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 20oC 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 20oC you may assume that it contains 20% sugar.
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 60oF and the Gay Lussac at 68oF. 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 60oF will not be the same value that it would be for that solution at 80oF. Specific gravity tables are published to apply at certain standard temperatures, such as 60oF (15.56oC) or 68oF (20oC) 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 78oF (25.6oC). Then, on July 1, 2012, I measured the specific gravity of the resulting white wine to be 0.992 at 77.4oF (25.2oC). Both of these measurements were obtained using the 60oF hydrometer, and both specific gravity measurements must be corrected for temperature. At around 77oF to 78oF the use of a 60oF 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.
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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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 20oC 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 20oC you may assume that it contains 20% sugar.
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| Two Hydrometers |
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 60oF will not be the same value that it would be for that solution at 80oF. Specific gravity tables are published to apply at certain standard temperatures, such as 60oF (15.56oC) or 68oF (20oC) 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 78oF (25.6oC). Then, on July 1, 2012, I measured the specific gravity of the resulting white wine to be 0.992 at 77.4oF (25.2oC). Both of these measurements were obtained using the 60oF hydrometer, and both specific gravity measurements must be corrected for temperature. At around 77oF to 78oF the use of a 60oF 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.
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| Distillation of Rose' Wine |
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I understand that you have been having an extremely dry year. Presumably this year's growing conditions will affect the sugar content of the grapes. Perhaps later when you have the data on sugar content in hand you can post a blog in which you project what effect this might have on the wine that ultimately results.
ReplyDeleteHello Russ,
ReplyDeleteYes, this was and continues to be a dry year. I pick my grapes when tests show that the sugar content is not changing much and the berries look as if they are beginning to dry. This year August 26th was the day that I picked my grapes and got a corrected sugar content of 23.6%. Sugar contents between this reading and 25% are ideal, so I am not doing anything but leaving the must as is. The result should be a dry, full bodied wine with good keeping qualities. What other qualities it will have are a matter of ameliorations at a later date. Usually, because it is a mix of two grape varieties it is somewhat lighter than a pure Merlot and has a blend of fruit and nut flavors which come from the growing environment.