Modern Homebrew Recipes (53 page)

In our example, this is 440 (40 • 11).

We now need to calculate the gravity point contribution provided by each fermentable in the recipe. It’s easiest to do this by first converting the original recipe to percentages:

Total grist weight = 7 + 0.75 + 0.4375 = 8.1875 lb.

7 lb. pale ale malt = 7 / 8.1875 = 85.5%

0.75 lb. crystal malt = 0.75 / 8.1875 = 9.2%

0.4375 lb. chocolate malt = 0.4375 / 8.1875 = 5.3%

Next we apply those percentages to the total gravity points required (440) to see what each fermentable needs to contribute:

85.5% pale ale malt = 0.855 • 440 = 376.2 points

9.2% crystal malt = 0.092 • 440 = 40.3 points

5.3% chocolate malt = 0.053 • 440 = 23.5 points

Almost done. We use the equation from calculation #6 (Estimating original gravity) to these gravity points. to work backwards and find the amount of grain that will give us these gravity points. The rewritten equation is:

Recalling that pale, crystal, and chocolate malts have potential extracts of 38, 33, and 31 points per pound, and that our mash efficiency is 75%, we convert the gravity point contributions to weights:

Pale ale malt: 376.2 / (38 • 0.75) = 13.2 lb.

Crystal malt: 40.3 / (33 • 0.75) = 1.63 lb.

Chocolate malt: 23.5 / (31 • 0.75) = 1.01 lb.

I would typically round those numbers to make them easier to work with. If using adjuncts, remember that a brewing sugar or malt extract have a mash efficiency of 100%.

8. Estimating Color
– The general formula for color (in SRM, or Standard Reference Method) from a single grain addition is:

where MCU (malt color units) for each grain addition are calculated using the weight of the grain (Weight
_fermentable
), the color of the grain in degrees Lovibond (Lovibond
_fermentable
), and the batch size in gallons (Boil Vol
_f
):

For example, calculating the color of the beer from the previous example:

Pale ale malt: 13.2 lb. • 2 L = 226.4 L pts

Crystal malt: 1.63 lb. • 120 L = 195.6 L pts

Chocolate malt: 1.01 lb. • 350 L =353.5 L Pts

9. Estimating IBUs
– The general formula for estimating IBUs from a single hop addition is:

The variables needed are the weight of hops in ounces (Weight
_hops
), the alpha acid of hops as a percentage (AA%), the hop utilization factor as a percentage (U%), and the batch volume in gallons (Boil Volf). The hop utilization is the percentage of alpha acids (AA) added to the kettle (by weight) that get converted into iso-alpha acids in the beer.

For example, if we add 2 oz of 5.5% AA hops at 60 min in 5 gal final volume, how many IBU do we add?

If we want 60 BU of 11% AA hops at 60 min in 5 gal batch, how many oz do we need?

What if you don’t know the weight of iso-alpha acids in your beer? You’ll have to estimate that factor instead!

The Tinseth method
³
is a set of formulas to estimate hop utilization (U%). Glenn Tinseth fit a curve to research data and produced a fairly accurate formula based on boil time and wort gravity. He calculated utilization as:

Hop utilization = (gravity factor) • (time factor), where

Gravity factor = 1.65 • 0.000125(wort gravity – 1) and

Time factor = [1 – e(-0.04 • time in minutes)] / 4.15

For example, if you have a 30 minute addition of hops in 1.060 wort, the utilization factor would be:

If that addition was 1.5 ounces of 6.3% AA Cascade hops in a 5-gallon batch, then the IBU contribution of that addition would be:

But this is not an actual calculation you typically perform. You normally look up utilization based on wort gravity and time factor from a table where the calculations were already performed. There is such a table in the previously mentioned link to the Tinseth method. Most homebrewers I know just use recipe software, which will have the calculations built in. The advantage to using recipe software is that it will perform an exact calculation based on wort gravity and time, while tables might not have the values you need. In that case, interpolation (picking a number in between nearest values) or approximation (rounding to nearest value) is what most would use.