This week I take a look at fermentation considerations when brewing a very high gravity beer such as an Imperial Stout or Barley Wine. Last week in Part 1, I covered several methods for achieving a very high starting gravity.
Selecting Your Yeast
Now that you have a high gravity wort, you need to consider how you are going to ferment it in a consistent manner to achieve your target finishing gravity. One of the worst things that can happen with a high gravity brew is a stuck fermentation where the final gravity is too high and you end up with a sickly sweet mess instead of the well balanced beer you were shooting for.
The first step in this is yeast selection. You want to pick a yeast strain that has both a high alcohol tolerance and good attenuation. All yeast strains have a limited alcohol tolerance, and many beer yeast strains can only handle 8-10% alcohol. This is obviously a problem if you are making a 12-15% stout or barley wine. So its time to actually do a bit of research and pull up the yeast specs on the web.
Ideally you want a yeast can handle higher alcohol, something in the 15%-18% range. Many of these yeasts are wine or champagne yeasts, and these yeasts are often quite suitable for beer. However, some Belgian yeasts as well as some California ale yeasts can reach into the 13% range.
Hand in hand with this you should look at the attenuation of the yeast strain. Higher attenuation yeasts will ferment a higher percentage of the available sugars and often do so more cleanly with less stress.
Pitching and Early Fermentation
When you pitch yeast into a new environment it takes some time to acclimatize and begin to properly regulate the membrane at the cellular wall. This is particularly true for dry yeast which has to both hydrate and regulate itself in a very short period of time.
At extremely high gravities, another concern is osmotic shock, which I detailed in this article. In very high gravity beers the sugar concentration of the wort can be so high it can actually cause the cell wall to fail before the cell has time to adjust, resulting in a high mortality rate for your pitched yeast. The solution as outlined in the linked article is to take time to hydrate dry yeast and also slowly introduce wort to your yeast so it has time to adjust to the full gravity.
Another consideration is aeration. Because of the high gravity, the yeast needs as much oxygen during early fermentation as possible. I recommend aerating your wort using pure oxygen or an oxygen wand before pitching your yeast. In addition, for beers above 1.080 in starting gravity it is very common to add a second dose of oxygen about 12 hours into the fermentation. The double dose of oxygen aids in rapid yeast expansion during the early growth phase.
Good temperature control is also important, particularly during early fermentation. Because of the high gravity the fermentation can run hot and if you don’t control the temperature it can result in higher production of fusel alcohols, esters and other off-flavors.
Fermentation and Aging
Assuming your fermentation went well, the next consideration is aging your beer as most high gravity styles require extensive aging. The first thing you want to do is make sure your fermentation is truly complete with a stable final gravity that does not change over a period of several days. I also like to leave my beer in the primary fermenter until most of the yeast has fallen out. You can aid this precipitation by cold crashing your beer once you are certain fermentation activity has ended.
Once the yeast has fallen out it is important to separate the yeast from the beer, as extended aging over a period of months can lead to off flavors if left on the yeast cake. On a conical fermenter this is as simple as drawing the yeast from the bottom of the vessel, but if you are fermenting in a carboy or bucket you will need to transfer the beer to a secondary fermenter or barrel if you are barrel aging.
HIgh gravity beers often require extensive aging. Ideally you want to avoid any oxygen during aging and maintain a controlled environment for the beer. For barrels it is a good idea to periodically top up the barrels with beer to replace the “angels share” that evaporates through the barrels.
It is very common to add spices or blend the finished beer. Due to the very high finishing gravity, a raw Barley Wine or Imperial Stout can often be too malty or sweet. To achieve proper balance you need tannins and acids from oak, wood, spices and dark malts.
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