All Grain: Grain Absorbtion

BeerSmith has incorrectly calculated my pre-boil volume for several batches now and I finally decided to figure out why.

Normally, I use the tools in the recipe screen to calculate water volume (i.e. equipment profile). I hunted around and I found the "Water Needed" tool under the Tools section. I plugged in all of my equipment profile data and saw that it was calculating grain absorbtion. Since that was the only place that water could mysteriously disappear, and I hadn't been considering it in the past, I figured that was the place to look.

According to Designing Great Beers, page 64, the amount of water trapped in the spent grain is predictable: 80% water, 20% spent grain. He says that while you can weigh your spent grain, it is safe to assume that the grain weighs 40% of its original weight after mashing and sparging. You then multiply this value by 4, since there is 4 times more water in their than grain.

Let's use a recent batch as an example. I started with 7.03 lbs. of grain. After mashing, it should weight 2.812 lbs, and have absorbed 4 times that amount in water: 11.248 lbs. Water weights 8 lbs. per gallon (also according to Designing Great Beers, page 64), so that means I lost 1.406 gallons, or 5.624 quarts. I then used the BeerSmith "Weight to Volume" tool to find that water weighs 8.35 lbs. per gallon, and confirmed that value with a google search. So the real amount lost to grain is 1.347 gallons or 5.39 quarts. he also has a table that you can refer to (Table 8.1, page 64). When you look up '7' under 'pounds of grain mashed', you get 1.5 gallons, or 6 quarts. This is even more off than the hand calculation, which itself was pretty flawed.

Now let's try BeerSmith's "Water Needed" tool. I plugged in 7.03 lbs of grain and got 3.37 quarts of water lost. According to BeerSmith, they use a static amount of water lost per pound of grain and you can't change it. This is a shame because, as we are about to see, everyone's system loses different amounts of water to grain. BIAB, in particular, loses less water.

What did I actually get in practice? Let's go over my method. I mashed in with 10 quarts of water, lifted the bag out and held it over the pot while it drained. Then I placed it over a bowl on top of a cooling rack to drain while I poured the sweet wort into a bucket. Whatever drained into the bowl (a couple of ounces) gets tossed in too before I stirred and took a gravity reading. I repeated this same thing with both sparge batches. I did not squeeze, poke or otherwise fiddle with the bag.

I started with 20.5 quarts of water. I ended up with 17.75 quarts of water in the kettle. Loss to grains was thus 2.75 quarts: significantly less than either calculation. That means I lost 0.3912 quarts per pound, while BeerSmith assumed I would lose 0.4794 quarts per pound and Mr. Daniels suggested that I would lose a whopping 0.8 quarts per pound.

The lesson here is that you should not assume your system will *ever* produce the same results as someone else's, no matter how predictable they claim the value is. Take recommendations with a grain of salt, use them as a starting point and DO YOUR OWN EXPERIMENTS.

Why is this even important? If you end up with more water in your kettle after mashing, that means you either have to leave some out, thus lowering your efficiency, or boil longer, or end up with more beer with a lower gravity. Boiling longer is the only choice if you want to try and create the same beer you were aiming for, but that can be pretty unpredictable too.

edit: My new calculation of 0.3912 quarts of grain loss per pound worked flawlessly. On the very next batch, I got the exact boil volume predicted, give or take a few ounces measurement error.