Since you report the wine “has become carbonated,” it sounds like it was still when you put it in the bottle and you wanted it to stay that way. It also sounds to me like you have experienced a classic (but unwanted) re-fermentation in the bottle. Essentially, re-fermentations are avoided either by controlling the microorganisms that perform them (yeast or bacteria) or by controlling their food source (sugars in the case of the former, malic acid, alcohol and other substrates in the case of the latter). Whether caused by yeast, bacteria or a combination of both, post-bottle refermentation is one of the toughest problems a winemaker can face and one of the issues we all should work our hardest to avoid. First let’s walk through the possible causes and their prevention, then let’s explore your options for saving the wine.
Re-fermentation, whether in the bottle or during bulk storage, occurs when ambient microbes find and metabolize a food source in wine at a time not intended by the winemaker. In the case of Saccharomyces cerevisiae (wine yeast) cells, a re-ferment can happen anytime there are yeast present and there is still fermentable sugar present in the wine. During bulk storage (in barrel, tank or carboy), a yeast re-ferment is not the end of the world because the fermentation can finish out and you still have an opportunity to rack off the settled lees and bottle a still, clear wine. In the bottle, however, a re-fermentation is disasterous because the carbon dioxide, along with a sediment of the dividing yeast cells as they live and die off, are trapped inside the bottle with no means of escape. This leaves you, the winemaker and wine drinker, with a fizzy, cloudy and sometimes smelly bottled wine. Not pleasant.
Yeast are the most common re-fermentation culprit because they are ubiquitous in the winemaking environment and have a much higher alcohol and sulfur dioxide tolerance than most bacteria. To control refermentation caused by yeast, a winemaker must take away the food source, i.e. make sure that wines ferment to dryness — most wineries consider under 0.3% residual sugar to be dry but I use the more stringent number of 0.1%. You can check for the completeness of the alcoholic fermentation by using a simple “sugar pill” test sold by many winemaking supply stores or by sending a sample to a wine lab for a more accurate glucose/fructose assay. The winemaker must also control the yeast cells. The easiest way to do this is to inoculate your juice or must with an off-the-shelf yeast strain that is suited for the sugar concentration you’ve got. Particularly strong yeast, like EC-1118 (also called Prise de Mousse), have been known to motor through predictable alcohol levels of 15% or more and still conduct a fermentation to dryness. It’s also smart to follow good, sound winemaking practices that will avoid a stuck fermentation and therefore, residual sugar being left in your wine. Make sure the temperatures of red musts don’t get above 85 °F (29 °C), that the yeast have proper nutrition and that you don’t have a concurrent microbial infection going on. Read “Wine Wizard” from October-November 2002 for more details on avoiding a stuck fermentation.
One culprit is a somewhat rare and especially nasty non-Saccharomyces yeast, Brettanomyces, which is able to survive in wine conditions even when all fermentable sugars are gone. Believe it or not, Brettanomyces can eat carbohydrates (sugars) present in the wood of barrels. These five-carbon sugars, or pentoses, are unfermentable to wine yeast and can fuel a Brettanomyces bloom. If you want to find out if Brettanomyces caused your re-fermentation, you may want to order a test kit from White Labs (www.whitelabs.com) or send a sample of your wine to another wine lab for culturing.
In addition to yeast, there are plenty of bacteria that will happily munch on your wine if the conditions are right. Unless sterile filtered (and sometimes even then), there will always be bacteria of some kind living in your wine, even when it’s bottled. The key is as before: control the food source and control the microbes and you’ll avoid a re-fermentation. Bacterial food source control is a tough one as there are more than one class of things these guys eat. The most important possible food source to focus on is malic acid. Grapes naturally have a healthy amount of malic acid in them right off the vine. There are also a lot of microbes in the environment, especially Lactobacillus and Oenococcus, that enjoy malic acid, digest it into lactic acid and spit out carbon dioxide and aromatic compounds (like diacetyl, which smells like popcorn butter) in the process.
The best way to prevent a malic acid refermentation in the bottle is the old-fashioned way — by encouraging bacteria to eat the malic acid before it goes into the bottle. You can test its disappearance with home test kits or by sending samples out to a commercial wine lab. As you might predict, the most reliable way to make sure you get a complete malolactic fermentation is to inoculate with a known, strong malolactic bacteria culture purchased from your favorite winemaking supply. Oenococcus oeni is a preferred strain out in the commercial realm right now though supply houses (see list below) constantly come out with new strains every year, some suited especially for certain challenges like a high-alcohol environment or a low pH wine.
If you don’t put your wine through the malolactic fermentation (sometimes called the secondary fermentation in older winemaking manuals) then you’re going to have to use either sulfur dioxide levels (use more the higher the pH of your wine) or filtration to knock down microbial populations. An 0.45 nominal micron pad, or even one of the newer 0.45 micron-pore inline filter cartridges, will go a great length towards reducing the number of yeast and bacteria present. They are most effective if used right before going to bottle because wine that is filtered and then put back to bulk storage can easily pick up just as much microbial life again. With adequate free SO2 levels (28–35 ppm) you should be able to keep most bacteria in check as they’re particularly sensitive to SO2. Use more SO2 with higher pH wines. For example, I would feel comfortable carrying a dry, white malolactic incomplete wine with a pH of 3.25 at 28 ppm FSO2 while a dry red wine with a pH of 3.75 I would keep at 32–35 ppm. For these reasons, sanitation and proper SO2 levels at bottling is especially critical. Analyze how you bottled your batch of wine and make sure you did everything you could’ve to minimize the number of bacteria and yeast in the wine as you went to bottle.
Incomplete fermentations are the most common cause of re-fermentations in the bottle for the micro-producer. In addition to shooting for dry fermentations, as you take your wine through each stage of its lifecycle, it’s important to think like a microbe and mentally troubleshoot. A classic example of this is what I call the “blending refermentation.” This occurs when the unsuspecting winemaker blends two seemingly still and stable wines together and then stands by shocked as the new blend begins to bubble and take on a life of its own. This happens because, while the two wines separate were microbially stable (no food source or no microbes willing to eat it), together, they provided an environment favorable for a refermentation.
Sometimes microbes can almost lie dormant in a wine and will “wake up” when something new and exciting, like a certain amino acid or a new infusion of vitamins, comes their way. Sometimes a new ambient microbe will be introduced to the mix because you didn’t sanitize your hoses properly. Other times microbes will wake up after sleeping all winter; some believe that sparkling wines were discovered in Europe when cold, sluggish yeast cells revived in the warmth of a Medieval spring thaw.
As for salvaging this batch of wine, I’ll let you decide which path to take but I’ll caution you that, after decanting and trying to re-ferment the wine, it’s unlikely this lot will ever be as good as it was when it went into the bottle the first time around. Stuck or sluggish fermentations, and their subsequent re-fermentations, are microbial playgrounds and it’s likely you’ll find a lot of the original, ephemeral aroma has gone only to be replaced by spoilage aromas.
If you do want to try to attempt to get the wine to settle down, I encourage you to find the cause so you’ll know what you can do with the wine and what you can’t. If you have residual sugar you can attempt to re-start a stuck fermentation and see if that helps. If you have residual malic acid, you can try to inoculate the wine with malolactic bacteria and see if that does it.
The lazy-winemaker’s answer to this problem would be to decant the bottles, wait until the wine is no longer producing carbon dioxide (whatever the cause) and has fallen bright (if it ever does) and then sterile-filter and bottle again. Whether or not you decide to go through all this trouble for the chance to drink a mediocre wine, or just wait until next harvest to do your next lot right from the ground up is, as always, your call.
Here is a list of wine laboratories that sell yeast and bacteria, provide analytical services and can help with troubleshooting difficult fermentations: