Northern Table is hosting this month’s Fermentation Friday, a last-Friday-of-the-month blogging event specially made for homebrew bloggers. This month’s topic: “Beer and Liquor”

One of the easier ways to add spice flavors to a beer is to steep the spices in vodka for a week or two and then add a measured amount of the resultant “potion” (as Randy Mosher likes to call it in Radical Brewing) to the beer at bottling time.

The advantage of this technique is that the recipe for the spice extract is easy to reproduce with precision, and it’s easy to make sure you’re adding the exact amount of spice flavor that you want. We’ll go over the details of this process in a future post.

The biggest disadvantage is the potential increase in alcohol content in the final beer, unless your personal philosophies dictate that this is not, in fact, a disadvantage. If alcohol content is a significant factor for you, it will pay off to learn how the addition of liquor to your beer will affect the final %ABV.

Get ready. This is gonna be mathy. Skip to the end if you’re not interested in the derivation.

Before mixing the liquor into the beer, we know the following quantities (note that percentages must be expressed as decimals, i.e. 35% is equivalent to 0.35):

, the %ABV in the beer
, the %ABV in the liquor
, the volume of beer
, the volume of liquor

From these quantities, we can derive the following:

The volume of alcohol in the beer: (1)
The volume of alcohol in the liquor: (2)

We can now derive our final equation. The final %ABV is equal to the total volume of alcohol in the beer and the liquor divided by the total volume of the beer and liquor:

Final %ABV:

Substituting in our equations for volumes of alcohol in the beer and liquor (equations (1) and (2), respectively), we get the final equation:

(3)

Now for an example. Suppose we add 8 fluid ounces of 80 proof vodka (40% ABV) to a 5 gallon batch of beer at 6% ABV. We need to be working in the same units for each volume, so let’s convert the volume of beer to ounces:

Plugging all of our numbers into (3), we get the final %ABV:

So the final %ABV will be 6.4%. As you can see, the difference will be small (though not insignificant) for even a half-pint of vodka. In reality, assuming you make a concentrated spice extract, you’re likely to need much less liquor than that, so in most cases, the change in alcohol content should not matter very much.

Fermentation Friday, Homebrewing, Technical 2 Comments »