I racked my wine after sweetening it back let it set for a week to make sure the fermenting had stopped, took my reading which was ok. bottled it, and in about a week it started to explode. I’ve been making wine for 8 years and this is the first time this has happen. What caused this to occur?
Name: Barbara E.
What has happened is somewhat straight-forward. The most common reason for a fermentation to stop is because it ran out of sugar to ferment. The fermentation is all about taking wine yeast consuming the sugars and converting them into alcohol and CO2 gas. If you add more sugar back to the wine, for whatever reason, the fermentation can start up again.
It only takes the slightest amount of fermentation to create a lot of CO2 gas and pressure. The pressure builds up and either pops the cork, or what happens in the case of screw-cap bottles, the bottle fails. This is what happened in your cause. The fact that you have not had this problem before in 8 years I find remarkable.
For future batches you can do just as you have done before. That is: let the fermentation complete and clear; then add sugar to taste at bottling time. But before bottling also add Potassium Sorbate. This is a wine stabilizer that will help keep the wine from re-fermenting. It is also important that the wine be given plenty of time to drop out all the inactive wine yeast after the fermentation before you sweeten and bottle. This will also help to keep a re-fermentation out of your future.
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.