A Craft Beer & Brewing Magazine reader recently asked us the following question:
I recently tried to brew a high-gravity imperial stout using all grain, but my original gravity came in much lower than predicted, and I don’t know why.
Brewing a high-gravity all-grain beer can be a challenge, and getting it right requires some forethought and planning. These beers typically involve original gravities of 1.080 or higher, which means you may be using double the amount of grain (or more) that you typically brew with. Not only does this push the limited capacity of your mash tun, but it also lowers the efficiency of your brewing system. Lower efficiency means lower gravities and even more grain to reach your target.
If we look at a normal lautering process, we see that the gravity coming out of the mash tun during the early runnings is quite high, and then it tapers off as we run more wort through the grain bed. In fact, it is not uncommon to get two-thirds of the gravity points extracted in the first half of the runnings and only one-third of the gravity points in the second half of the runnings. There is a method called “parti-gyle” brewing that does exactly this—makes one high-gravity beer from the first runnings and another low-gravity beer from the second half. (For more information about parti-gyle, see Practical Parti-Gyle Brewing)
Lets consider a 1.040 beer brewed on a 10-gallon (38 l) system. At 72 percent brewhouse efficiency, it would take about 15 pounds (6.8 kg) of grain to reach 1.040 original gravity assuming just over 13 gallons (49 l) of wort is produced pre-boil. If we up that to a 1.080 beer, we now need 30 pounds (13.6 kg), or double the amount of grain, to reach the higher gravity assuming the same efficiency. The problem is that we’re still producing only 13 gallons (49 l) of wort from the mash, so we’ve doubled the amount of grain but are making the same amount of wort.
Comparing the two cases of normal vs. big beer, we can see that doubling the amount of grain to produce the same amount of wort is much the same as the parti-gyle method above. When we make a normal beer, we have enough water running through the grain bed to get good extraction, but when we double the grain used, we’re taking the high-gravity runnings (the first two thirds of gravity) but discarding the tail end (the last one third), so mashing a big beer is much less efficient.
The bottom line is that when you brew a big beer, you will get lower efficiency. For an extreme case like this where we’ve doubled the grain used, it could be as much as 25–33 percent lower, which means our 72 percent brewhouse efficiency could drop as low as 50–60 percent. This means we would need another 10 pounds (4.5 kg) of grain to reach our 1.080 target! In practice, it may not be quite that bad, but even using a 60 percent efficiency number means an extra 6 pounds (2.7 kg) or 36 pounds total (16.3 kg) is needed for our 1.080 beer.
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