My Wine, Mead, Cider or Beer Won’t Quit Fermenting
The Winexpert instructions or recipe indicate that the fermentation should be complete, but the carboy or bucket continues to bubble and bubble and bubble. The first thing to check is the specific gravity. If it is indeed too high, give it a good stir and check the temperature of the product using a thermometer. Make sure the temperature is at the high end of the specified range in the Winexpert kit or as specified for the yeast strain doing the work.
If both the specific gravity and temperature are within the specified range you may be experiencing one of two phenomena. First, your fermentation may actually be complete, but due to temperature swings or barometric pressure changes, it is out-gassing carbon dioxide at a rate that actually looks like an active fermentation. This may be exacerbated if the fermentation occurred rather slowly as cool fermentations like lagering or cold weather snaps promote CO2 saturation in solution.
CO2 is soluble in a liquid solution in inverse proportion with the temperature of the solution. Think about a can of soda. If you pull the tab on a soda that is ice cold it barely hisses. But, if it is hot, when pulling the tab, the soda gushes from the can as the CO2 tries to escape as quickly as possible. The same thing happens with fermented beverages. If fermentation occurs under cool conditions and the product is warmed up either intentionally as in a diacetyl rest or temperature swings caused by the weather, it will expel the gas quickly mimicking active fermentation. All this CO2 is what wine, mead and cider makers are driving out of solution when de-gassing prior to bottling.
Now for the effects of barometric pressure on fermented products: The layer of air above you presses down on you and everything around you at about 15 PSI at sea level. The pressure of air will also squeeze the carbon dioxide into the fermented product. Under high pressure conditions, clear skies and sunny days, the CO2 stays in solution. When the weather changes (cloudy skies or rain) the barometric pressure drops and the CO2 comes out of solution, again mimicking active fermentation.
Another possibility is that the fermentation is being caused by an organism that can continue to access nutrients long after the cultured yeast is finished. These wild yeast or bacteria eat the normally-unfermentable sugars like dextrins. So the carboy or bucket continues to bubble until all the carbohydrates are fermented resulting in a product that has no body and very little taste. This could be random bacteria from the environment like lactic acid bacteria, Saccharomyces diastaticus or spontaneous malolactic fermentation (not a good thing), a mutant or indigenous yeast or yeast autolysis.
If the wort or must was insufficiently aerated when the yeast was pitched, slow or sluggish fermentation will result. Mead, cider and some fruit wines do not naturally contain sufficient nutrients to keep the yeast happy and healthy also resulting in a slow, sluggish or even stalled fermentation. When formulating the recipe for traditionally nutrient deficient musts, include Fermaid K Yeast Nutrient or Yeast Energizer in the recipe. It is rare that a beer will lack sufficient nutrients for a vigorous complete fermentation. The addition of Yeast Energizer to correct poor nutrient musts or worts may help.