Well, it seems like you have been paid a visit by a colony of Acetobacter, aka acetic acid bacteria. They love air, eat alcohol, and turn it into carbon dioxide and vinegar. Not fun. The biggest issue is that no matter what, once you’ve got them and they’ve produced a certain amount of acetic acid, (aka “VA” or volatile acidity) there’s little you can do to remove it from your wine. Bigger commercial wineries can hire in reverse osmosis filters to remove some of the VA for you, but sadly home winemakers make far too little volume to make this kind of treatment worthwhile.
The only option really available to you is to blend this carboy out with other wine to lessen the level of the vinegar smell. However, you and my loyal readers might know that I always advise you to never “blend a loser” i.e. don’t throw good money after bad. If you don’t love the new blend then you may be better off just tossing the offending Merlot batch and making a new one. That’s the beauty of kit winemaking after all, you can make wine any time of year when buying a kit off the shelf instead of relying on grapes.
So what can you do in the future to ward off future attacks of the sneak Acetobacter? Read on for some handy tips.
— Make sure your SO2 (sulfur dioxide) levels are high enough and are appropriate for the pH of your wine. SO2 becomes less effective as the pH rises, so for red wines, especially, (which tend to be lower acid- or higher pH- than whites) it’s important to store wines with at least 28 ppm free SO2.
— Be sure your pH is in the “safe” range not just for sulfur dioxide effectiveness, but because acidity naturally inhibits Acetobacter, which love a higher pH environment. I try to keep my red wines under 3.65 pH. It’s OK to add tartaric acid after malolactic fermentation if you need a little tweak downwards.
— Keep dry wine completely topped up, i.e. in completely full containers. Young wines like yours are especially vulnerable to Acetobacter attack since the protective layer of carbon dioxide gas produced during fermentation is no longer there. Here’s a trick for if you don’t have a completely topped-up carboy: Pitch some washed, dried glass marbles (or glass decorative pebbles, often found at a nursery or home decorating store) into your carboy. They’ll sink to the bottom and displace air, forcing the level of the wine up into the neck of your carboy. Just don’t use decorative rocks, even if they are round and clean — there’s no way of controlling what kinds of heavy metals or other undesirable toxic compounds could be leached into your wine!
— Immediately clean up spills and messes in your wine storage area. Having any kind of food (puddles of wine after racking, anyone?) for bacteria and spoilage organisms to munch on will only increase the chance of them getting a foothold in your winemaking area.
— Keep all equipment as clean as possible, even between uses.
— Watch wines that have lower alcohol levels, especially if they are under 10%. Alcohol acts as a natural antimicrobial agent (the reason why Saccharomyces produces it) inhibiting spoilage bacteria. If your Merlot is particularly lower alcohol, this could be a contributor to an Acetobacter outbreak.
— See a fruit fly? Try to get rid of Drosophila melanogaster as soon as you can. The best ways to control these little buggers that can carry all sorts of nasty spoilage bacteria on their dirty little feet is to exclude them from your winemaking environment altogether. They need a food source to survive so, as per above, make sure you’re cleaning up spills promptly and are getting rid of any sources of standing water. Make sure your winemaking and wine storing areas are completely screened off and if possible try to have some kind of positive air-displacement mechanism if you can manage it. I’ve even found that a fan positioned so it blows towards a door can help push these guys back and keep them from entering every time the door is opened.
—Store your wines in a cool, dry area. Lower temperatures and dryer humidity discourage acetic-acid causing bacteria as well as other potential spoilage yeasts and fungi.