Originally By Tony Ackland
There are two different original gravities (OG) that matter to a brewer: one is the pre-boil or extraction OG, and the other is the post-boil or pitching OG. And, ninety percent of the time, the pitching OG is what people are referring to because it determines the strength of the beer. When brewers plan recipes, they think in terms of the pitching OG, which assumes that the wort volume is the final size of the batch, e.g. 5 gallons.
But, when it comes to the efficiency of the mash and lauter, we want to think in terms of the pre-boil gravity. The Extract Efficiency section and table gave us the typical malt yields that allows us to evaluate our mashing process.
When all-grain homebrewers get together to brag about their brewing prowess or equipment and they say something like, "I got 30 (ppg) from my mash schedule", they are referring to the overall yield from their mash in terms of the amount of wort they collected.
It is important to realize that the total amount of sugar is constant, but the concentration (i.e. gravity) changes depending on the volume. To understand this, let's look at the unit of points/pound/gallon. This is a unit of concentration, so the unit is always expressed in reference to 1 gallon ("per gallon"). In mashing, you are collecting "x" number of gallons of wort that has a gravity of "1.0yy" that was produced from "z" pounds of malt. To calculate your mash extraction in terms of ppg, you need to multiply the number of gallons of wort you collected by its gravity and divide that by the amount of malt that was used. This will give you the gravity (points per gallon) per pound of malt used. Let's look at an example.
Palmer's Short Stout (target OG = 1.050)
6.5 lbs. of 2 Row
0.5 lb. of Chocolate Malt
0.5 lb. of Crystal 60
0.5 lb. of Dextrin Malt
0.5 lb. of Roast Barley
(8.5 lbs. total)
For our example batch, we will assume that 8.5 pounds of malt was mashed to produce 6 gallons of wort that yielded a gravity of 1.038. The brewer's total sugar extraction for this batch would be 6 gallons multiplied by 38 points/gallon = 230 points. Dividing the total points by the pounds of malt gives us our mash extraction in points/pound e.g. 230/8.5 = 27 ppg. This value is good, if not great; 30 ppg is basically what everyone shoots for. Comparing these numbers to lager malt's 37 ppg maximum gives us a good approximation of our mash efficiency: 27/37 = 73%, while 30/37 = 81%.
If we look at the maximum ppg numbers from Table 9 for each of the recipe's malts, we can calculate our actual mash efficiency:
Malts OG based on Max. PPG
6.5 lbs. of 2 Row 37 x 6.5 / 6 = 40.1
0.5 lb. of Chocolate Malt 28 x .5 / 6 = 2.3
0.5 lb. of Crystal 60 34 x .5 / 6 = 2.8
0.5 lb. of Dextrin Malt 32 x .5 / 6 = 2.6
0.5 lb. of Roast Barley 25 x .5 / 6 = 2.1
Total 49.9 points
In this case, our mash extraction of 1.038 means our percent efficiency was 38/49.9 = 76%. Usually I think you will find that your efficiency will be 80% or better.