Nowadays there are calculators for everything, so even the very best homebrewers don’t really understand the math behind calculations for things like grain absorption.
Of course, that’s perfectly fine and you can have a very happy career as a homebrewer without ever needing to understand this. However, I wanted to find out and I’m going to share my research in this article.
The general consensus among brewers is that the average grain absorption rate is 1 pint of water per pound of grain (1 liter/ kilogram). However, it’s important to be aware that many factors influence the rate of absorption & each brewer may experience a variation in their own home brewery.
This is actually a difficult question to answer in only a few sentences and there are quite a few factors to be aware of.
This is why I have written up everything that I found during my research so that I could share it with you. Please read on for more information.
What is grain absorption exactly?
Ok, first thing first, what do we mean by grain absorption and when does it happen?
In brewing, grain absorption is a term used to describe when the malt grains we are using during all-grain brewing takes on some of the water used during the mashing process.
The grain, because of its structure, can suck up a relatively large amount of water and so the bigger your grain bill the larger your grain absorption rate will be.
This is something you need to account for in your overall recipe and brewing calculations. For more information about how much water you need in brewing, please read my article.
If you can’t predict grain absorption accurately then you are going to have a very different final beer to the one you bargained for.
How does grain absorption affect your batch?
Well, if you weren’t aware of grain absorption, which you are as you’re reading this article, or at the very least the rate that water was being absorbed then your original gravity (OG) of your wort would be off.
If, for some reason, you only had a certain amount of water to hand you might find that after sparging your grain that you didn’t have the batch yield you were hoping for.
So, in reality, there isn’t really a big issue with grain absorption as long as you can plan for it and add the right quantity of water to hit your OG as stated in your recipe.
However, grain absorption and other forms of water loss can be very subjective and really do depend on your equipment and the way your grain was milled.
Changes in grain bill weight
According to Ray Daniels in his book ‘Brewing Great Beers‘, during the mashing phase grains which are heavy in pre-fermented sugars and other substances will be reduced to 40% of their original weight. This is why they are able to absorb such an amount of liquid per grain.
How to calculate grain absorption
As we’ve said, calculating grain absorption is very subjective and will depend on many factors. When it comes to the grains themselves, this can include the humidity on that day, how the grain is crushed, how it is mashed and, of course, the type of grain you are working with.
So, I want to talk about two methods below, one is a general rule and the other method will help you perfect a particular beer.
Calculating Grain absorption by ‘rule of thumb’
There are a couple of general rules that are spoken about around the homebrewing community. If you work within them generally your densities will be pretty much spot on.
Note: Denny Conn suggests that 1lb/1gallon is great for the system that he uses.
However, it’s generally accepted that the perfect rule of thumb for most homebrewers is:
- Grain bill: 6 pounds (2.72 kgs)
- Strike water: 5 gallons (19 l)
With this grain bill and the initial amount of strike water you would expect the grain to absorb roughly 6 pints of wort (2.83 l)
Calculating your true grain absorption
In all honesty, if you want to know the exact grain absorption rate you need to calculate this yourself by recording certain measurements on your next brew day. This is especially useful if you are trying to perfect a particular beer recipe.
First, let’s assume that you are already aware of the rate of water lost from other sources for your personal set up. If you don’t know this, then read the next section for more information.
To calculate your absorption rate you simply need to measure the difference in the strike water you put into your mash tun for mashing and the first running (wort you extract) during the sparging phase.
*First running – Mash Tun Loss
Other water loss causes
Although we are talking about grain absorption here today, it’s worth mentioning some of the other ways in which water is lost during brewing as it does factor in on the calculations.
Evaporation (boil-off rate)
This is how much wort you will lose through steam during your boil. Again it really depends on your heat source, but 1-3 gallons per hour isn’t unusual.
This means that you either have to top up your wort before pitching or you need to start boiling with more water than your final target yield. If you are unsure how to choose the correct size brew kettle for your home brewery, please read my full guide about all the things you need to consider here.
Mash Tun loss
Again, how much wort you will leave behind in your mash tun depends on the mash tun itself and the dead space built into it.
When using a false bottom or if racking your beer through a spigot, everything below those areas will be lost. Often the quantities are small but they are still significant to know about before you start calculating for your brew.
Luckily, once you know this number you can keep it as a constant for as long as you brew with that piece of kit.
Measuring Mash Tun Loss
The easiest way I know how to calculate this is to pour a known amount of water into your mash tun, preferably until the false bottom or spigot level are covered, then empty it.
Simply subtract the amount of water you can extract from the mash tun from the total, which will tell you the mash tun loss for your equipment.
Although generally something associated with fermentation, it’s worth talking about this here too. Trub is a blanket term for the material left over at the bottom of the fermentation vessel after you have racked your beer into bottles of a keg.
How much water loss you get really depends on the design of your fermenter and the amount of hops you used. Generally, you want to leave all the liquid below the level of the spigot, if your fermenter has one or enough to ensure you don’t pick up any solid matter when racking your beer.
Loss in tubes
You will lose a very tiny amount of wort in your tubing as you transfer it around, but really it’s not going to be very noticeable by the end of the brewing process.
Grain absorption calculators
There are several online calculators for grain absorption, often as part of overall water calculations, as well as some apps to choose from. Below is a small selection:
Is grain absorption different for different types of malt grains?
Although you can use a general rule of thumb measurement when it comes to calculating grain absorption, each grain is different and will be different each time you brew with it.
Firstly, the absorption properties of grain depend on how it is milled. Very fine grist will absorb a lot more wort than lightly crushed grain.
As I said before, the humidity of the ambient air and whether or not the grain is fully dry or not will also affect absorption as will the heat and length of mashing.
Generally, maize, millet, and sorghum (used a lot in Africa) will take longer to reach their saturation point, so they will absorb less than other grains during the mashing process.