How do I tell a friend his homemade wine is bad?

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Dear Dr. Vinny,
A friend gave me two bottles of homemade wine for Christmas. Both of them tasted off—I suspect bad corks. Should I say anything? I have a feeling he might be giving away more of this wine. What’s the right thing to do?
—Lee, Napa, Calif.
Dear Lee,
That’s a tough one. It partly depends on how close you are to your friend, but in most cases, I think you should say something. Times like this I like to imagine the roles reversed: Would I want to know if I were giving away flawed wine? Yes, I would.
I think the way to phrase this is from a place of gratitude: “Thank you so much for the wine. I couldn’t wait to open the bottles and try them, but I was concerned because both bottles seemed a little off to me.” Bonus points if you kept some of the wine and you could ask him to try it and see if it is consistent with what he expected the wine to taste like.
Of course, this scenario works best if your friend can recognize and understand the flaw, and agree with you.
I think there are two possible scenarios of what might be wrong based on your comment about corks. The first is TCA, or 2,4,6-trichloroanisol. That’s the technical name for a “corked” or “corky” wine—where the flavors take on musty, dank or moldy notes. It can develop in corks, which is why TCA is associated with tainted corks, but in fairness, it can also develop elsewhere—your friend might be making wine in a TCA-infected area. If he’s a home winemaker, I think that’s very possible. Most people don’t understand that household cleaners that contain chlorine increase the possibility of TCA, since chlorine is one of the ingredients in this non-toxic but wine-ruining flaw.
The other scenario is that the corks aren’t sealing properly, and that they are letting oxygen into the bottles, muddying the flavors and making them take on tired, nutty notes. An old batch of corks stored in a dry place, plus a hand-corking machine could very well result in oxidized wines.
In either case, I would be gentle about your observations and bring a bottle of wine with you to enjoy together as you figure it out. Good luck!
—Dr. Vinny