Well, according to specific gravity, your cucumber wine (sounds refreshing) is dry. For RS-dry (residual sugar dry) you want to look for an SG of 0.992 and 0.996 on your hydrometer. If your wine is dry, with no sugar left to ferment, and it’s still actively producing CO2, then the next thing I’d wonder about is this: Is there another microbe in there eating something and producing CO2?
Since I’m not too familiar with the chemical makeup of cucumbers, I did a little quick research and lo and behold, it turns out cucumbers are chock full of malic acid. And what common wine microbe eats malic acid and pumps out carbon dioxide gas? How about your friend and mine, malolactic bacteria? These guys are often purposely inoculated into wines for this specific purpose — to render wines more microbially-stable during storage and bottling, as well as to naturally de-acidify them. Typical of these bacteria are Oenococcus oeni and various species of Lactobacillus and Pediococcus.
If I had to put money on it I’d wager you’ve got a spontaneous secondary fermentation happening and you’ve got some bacteria in there that are actively metabolizing your wine’s malic acid and spitting out lactic acid and carbon dioxide, along with possibly some other flavor and aroma compounds.
Adjusting acidity is critical and so precipitation of solids just has to be put up with . . .
If you want to stop the malolactic fermentation from happening, i.e. if you like the acid balance and don’t want to risk any lactic acid bacteria off-flavors, you may want to add sulfur dioxide in the form of potassium metabisulfite powder, a Campden tablet, or liquid SO2 solution, whichever method you use. You could also chill the wine down — is it possible to store it in a fridge so you can retard the fermentation?
If you want to de-acidify a little bit and don’t
mind the aromas and flavors you’re getting, you could always let this
second fermentation go to completion. To test for completion, you can
send a sample off to a wine lab or you could go low-tech and wait for
these bubbles to subside before you rack and add SO2 (sulfur dioxide). You could also add SO2,
chill, and then filter if you are able. Not sure about you, but my gut
tells me that you’ll make a better cucumber wine if you don’t let it go
through malolactic fermentation (MLF). Basically, MLF, while it can
de-acidify a wine, also produces a wide variety of aromas, some of which
might not match the delicate aromas of your raw material. I’m not sure
how well diacetyl (butter) would go with cucumbers in a wine. I’d shoot
for fresh and crisp and not let any of the sometimes-weird secondary
aromas creep in.
If you just add SO2 and leave residual malic acid in your wine, you’ll have to be really vigilant on maintaining a free SO2 of around 30 ppm or so (more, if the pH of your wine is higher than 3.5-ish), to retard this fermentation through aging and into the bottle. Alternately, you could sterile filter your wine to exclude the bacteria.
A cucumber/veggie wine sounds very refreshing on a summer day! Way different than I make in California but maybe someone somewhere will make something cool for the tasting room one of these days . . .