Wine Wizard replies: First of all, you need to make sure that the fermentation is finished. You do this by checking your sugars with a hyrdrometer. Fermentation for a “dry” wine is completed when the hydrometer reads 0.0 degrees Brix (or 0.990 specific gravity) and remains there for several days (or weeks). An even better way for home winemakers to check residual sugar levels is to buy the Clinitest tablets that diabetics use to determine sugar in their urine. To do the test, simply drop a tablet into a sample of wine. The wine will change color, and you compare the color against the chart provided in the kit (reading for wine, not urine!). You might possibly be experiencing the joys of what I can only assume to be re-fermentation or secondary fermentation. This scenario can happen to red, white or rosé wines and are not necessarily a bad thing. Though it sounds like it’s a factor you didn’t expect and don’t enjoy too much.
Re-fermentation occurs when your wine has not gone completely dry (absolutely no sugar left for the yeast to ferment) and residual yeast (or ambient yeast picked up during a racking or filtration) hop in and ferment what’s left, creating the fizziness you’ve experienced. If your wine wasn’t dry when you filtered it for the first time, this is most likely what happened. Adding glucose to the carboy after the wine was filtered could have contributed to this by giving the yeast a new energy source.
Some winemakers actively encourage this re-fermentation, namely producers of sparkling wines. If you bottle the wine with a small percentage of residual sugar and bottle the wine with crown caps like a beer bottle, then you capture the carbon dioxide in the bottle itself as opposed to having it dissipate in the carboy. With the right base wine this extra fizz can make a ho-hum wine sizzle.
If your wine was dry at the time of your first filtration, then what you are most likely experiencing is carbon dioxide released by malolactic bacteria. Malolactic bacteria float around freely in the air we breathe — you don’t have to inoculate for them. These bacteria landed in your wine and began munching on the malic acid in your wine. You’d have to have a sterile filter and a sterile environment in your home winery in order to not have these guys hanging around — not even commercial wineries go that far! In fact, many winemakers actively encourage this secondary fermentation because it stabilizes the wine for bottling by pre-empting any ML bugs that might attack the residual malic acid when the wine is in the bottle. This creates fizzy wine in a bottle as opposed to fizzy wine in a carboy, which will dissipate with time. ML fermentation can also create some aromatic characteristics that are desirable for certain wine styles, namely the buttery-oaky New World Cabernet Sauvignon and Chardonnay table wines that are currently so popular.
If fizz really bothers you, you should try to ferment your wines dry and not bottle them until they’ve gone through malolactic fermentation. If you don’t like the characteristics that ML fermentation contributes to your wines and want to actively discourage ML fermentation, keep the pH below 3.30, the wine relatively cool in the cellar (below 60° F (16° C)), rack your wine cleanly during its cellaring lifetime, keep sulfur dioxide levels at 25 to 50 mg/L free SO2 and bottle as cleanly as possible (trying to avoid transferring any of the lees or too much air). You can monitor pH and sulfur dioxide with simple kits available at any home winemaking store.