certain oak trees (typically Quercus suber) that are twenty-five years old or more. Q. suber does not take well to cultivation so the plains of Portugal, the largest producer of cork, are not covered with tidy rows of cork trees like a palm oil plantation. They are haphazard and therefore production is limited. Demand for cork is not. Almost everyone making wine wants cork. The screw cap is gaining in popularity and may help reduce demand in the future. The best corks come from old trees whose bark is the densest and thickest with few flaws (gaps, holes, etc). These are the most sought after and expensive.

Home wine makers rarely get the best corks simply because we don’t want to pay for the best corks. So we use corks with a lot of flaws. We can also be sloppy about how we fill and treat our wines at bottling. Overfilling (less than an inch of wine below the cork) can result in barometric pressure pushing the wine out. Not letting the bottle rest upright for at least 24 (but 48 hours is better) after bottling is also important, as it lets the air pressure within the bottle equalize with the air outside. This rest period also allows the surface of the cork to dry, which prevents wicking of the wine through a wet cork.

Thus, the answer to your question is three-fold. The bottle of wine was overfilled and the compressed air inside was not allowed to equalize, pushing the wine through the cork. Further, the freshly inserted cork did not get sufficient time to dry out before being laid on its side and wine began to wick out. Finally, the winemaker used too narrow of a cork. The less expensive hand corkers do not handle the larger corks very well.  The wider the cork and the less flaws the less wicking and tighter the fit.

Notice how many gaps, slits and holes are in the stained cork. The other two do not show these faults. All too often bottling a wine is the last thing home winemakers do and it’s where they decide to be tight with their money. Quality corks cost.  Even commercial wineries spend as much as $1.00 – $1.50 for a single cork.  If this cost is too high for the home winemaker there will always be some risk of leaking or even wine loss.


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