High-gravity brewing is usually thought of as a way to produce a larger volume of beer than your fermenters will hold. As a homebrewer, however, it can also be a way to generate two or more kinds of beer from a single fermented batch. Just brew the strong beer, retain some of it and dilute the remaining beer to make a lighter version. The “classic” combo of beers to make would be an American-style malt liquor (around OG 1.064) and an American Pilsner (with a virtual OG around 1.044). But there are other pairs of beers that are of more interest to homebrewers.
English and Scottish Ales
The two “ranges” of beer styles that give the most opportunities are English pale ales and Scottish ales. English pale ales range from ordinary bitters to ESBs. The BJCP gives three beers in that range — ordinary bitter, best bitter and extra special bitter. All three share the same basic ingredients, with each successive style being (on average) a bit a stronger and hoppier than the next. Likewise, the range of Scottish beers from 60/- (sixty shilling) to 70/- to 80/- follows this same pattern. In both cases, these beers span a range of fairly low original gravities, which makes them well suited to being brewed using high-gravity brewing.
If you’re less picky about brewing beers exactly to style guidelines, there are a few other pairings that might work well enough. You could try, for example to brew an Octoberfest-like beer and a Vienna lager pair, or a hoppy Maibock and a Pilsner. And of course, you can pick any beer style and brew an “imperial” version of that style and then make the regular beer by dilution.
To figure out what your beers will be like, formulate or choose a recipe for the stronger of the two beers. Then, divide the relevant statistics by your expansion factor. For example, let’s say you’re brewing a Scottish 80/- at an OG of 1.050 and 25 IBUs, and you plan to expand it by 1.3X. Divide the 50 “gravity” points by 1.3 to yield 38.5, or an OG of 1.039 (when you round 38.5 to 39). Likewise, 25/1.3 = 19.2. This OG and level of bitterness fit within the guidelines for a Scottish 70/-.Then, brew the stronger of the two beers.
Balancing Act — Two Equal Volumes
If you want to yield roughly the same volume of each of the two beers, here’s how you can do it. If you plan to expand the strong beer by 1.2X, brew 5.00 gallons (18.9 L) of the strong beer and withhold 2.75 gallons (10.4 L) if it. Expand the remaining 2.25 gallons (8.52 L) to 2.70 gallons (10.2 L). If your expansion rate is 1.3X, brew 5.00 gallons (18.9 L) of the strong beer and withhold 2.85 gallons (10.8 L). Expand the remaining 2.15 gallons (8.14 L) to 2.80 gallons (10.6 L). And finally, if your expansion rate is 1.4X, brew 5.00 gallons (18.9 L) of the strong beer and withhold 2.90 gallons (11.0 L) if it. Expand the remaining 2.10 gallons (7.95 L) to 2.94 gallons (11.1 L).
Balancing Act — Three Equal Volumes
If you’re ambitious and want to have roughly equal volumes of three versions or your beer, here’s how. If you’re expanding the beer by a factor of 1.2, brew 5.00 gallons (18.9 L) and withhold 2.00 gallons (7.57 L) of the strong beer. Expand the remaining 3.00 gallons (11.4 L) of beer to 3.6 gallons (13.6 L) of beer. Withhold 2.00 gallons (7.57 L) of the moderate strength beer and dilute the remaining 1.6 gallons (6.06 L) to 1.92 gallons (7.27 L).
If you’re expanding the beer by a factor of 1.3, brew 5.00 gallons (18.9 L) and withhold 2.10 gallons (7.95 L) of the strong beer. Expand the remaining 2.90 gallons (11.0 L) of beer to 3.77 gallons (14.3 L) of beer. Withhold 2.10 gallons (7.95 L) of the moderate strength beer and dilute the remaining 1.67 gallons (6.32 L) to 2.17 gallons (8.21 L).
And finally, if you’re expanding the beer by a factor of 1.4, brew 5.00 gallons (18.9 L) and withhold 2.25 gallons (8.52 L) of the strong beer. Expand the remaining 2.75 gallons (10.4 L) of beer to 3.85 gallons (14.6 L) of beer. Withhold 2.25 gallons (8.52 L) of the moderate strength beer and dilute the remaining 1.60 gallons (6.05 L) to 2.24 gallons (8.48 L).
So if you’re interesting in your 5.0-gallon (19-L) fermenter yielding more than 5.0 gallons (19 L) — and perhaps two or three different kinds of beer — give high-gravity brewing a try.