Brewing a drinkable very strong beer takes a lot of work. (For the sake of this article, let’s define “very strong” as over 12% ABV.) Everything — from wort production, to raising the proper amount of yeast, to running the fermentation — requires more. More ingredients. More time. More care. One technique can make brewing the absolute biggest beers a bit easier — “feeding” the beer as fermentation progresses.
The basic idea behind “feeding” is to brew the beer at an OG lower than planned, but add the remaining fermentables to the beer during fermentation. For example, one way to brew a 14% ABV beer would be to make some wort with an original gravity around SG 1.140, then ferment it down to an FG around 1.035. Another way would be to produce wort with an OG 1.120 — which would yield a roughly 12% ABV beer, assuming the same degree of attenuation — but “feed” the fermentation with the additional carbohydrates that would have been required to make the wort 1.140. This adds extra steps to the fermentation, so why would you do it?
The rationale behind “feeding” is that the yeast never encounter a sugar concentration over a certain level. For example, in the second example above, the yeast never have to ferment wort over SG 1.120. After the yeast is pitched, the specific gravity would drop. The fermentables added later would boost the specific gravity, but not beyond the original wort strength. For example, perhaps the wort fermented down to 1.080, then the fermentation was “fed” so that the specific gravity was boosted to 1.100. When this beer finished fermenting, it would have the strength of a 14% ABV beer, but the yeast would never have had to deal with anything beyond a projected 12% ABV level of wort sugars. Very high gravity fermentations can be stressful and feeding is a way to ease some of this stress. (A side benefit is that, because your original boiled wort is lower in gravity, you may experience slightly higher hop utilization.)
You can feed your fermentation pretty much anything that would work as a kettle adjunct. The most obvious “food” would be dissolved malt extract. You could dissolve the extract to make a very thick wort, heat it to 170 °F (77 °C) to sanitize it, cool, and add it to your fermenting wort. Likewise, thick solutions of simple sugars such as cane sugar (sucrose), corn sugar (glucose), or honey would also work. Feeding the fermentation simple sugars, which are 100% fermentable, would not raise the expected FG of the beer as much as malt extract (which is not 100% fermentable).
The sugar solution you make needs to be thick enough to raise the overall “ virtual OG” of your beer, but not so thick that it doesn’t dissolve readily in fermenting wort. When you add the sugar, it should be stirred in with a sanitized spoon or racking cane. However, the wort should not be aerated at this point. Adding a small amount of yeast nutrients at each feeding is a good idea. However, the amount of yeast nutrients added for the final feedings should be tiny enough that the yeast can be reasonably expected to use them all up. If the yeast don’t consume the nutrients, any contaminants in your beer will have a source of nitrogen and other minerals to utilize.
Early next week, I’ll give an example of how to develop a recipe, how to produce the initial wort, and how to best feed the fermentation. One complication to the process is that, whenever you feed the fermentation, you also add volume to your fermenting beer. This needs to be accounted for, but it is not that difficult if you plan correctly. Another important point is how frequently to feed the fermentation and the best time to do it. Adding all the extra sugars at once is fine if the overall amount you’re adding is small. For larger amounts, when brewing correspondingly stronger beers, it’s best to use multiple feeding sessions.