Hi from East TN. My sister made wine back in 2018 and I have a few questions

[brewingintenn.]

Hello,
Came here hoping for some answers, I'm looking into making beer, wine and hard ciders. My sister started making wine 2018 but has stopped, she doesn't drink and lossed interest in making it, anywho, One of her bottles from 2018 I think it was red wine from grape juice, has a white ring on the inside of the bottle, when you tilt the bottle the ring stays put so it's stuck to the glass not the wine, we're trying to figure out, 1. What is it? 2. Is it safe to drink? 3. How did it happen? none of her other bottles have this, but one of her bottles was more sour, vinager like then the others, so I'm making vinager out of it, (hopefully)
anyway, Thank in advance.

 

[lumpher]

Most likely mold, since it's sour and vinegary. It wouldn't kill you to drink it (most likely), but I sure wouldn't. That usually happens because, when sanitizing the bottle, you miss 1 little speck of something that has bad bacteria in it.