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)