Sugar Extraction and Predicting OG

When you make beer, there's two main ways that you can go about doing it, one method is making it from extract and the other is via the all grain (AG) method.  When you first start brewing it's typical to go out and buy extract, it really speeds up the brewday.  When I talk about extract I refer to the sugars that go into our beer compressed into either a powder or syrup:

You can buy these by the pound from your Local Homebrew Shop or online.  It is a little more pricey but you're paying for a shorter brewday because you can just start the brewday by boiling the water.  On the other side we have the all grain method.  The AG method involves an extra step called the "mash", which can take more than an hour to complete in most cases.  It is basically steeping the crushed grains in hot water for an hour in order to extract the sugars from them.  And remember from the Determining Percent Alcohol post, the more sugars we can get from the grains, the higher the Original Gravity will be.  But I'm going to tell you a little secret *drum roll*... I've already predicted how much sugar will be extracted before I even steep the grains.  I mean, what kind of scientist would I be if I couldn't predict the future with math?... not a very good one.

The mash, as I said before, is steeping the crushed grains for an hour in hot water.  How hot?  About 150-158 degrees F.  Why?  That's the temperature range the enzymes in the grains are active.  These enzymes breakdown the starches from the grains into simple sugars that we can ferment into alcohol, so in summary, these enzymes are pretty freakin' important if you want to make beer.  It goes something like this:

The way we predict how much sugar will end up in the wort is through a term I like to keep track of called Mash Efficiency.  Mash Efficiency is recorded as the percentage of sugar extracted from the grain.  Each type of grain has it's own Grain Potential (reported as 1.0xx), which is what you would read on the hydrometer if you were to mash 1 lb of that certain grain and drain out 1 gallon of water with 100% extraction of the sugars.  This number may look very similar and in fact the xx actually are what we call "points", or another term, Gravity Units (GUs) from the BU/GU ratio post.  We refer to these as points sometimes because a term commonly used by brewers is ppg, or points per pound per gallon, which is what I described earlier about the 1 pound and 1 gallon extraction.  The terms are interchangeable, I could say "2-row has a Grain Potential of 1.036" or "2-row provides 36ppg", they mean the same thing.  For calculations we'll use the ppg value (or GUs, if you're more comfortable with that terminology).

Let's work through a real life example.  For the next brew that we have coming up, it's a relatively simple pale ale, containing only 2 types of grains. Here's the grain bill:

7.5 lbs 2-row (Grain Potential = 1.036)
0.5 lbs 60L Crystal Malt  (Grain Potential = 1.034)

Total = 8.0 lbs

The plan is to mash this 8 lbs of crushed grain with about 155F water for one hour.  How much water?  Up to you, it sometimes depends on the style of beer, but for the most part I've seen people use from 1.00qt/lb to 1.75qt/lb.  We usually go with 1.15 quarts of water per pound of grain, which works out to be 9.2 quarts or 2.3 gallons in this case.

Moving on... how much sugar will we get from this?  Well, I'm glad you asked.  Since we plan to make 5 gallons from this recipe, we'll probably need 6 gallons of wort generated from the mash, this will be called our pre-boil volume (it is typical for 1 gallon to be boiled off over the course of 60 min).  The pre-boil volume is whatever volume we get from the mash, so that will be our "per gallon" value in ppg, in this case 6 gallons.  Next we need to calculate how much sugar can be extracted if we extract 100% of the sugars from the grain.  Since the 2 grains we're using have different Grain Potentials we can calculate them separately, then sum them in the end:

2-row:                 7.5 lbs x 36 = 270 GUs
Crystal Malt:       0.5 lbs x 34 = 17 GUs

270 + 17 = 287 total GUs

So we can get a total of 287 GUs from all 8 lbs of grain.  Now we divide by the pre-boil volume:

287 GUs ÷ 6.0 = 48 GUs

What this tells us is that after the mash is over, and we're left with 6 gallons of wort, if we extracted 100% of the sugars our hydrometer should read 1.048.  But as I said earlier, that's not very likely to extract all 100%, so now we can multiply it by the Mash Efficiency.  In our setup, the mash efficiency is close to 80%, meaning we extract 80% of the sugars from the grains everytime we mash:

48 GUs x 0.80 = 38 GUs

Or in a real situation we would actually expect the hydrometer to read 1.038 because we would have extracted 80% of those sugars.  Doing one last basic step, we'll use the equation for dilution to find our final concentration of our wort after our boil from 6 gallons to 5 gallons (where Mx is concentration and Vx is volume):

M1V1 = M2V2
M2 = M1V1 ÷ V2
M2 = (38 GUs)(6gal) ÷ (5gal) = 46 GUs

So our final concentration of our wort (or the Original Gravity) is predicted to be 1.046.  How about that?  And we haven't even brewed yet!  A lot of times you'll here experienced brewers say "Oh, I was only 2 points off from my Target OG".  This OG calculation that I've just gone through is what they're referring to as the Target OG.

Mash Efficiency can vary from brewer to brewer and no one can tell you your mash efficiency for you.  You just have to brew and find out.  The only numbers you need to measure on brewday are the hydrometer reading just before you boil the wort and the exact pre-boil volume.  Along with the grain potentials that are available online, you can calculate your own mash efficiency for the next time you brew.  For our example, let's say that we mashed the grains mentioned above and got 6 gallons drained out and our pre-boil hydrometer reading was 1.036:

(Our hydrometer reading in GUs) ÷ (hydrometer reading for 100% of sugars extracted in GUs) = 36 ÷ 48 = 0.75 = 75%

If that was our hydrometer reading then we would have a mash efficiency of 75%.  If you're just starting to do all-grain brewing they say assume 75% mash efficiency when brewing for the first time, then you can calculate your actual mash efficiency from that.

Lots of factors affect the mash efficiency, such as crushing the grain (too fine or not crushed enough), water temperature, dead space where water gets trapped in your mash tun, etc.  So you really just have to experiment and find a balance that works for you.