Definition:
High Gravity Brewing (or Blending) is a technique that allows you to brew a larger end-batch volume without buying larger fermentation equipment. A higher specific gravity beer (strong beer)is first fermented and then blended with water to create a lower alcohol beer with less IBU and SRM than the originally fermented beverage.
High-gravity brewing (or blending) is a technique the big US brewers use. American Pilsners are brewed with an original gravity (OG) around 14–16 °Plato (SG 1.056–1.064). After fermentation, the strong beer is diluted to a virtual OG of around 10–11 °Plato (1.040–1.044). The big breweries brew this way because they can produce more beer with high-gravity brewing than if they brewed it all at working strength. A fermenter that holds 500 barrels of beer will end up producing 700 barrels of beer. For homebrewers, a 5-gallon (19-L) batch of strong beer can easily be diluted to yield 6–6.5 gallons (23–25 L) of finished beer, more if you’re careful.[Colby,2004]
Recipe Formulation:
The easiest way to formulate a high-gravity brewing recipe is to take a recipe for one volume and expand it to the larger target volume. However, you should brew the beer at the original volume. To calculate how much to expand the recipe, divide the larger volume of target beer by the smaller volume of strong beer. [For example, let’s say you have a 5-gallon (19 L) pale ale recipe with 10 lbs. (4.5 kg) pale malt, 1 lb. (0.45 kg) crystal malt and 2 oz. (56 g) of bittering hops. If you want to end up with 6 gallons (23 L) of beer, multiply all the ingredients by 6 divided by 5, or 1.2. You’d end up with 12 lbs. (5.4 kg) pale malt, 1.2 lbs (0.54 kg) and 2.4 oz (68 g) of hops. Brew 5 gallons (19 L) of this beer and dilute it to 6 gallons (23 L)] A drawback to this simple calculation method is that hop utilization slightly decreases at higher wort gravities. Thus, using proportionally more hops in your high-gravity base beer may lead to an underhopped diluted beer. However, in most cases, this discrepancy will be minimal.[Colby,2004]
Brewing the Strong Beer:
Brew your base beer as you would any strong beer. The quality of your final beer will depend primarily on how well the fermentation of your strong beer went. Fermentation byproducts, such as esters and higher alcohols (fusel oils), are produced at a disproportionally greater rate in “thick” worts than in “thin” ones. At higher dilution rates, you may notice that your finished beer is a bit more estery than it would have been had you brewed it normally. If your final beer tastes too estery, has fusel notes or other unwanted fermentation byproducts, you’ve pushed the technique too far. At higher dilution rates, you may also need to tinker with your recipe a bit to get the beer to taste as you want it. The big breweries typically do not exceed 16 °Plato (SG 1.064) when brewing their strong beer for dilution. Above this gravity and the resulting blended lager is too estery. For most lagers to be blended, 16 °Plato (SG 1.064) is a reasonable maximum starting gravity for the strong base beer.[Colby,2004]
--By WortMonger, member of HBT.com 14:37, 18 August 2008 (CDT)
