Fermentation Failure: Too Much Alcohol Already In The Wine
Alcohol in Wine is a Preservative
One thing that must be understood when making wine is that alcohol is a preservative. By definition, a preservative is something that helps a perishable to remain in its current state--to not change, but remain as is. How this translates to wine making is that the alcohol itself can be the reason a must is no longer fermenting.
Musts that are just beginning fermentation and are still at lower levels of alcohol concentration, say 5 percent, do not experience much problem hosting a fermentation. But, as a must's alcohol level rises, one will usually begin to notice a slowing of the fermentation activity. This slowing is primarily due to the increased preserving effects the rising alcohol is having on the must.
And, as the alcohol level continues to rise throughout the latter part of the fermentation, 10-12-14 percent, the fermentation becomes slower and slower until it simply can no longer overcome the increasing effects of the alcohol.
To put an analogy to this, if you have ever seen a tractor pull, you can think of a fermentation as a single pull. It takes off with little hesitation, but as it grinds on further and further and the resistance from the weighted trailer becomes greater and greater, there comes a point when the wine yeast can't pull any more and it simply has to shut down.
And, it will shut down regardless of how much sugar may be left in the must. So, it is possible for a must to have more sugar than the wine yeast can turn into alcohol, resulting in a wine with significant alcohol but way too much sugar to be drinkable.
How to Use a Hydrometer to Measure Alcohol in Wine
Using a hydrometer is the key to knowing how much sugar can be safely added to a wine recipe. By using a hydrometer you can add the correct amount of sugar to a wine recipe to shoot for a specific alcohol level that is reasonable and achievable.
Expect a wine yeast to produce up to 12-13 percent alcohol with minimal effort, this is assuming that all other environmental conditions for the yeast are optimal, such as: temperature, nutrients, and so on. Anything beyond 13 percent, you're on your own.
If you want to experiment with producing higher alcohol wines beyond 13 percent, you will want to carefully read the article, "Making High Alcohol Wines" listed below.
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Getting To Know Your Hydrometer