Brewing High-Gravity Beers

After enjoying some of the high-gravity beers available in Ohio (and in Colorado during a visit), I began wondering how brewers were able to coax yeast beyond the range I had managed up to that point. I'd gotten a beer as high as 14% ABV without much trouble, but I'd never tried going beyond that.  I wondered how high I could get it.  I succeeded beyond my expectations, pushing the beer in question beyond 20% ABV.  That's higher than one of the professional brewers I spoke with, who regularly brews high-gravity ales and has done so for several years.

Below is a condensed series of tips I've collected. These have come from the White Labs WLP099 product listing, online forums, other articles on the Internet, and verbal conversations with other brewers.

Grist and Mash

• Realize that you're probably going to get a lower brew house efficiency on these high gravity beers than you get on lower-gravity batches. You'll have to compensate for that with more grain, adjuncts, malt extract, or a longer boil in some cases.
• Depending on your system, you may not have enough capacity for grain to reach your target gravity. You can compensate for this by:
• Using malt extract
• Using adjuncts like honey, maple syrup, candi sugar, corn sugar
• Using a multi-stage brewing process, where the wort from the first batch becomes the mash and sparge water for the next
• Boiling much longer than usual to further concentrate the wort
• Pulling off some of the wort and boiling it much harder in another kettle
• You want to aim for a very fermentable wort, because there is a good chance the yeast could give out before you reach your target gravity. You'll want to make it easier on them by breaking down the sugars as much as possible.
• One way you can make the wort extremely fermentable is to use glucoamylase enzyme or papain, often used in Brut-style beers.
• Wort pH during the mash should stay between 5.0 and 5.3
• Aim for an original gravity of 1.106 to 1.120 initially, and add fermentables later. This will keep osmotic pressure on the yeast low.  (White Labs suggests starting with a wort that would produce a 6-8% beer and adding fermentables during the first 5 days of fermentation.)

Boil

• As noted above, boiling longer will help concentrate a weaker wort
• Hop utilization in high gravity brews is lower, so hop the beer more than you think you should. There is a kind of sweetness produced in the high-gravity beers, too, and the hops will help balance that out. (I've not yet worked out just how much you want to hop these beers more than normal to maintain balance.)
• Add 2-5 times the amount of yeast nutrient you'd normally add during the boil.

Fermentation

• The key here is to reduce stress on the yeast as much as possible. At a high level, this means:
• Minimizing osmotic pressure by starting with a lower-gravity wort and adding concentrated fermentables
• Giving the yeast a healthy environment with plenty of oxygen and nutrients
• Inoculating the wort with a lot of yeast cells to compensate for the stress of the higher gravity wort
• Keeping temperatures low to prevent off-flavors and ensure yeast health
• Agitating the wort for the first several days to keep yeast in suspension
• Pitch 3-4 times as much yeast as you would normally pitch for the size and gravity
• Keep the fermentation temperature down to the lowest end of the yeast's optimal range. This will minimize the production of fusel alcohols and other potentially-unwanted byproducts from the yeast. It will also minimize stress on the yeast.
• Until the yeast reaches 67% attenuation (approximately), you should do the following daily:
• Add oxygen:  Aerate 5-10 minutes with a pump or 30 seconds with pure oxygen. Aerate about 4 times as much as you normally would.
• Add nutrients:  Add a fresh dose of nutrient
• Agitate:  Swirl the fermenter to get the yeast into suspension and help distribute the oxygen and nutrients
• Add fermentables: Add sugars and/or concentrated wort daily for the first 5 days, until your target gravity is achieved.  After all fermentables have been added, keep doing the other steps above until 67% attenuation is reached.  
• Be sure to keep detailed notes on these additions, including weighing your fermentable additions. If you wind up with a great beer, you will want to be able to reproduce it, and these notes will prove important.
• After 67% attenuation, I recommend continuing to agitate the yeast until final gravity is reached or the yeast stalls out.
• If fermentation seems to be slowing or stalling, add another yeast strain with higher attenuation and alcohol tolerance to help keep the process going.  
• Champagne yeast is one option
• CBC-1 Cask and Bottle Conditioning yeast is another
• If it's appropriate to the style, adding Brettanomyces is another option
• Be patient. It took about two weeks to get my one-gallon test batch to 20.7%.
• Adding some sanitized wood chips is a way to offset some of the sweetness of the beer and introduce barrel-aged flavors.

Post-Fermentation

• The beer may show a lot of "green flavors" when it is young. The solution is to age it for a longer time, possibly even a few years, until it mellows and becomes pleasant to drink.
• Note that hop character changes over time, so if your style is one that relies on hop flavors, you'll want to taste the beer (perhaps monthly) to figure out its optimal aging time.
• Keep notes on what you did to brew, ferment, and age the beer so that if you decide to re-brew it you will be able to reproduce the beer as you liked it best.
• It's difficult to create balance in a high gravity recipe. Starting with an established winning recipe could help. 

I plan to do some additional high-gravity experiments this year and talk with pro brewers when I can, and may update this post as I learn new things about the process.