The recipe calculator contains some simplifying assumptions. For example, Cascade hops may range from 4.5 to 7.0% alpha acids depending upon the growing conditions and other influences. This variation poses a technical problem for the calculator. The simple solution was to average these high and low values. The Cascade hop value that the calculator uses is 5.75%.
So what should you do if your Cascade hops are 6.5% alpha acids instead of the 5.75% value used by the calculator? For most homebrewers these small adjustments won't matter much. "Relax, don't worry" as Charlie Papazian might say. For others, a more precise result can be determined with relatively little effort. Read on if you are in this latter group of homebrewers who would like more precision.
The calculator provides the default values as part of the output. Here is the output from the cream ale example that was used in the recipe instructions.
For this example let's focus on the gravity points of the American two-row base malt. The output shows that the calculator uses a default value of 1.037 for this malt. If the gravity potential of your base malt is also 1.037 then there is no need for adjustment.
Let's assume, however, that your base malt is 1.038 instead of the default 1.037. A more exact calculation for your brewing circumstances could be made. The recipe needs 206.3 gravity points for the American two-row malt. This gravity point value can be used to recalculate the weight needed using the following formula.
206.3 gravity points / [38 (From your grain's 1.038 specific gravity) * .75 (your efficiency)] = 7.24 lbs of American two-row malt
Your recipe will need slightly less grain than the value given by the calculator due to the slightly larger extract potential of your grain. The difference in this example, 7.43 lbs vs 7.24 lbs, is trivial. Such differences might be significant though if there is a large value difference between your grains and the default values used by the calculator. A larger volume batch might also see some substantial differences in a situation like this example.
A similar process can be used to make adjustments to the specialty grains. The differences are likely to be small for these grains because specialty grains are usually 10% or less of the grain bill. The small differences might not matter much at the homebrewing level.
A similar process can be done for the hop calculations. Let's say that your Liberty hops are 4.0% alpha acids, which is lower than the 4.5% alpha acid value used by the calculator for the cream ale recipe example. Let's adjust this example recipe to be more exact.
The far right side of each hop addition has the alpha acid units (AAU; also called Homebrew Units or HBU by some authors) that are needed. This value can be used to fine tune the hop additions. Alpha acid units are simple: Hop weight in ounces multiplied by the alpha acid percentage. For example, one ounce of Liberty hops at 4.0% alpha acids would be 4.0 AAU.
The cream ale example shows that we will need 4.34 AAU of bittering hops. The adjustment from 4.5% (default value) to 4.0% (our hops) is to divide the 4.34 AAU that we need by the 4.0 AAU of our hops.
4.34 AAU needed / 4.0 AAU from our hops = 1.09 ounces
We will need to increase the required weight of the bittering Liberty hops from .97 ounces to 1.09 ounces to compensate for the lower alpha acids of our hops.
Once again, this adjustment is very small and may not matter much for most homebrewers. It might be significant though for larger batches or in situations where the difference between your hops and the default value is large.
The recipe calculator determines the yeast pitching rates for dry and wet yeast by following the recommendations made by Lallemand/Danstar and Wyeast, respectively. The yeast pitching rates recommended by other companies such as Fermentis (dry), Mangrove Jack (dry), White Labs (wet), and others should be similar but may vary somewhat from the values produced by the calculator.
Homebrewers who use yeast from these other manufacturers can fine tune yeast pitching values by using the yeast pitch rate calculator page. Enter the original gravity, volume, and yeast type to get a recommendation that is specific to your yeast provider. This page also gives the commonly used repitching values of .75 million cells/ml/degree Plato estimates used for ales by some homebrewers and commercial breweries.
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