Bottling Beer With a High Final Gravity

My clone of Portsmouth Brewing's Kate the Great Russian Imperial Stout was brewed in early February, and got off to a rapid start.  I used a roughly 1000ml starter of WLP060 - American Ale Yeast Blend to contribute some lager-like flavors to the finished beer, and I watched the gravity drop significantly in the first few weeks of fermentation.  After thirty days I transferred to secondary and added two ounces of medium toast American oak cubes that had been soaking in bourbon.  At that point, the gravity had gone from 1.120 (28.01°P) to 1.030 (7.56°P) and airlock action had stopped.  Since then, I've consistently checked the gravity every few weeks and there's been no change.

I've never bottled a beer with such a high finishing gravity before.  The threat of over carbonating and creating bottle bombs has been pounded into my psyche by my homebrewing forebears since I started firing up a kettle almost a decade ago. Besides simply tasting the flavor profile, there are a few factors to consider when deciding whether or not to bottle a beer with a high FG and they basically fall into two categories:

1) the diligent application of environmental controls (pitch amount, temperature, ph, nutrient availability, aeration, etc.) and

2) yeast selection and the associated characteristics (below) that accompany that choice:

• ALE vs. LAGER:  This simple stylistic choice is pretty important for determining when a fermentation is finished, as lager yeast strains ferment a larger variety of sugars (specifically the trisaccharide raffinose composed of frustose, galactose and sucrose) which leads to greater attenuation.   
• ATTENUATION:  This refers to the amount of sugars that your yeast is able to convert during fermentation.  Yeast have the potential to consume all available sugars and attenuate completely, but rarely do.  Homebrewers are interested in desirable flavors and therefore commonly ferment at a lower than optimal temperature for ideal attenuation.  Likewise, flocculation (basically clumping together and sinking) of yeast is a desirable characteristic, contributing significantly to a beer's clarity.  If attenuation was the sole concern, then yeast rousing could force the yeast to stay active.  This also plays a role in determining whether to add fresh yeast for bottling.
• ALCOHOL TOLERANCE:  At some point the yeast produces enough ethanol that the environment becomes toxic and it begins to die.  Alcohol tolerance is a consideration when deciding whether to bottle with significant gravity remaining because it reveals how high alcohol can limit extended viability.
• STYLE:  If you begin with a very high gravity, you should expect to treat the yeast and fermentation a little differently.  Likewise,  with the above considerations in mind some beers are meant to finish higher.  Russian Imperial Stouts can acceptably finish between 1.018 and 1.030 according to the BJCP.

It seems that having stable final gravity within the acceptable range is most important.  Since my KTG clone had maintained a stable gravity during the entirety of secondary fermentation/oaking, I felt relatively confident that the yeast was done.  The stats associated with WLP060 - American Ale Yeast Blend seemed to agree.  WLP060 attenuates to between 72-80%, with medium-high alcohol tolerance.  My KTG clone finished at 73% attenuation and 12% alcohol.  With the FG being at the very top of the style range, I decided to go ahead and bottle.

I added a package of Safale US-05, providing fresh yeast to do the work of carbonation, and 5/8 of a cup table sugar dissolved into a simple syrup.  I want lower than normal carbonation in this beer as too much masks some of the deeper flavors.  I plan to age it for an extended period of time, drinking one now and then to taste how it changes.

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As an aside, I saved the oak cubes that I used in KTG and added them to Crumple Car Flanders Red.  Since this is their second application they should provide some moderate complexity prior to blending.  

Oak cubes retained from KTG.