It’s hard to tell exactly what may be the issue because wines that just finish fermenting and are so young often have “funny” smells and do indeed not smell like the finished wine that you are used to buying off a shelf. It’s hard to know if you are experiencing a bacteria or yeast-caused issue. If you didn’t complete primary fermentation it’s possible that you still may have active yeast that are possibly finishing a very long, protracted alcoholic fermentation. This would explain the “funny” odor and a release of carbon dioxide.
Your wine may also be going through malolactic fermentation (MLF), which is brokered by lactic acid bacteria. I’ve seen the volatile acidity creep on wines undergoing MLF, especially if there happens to be a bit of residual sugar sitting in the wine.
If, however, you’ve sent the wine out for analysis and your residual sugar is less than 0.20 g/100 mL (dry), your wine is ML complete, and your volatile acidity is climbing, I would suspect bacteria.
Acetic acid, or vinegar, in wine is most commonly caused by bacteria called “Acetobacter” eating alcohol and turning it into vinegar. Acetic acid can also be caused by yeast during primary fermentation, though this is less common. Air can exacerbate the production of acetic acid because the acetic acid bacteria are “aerophilic,” i.e. they need air to survive. Unfortunately, acetic acid bacteria are ubiquitous in the environment. You probably are breathing some in right now. When conditions are right, they find a food source, are exposed to air and are warm and “happy”, vinegar is an inevitable result. Here are my best tips for minimizing the production of acetic acid over the lifetime of your wine:
• Practice good cleaning and sanitation methods in winemaking
• Always inoculate with known organisms
• Aim for complete primary and secondary fermentations
• Keep dry (finished fermentation) wine away from oxygen
• Keep containers “topped up” to exclude air contact
• Store wine with free sulfur dioxide levels at least 25 ppm, higher if low alcohol, high pH or with residual sugar.
