When you talk about how or why a wine turns to vinegar, you have to start with the acetic acid. This is the stuff that makes vinegar taste like vinegar. It’s the tang. If you had acetic acid in a jar and poured some of it into a fruit drink, it would immediately start to take on the characteristics of vinegar. It’s what makes vinegar, vinegar.
Acetic acid is made by a bacteria known as acetobacter. This bacteria is everywhere: in the air, on fruit, on grape presses, etc. When acetobacter gets into your wine it can slowly turn the alcohol into acetic acid, if left unhindered.
The key takeaway here is: if you control the acetobacter; you control the vinegar.
During a fermentation the wine is relatively safe. The gases from the fermentation help to keep acetobacter fall-out from getting into the wine must. Also, domesticated wine yeast are bred to inhibit bacterial growth, giving them the term killer yeast. It is not until the fermentation is over that caution needs to be taken.
Fortunately for us winemakers, acetobacter can be very easily destroyed before they have a chance to colonize and produce acetic acid. All it takes is the addition of sulfites in the form of either Campden Tablets, Sodium Metabisulfite or Potassium Metabisulfite. Use any of these in the wine, and the acetobacter will be very easily destroyed.
I would suggest using sulfites 24 hours before adding the wine yeast. Do not use an air-lock or lid on the fermenter during this time. You need the sulfite gases to dissipate into the air during the 24 hours, otherwise it will remain in the wine must and destroy some or all of the wine yeast when you add it. I also suggest adding sulfites just before bottling to keep the wine fresh while in the bottle. You can find more information on our blog about when to add sulfites to your wine.
Make sure you use a domesticated wine yeast to keep your fermentation clean. And, always sanitize your homemade wine equipment with a sulfite/water solution before using it.
The bottom line to the question: “why does wine turn to vinegar?” is because acetobacter was allow to grow in your wine. Stop the acetobacter, and you’ll stop the vinegar.
Happy Wine Making,
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.