EXTRACT CALCULATIONS
The specific gravity resulting from a given weight of malt or adjunct used in a given volume of wort is (9)
SGpoints= weight(lb) x %yield x 46.31/batch size (gal)
which, after solving for the raw material weight, becomes
weight (lb)= SGpoints x batch size(gal)/%yield x 46.31
where SGpoints is the specific gravity in points (SG 1.050 = 50 points), percent yield is the yield of extract (w/w) from the raw material, and the constant 46.31 is the increase in specific gravity (points) for 100% yield from 1 lb of raw material in 1 gal of solution. In recipe calculations, percent yield is determined for the mixture of malts and adjuncts, including kettle adjuncts, and must take into account the mash extract efficiency. On the worksheet, this value is shown as total extract. For metric units, the constant becomes 386.5, for 1 kg in 1 L of solution (with batch size in liters, the weight is expressed in kilograms).
The equation is actually an approximation, but the error is negligible for specific gravities at or below 1.060. At SG 1.090, error is ~0.5%; at SG 1.140, error is ~1%.
COLOR
The Lovibond rating of a malt or adjunct grain is given for 1 lb in 1 gal. The beer color is obtained by adjusting to the actual weight of raw material and to the expected final volume:
color(°L)=rating (°L/lb/gal) x weight (lb)/batch size (gal)
In the recipe calculation, the Lovibond rating above must be found for the mixture of malts and adjuncts used, and
this value appears on the worksheet as the total color. To use metric units (with weight in kilograms and batch size in liters), a Lovibond rating must be found in °L/kg/L. Conversion of malt color data to metric units is simple: multiply the rating (in °L/lb/gal) by 8.346 to get °L/kg/L.
HOP BITTERING UNITS
The chosen measure of bitterness, IBU, represents a measurement of the isomerized a-acids in the finished beer in mg/L. In recipe calculations, the IBU level can be estimated based on the quantity of a-acids added (mg) to the boil per unit volume of beer and a utilization factor that estimates the fraction that will become isomerized and remain in the finished beer. In metric units, then,
IBU=weight(g) x %a-acid x 1000 x utilization factor/batch size(L)
Solving for the weight of hops required gives
weight(g)=IBU x batch size(L)/%a-acid x 1000 x utilization factor
In the calculation procedure described here, percent a-acid X 1000 X utilization factor is the total iso-a-acid determined for the mixture of hops, used according to the schedule.
Converting to U.S. units gives
weight(oz) = IBU x batch size(gal)/%a-acid x 7490 x utilization factor
The factor of 1/7.49 converts g/L into oz/gal. This equation can be used to find the weight of hops implied by a specified IBU contribution, given a batch size, an a-acid content, and a utilization factor derived from the boil time.
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