When I talk about high gravity brewing, I am talking about OG’s in excess of 1.095. I am talking the really big beers, 10% or higher. Sounds easy right?
If you are an extract brewer, it is easy. Just keep adding malt extract until you reach your desired gravity. But if you are an all grain brewer it’s not as easy as adding more grain. Here is why.
The first time I tried to brew a high gravity beer I was shooting for an OG of 1.115. This should have resulted in an 11.5% abv beer. I designed my recipe, packed 27.85 lbs of grain into my 17.5 gallon mash tun, added 8.7 gallons of mash water, and mashed at 149 for 90 minutes. Vorlaufed and fly sparged with 3 gallons of wort to collect a pre-boil volume of 7.75 gallons which will be boiled down to 6 gallons, my batch size.
My pre-boil gravity should have been 1.090 but it was 1.075. I quickly ran my mash efficiency calculations and realized I had only got 58% mash efficiency. I had calculated 75% mash efficiency in my recipe so what happened? When I get 75% mash efficiency it’s because I usually have 5 to 7 gallons of sparge water in a regular size batch. With the huge grain bill there was less than 3 gallons of sparge water and I was not able to thoroughly rinse the grain bed. This had a huge impact on efficiency and my finished product. I wound up with an OG of 1.095 and a 9.2% beer.
What do I do now? I could of added malt extract to the wort until I hit the gravity I wanted, and it still would have tasted great! But if you are going to brew all grain then I want to use all grain. So, back to what do I do now?
I went ahead and boiled it and just lived with the lower gravity which was still a good beer. But the next time I brewed this beer, I mashed and sparged exactly the same and then I added 5 gallons of wort back into the mash tun, stirred vigorously for a few minutes, vorlaufed and drained. Wouldn’t you know, this time the gravity was a little high but by the time I topped up with water to compensate for the extra grain absorption I was at 1.114 post boil OG.
That is how I compensated using the capabilities of my system. If you have a smaller mash tun, or if you brew in a bag in a smaller vessel you may have to perform multiple mashes, or reiterated mashing.
The basic idea is mash half your grain with all your water and then conduct a second mash with the other half of your grain using the wort you collected from the first mash. This can be expanded with even more mashes to get to the desired high gravity. This process allows you to brew high gravity no matter the size restrictions of your equipment.
I have found two great sources of information on reiterated mashing. One is an article from Brew Your Own Magazine and the other is a podcast from the Brewing Network where John Blichmann, Jamil Zainasheff, and John Palmer discuss the process. Cheers!