Modern Homebrew Recipes (52 page)

Algebraically, the reverse equation is:

4. Calculating efficiency
– When homebrewers discuss efficiency, they are talking about
mash
efficiency, or how completely sugar is extracted from the grains in the mash. To calculate this efficiency, determine how many gravity points per pound your grain would give you if its starches were fully converted into sugars. Most base grain contains around 36–38 points per pound (37 is a good average), crystal and roasts are less (roughly 32–34 for crystal and 25–31 for dark malts; 33 and 27 are good averages, respectively). Starchy adjuncts (like flaked wheat and flaked barley) are similar to base malts. Malt extracts and sugars are discussed in calculation #5. You can look up these values in most brewing software.

Start by calculating the theoretical limit of your extract by multiplying the amount of each fermentable (Weight
_fermentable
) by the potential extract of the same fermentable (PE
_fermentable
) in gravity points per pound:

This value is calculated for each fermentable in the mash, then summed to get the total potential extract. You can simplify the calculation by combining fermentables that have the same potential extract (add their weights together, then multiply by the potential extract).

Compare that against your actual extract (total gravity points you get in your kettle). Your actual extract is obtained from your pre-boil specific gravity reading and your pre-boil volume; this is the first equation used in calculation #3 (Estimating post-boil gravity):

The actual extract (Gravity Pts
_Total
) divided by the potential extract (Gravity Pts
_Theoretical
) is your mash efficiency (Efficiency
_mash
) in percent.

For example, one of my dark mild recipes uses 7 lb. of English pale ale malt, 12 oz (0.75 lb.) of English crystal malt, and 7 oz (0.4375 lb.) of English chocolate malt. Using the ingredient database in my brewing software, I see that English pale malt has 38 gravity points per pound, English chocolate malt has 31 gravity points per pound, and English crystal malt has 33 gravity points per pound. So my total theoretical gravity points are about 304 ([7 • 38] + [0.75 • 33] + [0.4375 • 31]).

I ran off 8 gallons of wort, and it measured at 1.028. I have 224 (28 • 8) gravity points in my kettle. As a result, my mash efficiency is 74% (224 / 304) for that batch.

When looking at different recipes, you’ll sometimes see terms such as
mash efficiency
,
total efficiency
,
brewhouse efficiency
, and simply
efficiency
; these terms have different meanings. On the homebrew (and typically the craft beer) scale, we are always talking about mash efficiency even if incorrectly using different terms. On the large commercial scale, brewhouse efficiency
²
is often used but it’s a different (and more complex) calculation and is often in the range of 90% or more.

5. Calculating fermentable or water additions
– If you want to add additional fermentables, know that most liquid fermentables (liquid malt extract, honey, syrups, etc.) add about
36 gravity points per pound
and most dry fermentables (dry malt extract, sugar, etc.) add about
45 gravity points per pound
. Specialty sugars and syrups for brewing (such as Belgian Candi syrups) often have the gravity points listed on the label. The common syrups add
32 gravity points per pound
, while soft sugars add
37–38 points per pound
. If you need to add to your total gravity points, use these to calculate the weight of your additions. Water adds 0 gravity points per pound and can be added to dilute the gravity.

Armed with this information, we can use the same formulas from calculation #3 to adjust our gravity. Suppose you have a final boil volume of 6 gallons with a specific gravity reading of 1.052. If you wanted to have a gravity of 1.064, then you’d have to add fermentables. You have 312 (52 • 6) points in your kettle,
but you want 384 points (64 • 6). Your deficiency is 72 points (384 – 312). Knowing the points per pound of your fermentables allows you to calculate the quantity needed. If you use liquid extract (36 points per pound), then you’ll need 2 pounds (72 / 36) to hit your gravity target. If you used dry malt extract, then you’d need 1.6 pounds (72 / 45).

If your gravity reading is too high, you can add water to bring it down. To properly dilute with water, you’ll need to calculate the volume. Suppose you have the same 6 gallons of 1.052 wort but you really wanted a gravity of 1.045. You have the same 312 points (52 • 6) in your kettle, but need to know what volume will give you 45 points per gallon. Take the total points and divide by the points per gallon target to reach the volume needed: 312 / 45 = 6.9 gallons. You need 6.9 gallons but only have 6 gallons, so you need to add 0.9 gallons (6.9 – 6.0) of water.

6. Estimating original gravity
– In order to estimate the original gravity of your recipe, you need to know the pounds of grain used (by type), your mash efficiency, and your batch size. You start by calculating the gravity point contribution of each grain addition:

For example, let’s assume my mash efficiency is 75% (0.75), my batch size is 5 gallons, and that I’m the using 7 lb. pale ale malt, 0.75 lb. crystal malt, 0.4375 lb. chocolate malt with potential extracts of 38, 33, and 31 points per pound, as my grist. The gravity point contribution calculation would be:

7 • 38 • 0.75 = 199.5 (pale ale malt contribution)

0.75 • 33 • 0.75 = 18.6 (crystal malt contribution)

0.4375 • 31 • 0.75 = 10.2 (chocolate malt contribution)

199.5 + 18.6 + 10.2 = 228.3 (total gravity points in kettle)

To estimate the original gravity, use this equation from #3 (Estimating post-boil gravity):

In our example, the estimated original gravity calculates to 46 (228.3 / 5). This translated to a specific gravity of 1.046.

7. Calculating grist additions
– To calculate the weight of grain additions for a specific batch size of your recipe, you essentially use calculation #6 (Estimating original gravity) in reverse. Let’s continue to use the same recipe as in the previous example, but change the batch size to 11 gallons and the target gravity to 1.040 (or 40 gravity points per gallon). Let’s also assume our mash efficiency remains at 75%. This example will not only show you how to calculate grain additions, but also how to scale the grist to a different batch size and gravity.

If you were using this approach on a new recipe, you would start with your grist in percentages and then calculate the weight of grain additions based on the target parameters.

We begin by calculating the total gravity points needed in the kettle using the equation from calculation #3 (Estimating post-boil gravity):