Measuring Alcohol Content in Wine by the Potential Alcohol Method

In an earlier blog, Measuring the Alcohol Content in Wine, some of the general points are covered relative to the alcohol content of wines and the various methods for its measurement.

Specific Gravity Materials

I use the potential alcohol method to measure the alcohol content in my wines.  This method is simple and it gives acceptable results.  First, I set up the area where I am going to do the measurement by bringing together in the same space the wine or must, the hydrometer, the transparent cylinder that I will use, and a thermometer.  The cylinder will contain the wine or must and the hydrometer.  These items are left for about three or four hours before making any measurements so that they are all at pretty much the same local temperature.

When I am ready to measure potential alcohol (see the video above), I place the hydrometer into the cylinder holding it by its upper tip.  I add the wine/must to the cylinder until the hydrometer floats, give the hydrometer a spin to get rid of bubbles that might be adhering to the glass, and let it come to rest on its own.  When the hydrometer stops moving and is floating freely not touching any part of the cylinder, I record the specific gravity reading.


As an example, for the rose' must, I measured its specific gravity on August 31, 2012 (using the hydrometer calibrated at 60^oF) and got 1.106 at a temperature of 27^oC (80.6^oF).  At this temperature the specific gravity requires a correction of 0.002 to be added to the reading giving 1.108 as the corrected specific gravity.  According to the table of potential alcohol for this specific gravity reading a wine of 14.2%  potential alcohol by volume should result.  On August 9, 2013, almost one year later, I measured the specific gravity of the rose' wine that this must produced and got 0.992 at 28^oC (82.4^oF) which requires a correction of 0.003 giving 0.995 as the corrected specific gravity.  But this reading is below 1.000 which indicates that all the sugars have been consumed and this is an essentially dry wine of 14.2% alcohol by volume.  If this had not been the case, the latter potential alcohol reading would be subtracted from the former to get the alcohol content:  14.2% - 0% = 14.2%.


Potential alcohol tables are readily available on the internet along with detailed instructions for their use.  Just use the search terms potential alcohol table and you will get a long list to choose from.  In fact, the hydrometer that I use came packaged with instructions for its use along with a potential alcohol table and instructions for making temperature corrections to specific gravity readings.

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Measuring the Alcohol Content of Wine

Each year I try to make four different wines produced from locally grown gapes: a white made from Muscat grapes, a rose' made from Merlot grapes, a Merlot, and an estate wine made from a mixture of Merlot and Cabernet Sauvignon grapes.  This is the time of the year that I bottle my wines, but before I do I measure their alcohol content for no other reason than when people ask me how strong is the wine, I can give them the answer.


There is more than one method used to measure the alcohol content of wine.  Some require expensive equipment and detailed procedures, and others require no more than a hydrometer, an instrument that all serious home wine makers have as standard equipment.  The hydrometer (there are different kinds of these and they come in different ranges) is used to measure the specific gravity of water solutions.  Two important water solutions in wine making are sugar in water and alcohol in water.


Technically, and in the limited sense that it applies to wine making, specific gravity is the ratio of the mass of a volume of solution of sugar in water (must) [or alcohol in water (wine)] to the mass of an equal volume of water measured at a specific temperature.  The specific gravity is then related to the exact composition of the solution, that is, the percent of sugar in the must or the percent of alcohol in the wine.  So, as an example, at 20^oC the specific gravity of a 20% solution of sugar in water is 1.083 while that of a 24% solution is 1.101;  thus, if you measure and find that the specific gravity of a must is 1.083 at 20^oC you may assume that it contains 20% sugar.

Two Hydrometers

The hydrometer is a long cylindrical glass instrument which has enclosed within the glass housing a visible scale or scales.  The longer one of the two shown actually has three such scales:  one for measuring specific gravity, one for measuring percent sugar by weight, and one for measuring potential alcohol by volume.  The other, shorter hydrometer is a Gay Lussac and measures the percent by volume of alcohol directly.  Each hydrometer is designed to give direct  readings at a particular temperature.  The longer one is intended for use at 60^oF and the Gay Lussac at 68^oF.  Hardly ever are these exact temperature conditions reached in actual practice.


To measure the amount of alcohol in my wines, I have used three methods.  The simplest of these is to measure the specific gravity and temperature of the must just prior to the start of fermentation, and then after almost one year later to measure the specific gravity and temperature of the resulting wine.  The two specific gravity measures are converted into what is called potential alcohol, and the difference between the before fermentation potential alcohol reading minus the wine's potential alcohol reading gives the alcohol content of the wine.  The temperature is recorded because specific gravity depends on temperature.  In other words, the specific gravity of a solution at 60^oF will not be the same value that it would be for that solution at 80^oF.  Specific gravity tables are published to apply at certain standard temperatures, such as 60^oF (15.56^oC) or 68^oF (20^oC) as well as others, and in order to use these tables accurately most specific gravity measurements must be corrected for temperature.


Consider this example, on September 7, 2011, I measured the specific gravity of my white must just before the start of fermentation and found it to be 1.102 at 78^oF (25.6^oC).  Then, on July 1, 2012, I measured the specific gravity of the resulting white wine to be 0.992 at 77.4^oF (25.2^oC).  Both of these measurements were obtained using the 60^oF hydrometer, and both specific gravity measurements must be corrected for temperature.  At around 77^oF to 78^oF the use of a 60^oF hydrometer calls for a correction of 0.002 to be added to each specific gravity;  thus, we get 1.104 and 0.994, respectively, for the corrected specific gravities.  With these corrected specific gravities, the potential alcohol table that comes with the hydrometer gives for the first reading 13.7% and for the second 0%.  The difference between these two readings is 13.7% alcohol by volume, so my white Muscat wine is 13.7% alcohol by volume.

Distillation of Rose' Wine

Earlier I said that I determine the alcohol content of my wines by different methods.  The potential alcohol is one method that I use, but I also use a weight method and a distillation method, too.  By these two latter methods, the alcohol content of my white Muscat wine is 14.1% and 12.7%, respectively.  I present these results as an illustration of the fact that the methods available to the home wine maker for alcohol determination are approximate, and, by far, the potential alcohol method is the easiest and least expensive method to use, and it gives very acceptable results.

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