This is the question every budding home wine maker wants to know, "How can I tell how much alcohol is in my wine?" The problem is, this question is usually asked about the time they're ready to bottle their wine. Unfortunately, for the amateur winemaker, this is far to late in the process to make any accurate determinations.
What Needs To Happen
To know how much alcohol is in your wine, you need to take two readings with what's known as a gravity hydrometer. One reading is taken before the fermentation has started and the other reading is taken anytime after the fermentation has completed. By comparing these two hydrometer readings you can determine, with great accuracy, how much alcohol is in your wine.
Very simply put, a hydrometer is a long, sealed glass tube with a weight on one end. By observing how high or low it floats in a liquid you can determine a reading.
"And what are we reading?", you might be asking at this point. Essentially, we are trying to figure out how much sugar is in the wine or wine must. The higher the gravity hydrometer floats, the more sugar there is in the liquid, and the opposite holds true as well.
During a fermentation, sugar is what yeast turns into alcohol. If we know how much sugar there was in the wine must before the fermentation, and we know how much sugar there is in the wine after the fermentation, we then know how much sugar was consumed by the yeast during the fermentation. From this information we can determine how much alcohol is in the wine.
It all sound complicated when it is all explained in detail this way, but in practice it is very easy to accomplish. All you need to do is:
1. Take a gravity hydrometer reading at the same time you add the yeast to your wine must. The gravity hydrometer has a scale along it called "Potential Alcohol". At this point in the wine making process you should be getting a reading of around 10% to 13%. The reading is the point where the surface of the wine must crosses the scale. This reading indicates how much alcohol the wine can have if all the sugars are fermented. Write this number from the gravity hydrometer down and save it for later.
2. Take another reading with the gravity hydrometer once the fermentation has completed. This reading should be somewhere around +1 to -1 on the Potential Alcohol scale. By comparing these two gravity hydrometer readings you can determine your wine's alcohol level. Take the first number you wrote down and from that, subtract the second number.
As an example, if your reading before the fermentation was 12% and the reading after the fermentation was 1%, this means that your wine has 11% alcohol (12 minus 1). If your first reading was 12% and your second reading was -1%, that means your wine has 13% alcohol (12 minus -1).
Another way to think of it is you are monitoring how far along the gravity hydrometer's Potential Alcohol scale the fermentation is traveling. It started at 12 and end up at -1. That's' 13 points across the scale.
You can find more information about using a hydrometer to make wine in the book, "First Steps In Winemaking." Also, the article, "Getting To Know Your Hydrometer" as lot additional information about using your hydrometer when making 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.