How To Stop A Fermentation
By Ed Kraus
Quite often we'll have someone ask us how to stop a fermentation before it is ready to stop on its own. Usually the reason for asking is because they have tasted their wine and they like the amount of sweetness it currently has--sounds reasonable.
Stopping the wine from fermenting any further would preserve the current level of sweetness. And likewise, allowing the wine to continue fermenting further would only make the wine less sweet with each passing day.
Eventually the wine would become completely dry at which time the fermentation would stop on its own. This is because during the fermentation process wine yeast turns the sugar into alcohol.
Wanting to stop a fermentation is all good in of itself. But unfortunately, there is really no practical way to successfully stop a fermentation dead in its tracks.
Using Sodium Bisulfite or Campden Tablets
Many winemakers will turn to sulfites such as that found in Sodium Bisulfite or Campden Tablets for the answer. But, these two items are not capable of reliably killing enough of the wine yeast to guarantee a complete stop of the activity--at least not at normal doses that leave the wine still drinkable.
Once the bulk of the sulfites from either of these home wine making ingredients dissipate from the wine into the air--as sulfites do--there is a very strong chance that the remaining few live wine yeast cells will start multiplying and fermenting again if given enough time. And, I might add that this usually happens at a most inconvenient time, like after the wine has been bottled and stowed away.
Using Potassium Sorbate
Potassium Sorbate is another home wine making ingredient that many winemakers consider when trying to stop a wine from fermenting any further. There is a lot of misunderstanding surrounding this product.
It is typically called for by home wine making books when sweetening a wine. This is a situation where the fermentation has already completed and is ready for bottling. You simply add the Potassium Sorbate along with the sugar that is added for sweetening.
The Potassium Sorbate stops the wine yeast from fermenting the newly added sugar. So, many winemakers assume Potassium Sorbate can stop an active fermentation as well. But, nothing could be further from the truth.
Potassium Sorbate does not kill the yeast at all, but rather it makes the wine yeast sterile. In other words, it impairs the wine yeast's ability to reproduce itself. But, it does not hinder the wine yeast's ability to ferment sugar into alcohol.
Potassium Sorbate puts a coating on the cell wall of each individual wine yeast in such a way that budding or multiplying is next to impossible.
The idea here is that if you happen to have few cells of live wine yeast remaining in your finished wine, they will be rendered harmless if they are unable to regenerate themselves to great enough numbers to invigorate a fermentation of any kind. This is true even if more sugar is added to the finished wine.
So, What Do You Do?
Well, remember the original goal here is to have a wine that is sweeter than what a natural fermentation will normally provide. And what the above tells us is that stopping a wine's fermentation in mid-stream when it is at the sweetness you like is not the answer for the average home wine maker.
The most successful way for a home wine maker to have a sweet wine is to let it finish fermenting completely to where it is dry. Then let the wine yeast settle out to the bottom on its own over a 2 to 3 week period. The settling process can be sped up with the use of a clarifier such as Speedy Bentonite.
Once this happens you can then siphon the wine off of the wine yeast settlings and add Potassium Sorbate and Sodium Bisulfite as directed on their packages. Once you have done this you can then simply sweeten your wine to taste with a sugar mixture of your choice.
It is important that the wine's fermentation process be complete before adding more sugar along with Potassium Sorbate and Sodium
Bisulfite to a wine. One way to make absolutely sure is to check the wine with a wine hydrometer. The wine should have a reading of 1.000 or less on the hydrometer's Specific Gravity scale.
You might want to take a look at another article on our web site titled, "Making Sweet Wines." It covers in detail the process for making a sweet wine.
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.