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Homebrewing – Measuring Alcohol Content of Your Beer

One of the first types of tests that most beginning home brewers measure is the alcohol content of their beer. This is an easy test to do, and it is something many people like to know before they start drinking. I find it is one of the first questions anyone asks before they take their first sip of by brew, “How much alcohol is in this stuff?”

Before we go into the methods of measuring alcohol, a little explanation is in order. There are no devices that measure the alcohol directly. The best we can really do is infer the amount of alcohol based on other observable data.

Yeast produces alcohol by metabolizing simple sugars (Dextrose) into two primary by-products: ethanol and carbon dioxide. All the other by-products occur in trace amounts that are irrelevant to our interests here. The carbon dioxide (CO2)is released into the atmosphere, and the alcohol remains.

Because the CO2 has left the liquid, the beer becomes less dense. The more sugar the yeast metabolize, the higher the amount of CO2 that is released, and consequently the higher the percentage of alcohol.

In order to determine the alcohol content of your beer, you need to measure the beer before it ferments, and after it ferments. By determining the difference between these two numbers, you can infer the amount of alcohol in the brew.

There are two ways of doing this measurement. You can measure the density of the beer, or the amount of sugar that is in the solution. We refer to the measurement of the density of the beer as it’s Specific Gravity, and the amount of sugar is expressed in units called Brix.

Specific Gravity

The Specific Gravity of water at 60 degrees Fahrenheit is 1.000. Everything else is either more or less dense than water. You use a simple little tool called a Hydrometer to measure the Specific Gravity of a solution. It is essentially just a glass cylinder with a weight on the bottom and lines showing the different gravity points going up the side. Technically, you are supposed to cool the beer sample down to 60 degrees in order to get an accurate reading. However, as long as the beer is around room temperature, your reading will be reasonably accurate. Just keep in mind; a hydrometer reading of boiling liquid will be way off.

When brewers talk about Specific Gravity they will usually specify either the Original Gravity (OG), or Terminal Gravity (TG). The OG is a measurement of the beer before it ferments, and the TG (also called Final Gravity – FG) is the measurement that is taken after fermentation is complete. The difference between the two tells you how much alcohol is in the brew. As an example, a typical Pale Ale will start off with an OG of around 1.045 and finish off with an TG of around 1.008.

Calculating the percentage of alcohol is as simple as plugging some numbers into the following equation.

% Alcohol = ((1.05 x (OG – TG)) / TG) / 0.79

So, given a few numbers suggested above:

OG = 1.045
TG = 1.008

The equation would look like this:

.0487 = ((1.05 x (1.045 – 1.008))/1.008) / 0.79

So, this beer would be about 4.9% alcohol.

Brix

Another great way to measure alcohol content is by measuring the sugar level. You measure Brix using a nifty little device called a Refractometer. These things are great because all you need is a drop of wort or beer to take a reading, and you don’t have to cool it off before you take a reading or perform mathematical gymnastics to compensate for temperature variances.

There are complex formulas to out there on Internet Land if you want to get super duper accurate, but the simple formula is to just take your Brix level and multiply it by four to get a Specific Gravity.

For our example, let’s assume a Brix reading of 11.25. Therefore we find the Specific Gravity this way:

4 x 11.25(Brix) = 45

So, the “45” in this case means 1.045. In brewing language, “45” actually means 1.045 – which is why this little trick works. You can now take this number and plug it into the formula given above to calculate your alcohol level.

My personal preference is to use the Refractometer instead of the Hydrometer. There is just no way to express the ease of putting a drop of wort on the Refractometer while I’m mashing my grains, or to make sure my gravity is spot on when the wort goes into the fermentor. The alternative of using a Hydrometer is such a pain in the ass that quite frankly I almost never take gravity readings.

On the downside, Refractometers cost much more than Hydrometers. You will pay about $60 – $120 for a Refractometer versus $12 – $25 for a Hydrometer. If you are considering all-grain brewing, then I would highly suggest a Refractometer.

You can get more information on Homebrewing in the Homebrew Section of www.DrunkMansGuide.com

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December 19, 2007 Posted by | beer, Homebrewing | , , , , , | 37 Comments