In most cases, the thing that separates a good Russian imperial stout from a bad one is a well-run fermentation. In order to conduct a good fermentation, you need to select the right yeast strain, pitch an adequate amount of it, and create a healthy environment for the cells.
Russian imperial stouts start at a high original gravity (OG). When selecting a yeast strain, you want one that will deliver a moderate to high level of apparent attenuation (75–80%), especially for the biggest examples. This will ensure the final gravity (FG) is low enough that the beer is not overly sweet. You may want to select a yeast strain that produces a moderate amount of fruity esters, or you may wish for a cleaner ale strain. In general, a “fruity,” estery yeast strain will be more appropriate in a traditional English-inspired Russian imperial stout, while a more “Americanized” version should be fermented with a clean ale strain. If your want your Russian imperial stout to be at or near the high end of the hopping range, you should also select a yeast strain that does not mute the hop bitterness.
If you are going for an “English” interpretation, White Labs WLP002 (English Ale), White Labs WLP007 (Dry English Ale), Wyeast 1968 (London ESB), and Wyeast 1028 (London Ale) are all good choices. If you go with the WLP002/1968 strain, be sure to make an adequately sized starter or the yeast may flocculate early.
For a bigger, hoppier “American” version, White Labs WLP001 (California Ale), Wyeast 1056 (American Ale), or Fermentis Safale US-05 dried yeast will is a common choice. Scottish ale strains — White Labs WLP028 (Edinburgh Scottish Ale) or Wyeast 1728 (Scottish Ale) — can also deliver a clean fermentation if held in the right temperature range. However, these deliver a lower level of apparent attenuation, so they wouldn’t be a good choice in the upper half of the original gravity range.
Some Belgian yeast strains can ferment fairly cleanly with elevated pitching rates and good temperature control. The high level of alcohol tolerance in these strains make them a tempting choice. (Wyeast suggests their 1762 (Belgian Abbey) strain as one of the yeast choices for a Russian imperial stout.)
In order to grow an optimal amount of yeast for a 5.0-gallon (19-L) fermentation, you will need to make a 2.9–7.0 qt. (2.7–6.6 L) yeast starter. The smallest size would be for the smallest Russian imperial stout (around 8% ABV) and the largest size would be for the strongest beer (around 12 % ABV). In the middle of the starting gravity range, for a roughly 10% ABV beer — you would need a 4.7 qt. (4.4 L) starter. In general, pitching more yeast leads to a faster start, less chance of a sluggish fermentation, and a lower final gravity (FG). Higher pitching rates also lead to less ester production from the yeast. This effect is most noticeable in yeast strains that produce a lot of esters.
For the hoppiest Russian imperial stouts, you may want to pitch between half and three quarters of the optimal amount of yeast. This will lead to less hop bitterness being lost to the yeast.
Because of the large yeast starter sizes required to raise an optimal amount of yeast, some homebrewers try to get away with making a smaller starter. If you don’t have a vessel that is suitable for making a 2.9–7.0 qt. (2.7–6.6 L) starter, try simply making your yeast starter in your fermenter. Ferment the starter, then pour off the starter beer before racking your Russian imperial stout wort to the fermenter. Another option is to brew a small beer, such as a dry stout, and use the yeast from that to pitch your Russian imperial stout. Yet another option would be to pitch a liquid yeast package that will give you character you want in your beer, then boost the cell count by adding a neutral dried yeast. For a 5.0-gallon (19-L) batch under OG 1.095, one package of liquid yeast (at 100 billion cells) and one 11.5 sachet of dried yeast (at around 200 billion cells) would be near optimal.
Your yeast stain and pitching rate both influence the level of esters in your beer. For a clean fermentation, select a clean yeast strain and pitch at least three quarters of the optimal number of yeast cells. For a more estery beer, select a yeast strain that is known for its ester production, and control the level of ester production by increasing or decreasing the pitching rate. (You can purposely overpitch to get a beer with a diminished level of yeast character.) In a big beer like a Russian imperial stout, selecting and estery yeast strain and purposely underpitching will lead to a very fruity beer.
Tomorrow, I’ll discuss giving the yeast the proper environment — temperature, oxygen levels, and nutrient levels.