Seems like an attack of your friend and mine, the acetic acid bacteria. These bacteria live in wineries, on winery equipment and in the air. In fact, you’re probably breathing some in right now. Unfortunately, when these little guys come in contact with wine and oxygen, they tend to produce acetic acid, the stuff that makes vinegar smell and taste, so, well, vinegary.
Even though your customers sound like fine winemaking folk, even the best of us come up against acetobacter once in a while. As you pointed out, unclean equipment can be a contributing factor to this vinegar smell. This seems to be only part of the issue, though. Like I said above, acetic acid bacteria are everywhere. It’s very difficult for winemakers to totally eradicate them from the winemaking environment. All we can do is find ways to live with them.
Acetic acid bacteria need the following things to survive: oxygen, a hospitable environment and a food source. By controlling these factors we can reduce the chances that acetic acid bacteria will find, infect, survive in, breed in and make acetic acid in our wines. We can do this by the following:
- Keep equipment scrupulously clean at all times.
- Keep containers as full as possible, as acetic acid bacteria thrive in half-empty containers that are, by definition, half full of air.
- If there must be headspace in a barrel or carboy, blanket the surface of the liquid with carbon dioxide or nitrogen gas, if available.
- Keep pHs low (under 3.7) so that microbes will not be able to survive as well in your wines.
- Use sulfur dioxide as an antimicrobial agent, keeping free SO2s between 20 to 35 ppm (mg/L).
- Store your wines in a cool, dry area. The lower temperatures and dry air will discourage not only acetic acid bacteria but molds and fungi as well.
- Watch wines that have low alcohol levels (below 10 percent). Alcohol acts as an antimicrobial agent to some extent, and wines with low alcohol levels are especially susceptible to attack by bacteria.
- Finished wine, or wine that has just finished fermentation, is the most vulnerable to acetobacter attack since the protective layer of carbon dioxide produced during fermentation is no longer present. Keep these vulnerable wines especially clean and topped up.
- Acetobacter are often transmitted to wines by insects like fruit flies. Do your best to clean up all spilled juice, must, skins and wine before you give fruit flies – and acetic acid bacteria – a chance to thrive in your winery. Immediately clean up spills wherever they occur, and especially keep tops of barrels, carboys and fermenters clean and free of residue.
Those tips should help both of your customers. Your first customer, who keeps his wine in filled bottles but is having problems with haze, needs to check his pHs and make sure they’re not over 3.6 to 3.7.
High pH wines lend themselves readily to both bacterial attack and haze formation. In fact, haze and instabilities are often the result of bacterial attack. Your second customer should store his wine in a more air-tight way. Even though airlocks and lids serve their purposes, air laden with oxygen and bacteria enters the carboy every time the fermentation lock is taken off to siphon out a carafe. I strongly suggest he bottle his wine or that he rack into smaller, one-gallon (3.8 L) jugs as the level in his carboy gets lower.
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