Bottle and Keg Conditioning: When to Add Yeast

If a beer is young enough or the fermentation conditions aren’t particularly hostile, bottle conditioning shouldn’t require additional yeast. Even with crashing and most finings, enough cells should still be in solution to adequately produce carbon dioxide. Beyond the slower speed of carbonation, the act of packaging can potentially introduce oxygen into the equation — a healthy amount of yeast will be able to scavenge this oxygen quickly, but a weak or slow conditioning period may lead to staling.

However, there are some cases in which it is beneficial to use fresh yeast for effective carbonation. If you are working with an acidic wort, high alcohol (at least 8% ABV), or the beer has had a long conditioning period (a month or more), you’ll find the best results when adding fresh yeast at packaging time. If possible (and desired), use the same strain or blend that was used for your primary fermentation. This ensures that the yeast will only consume the sugars you’ve added for conditioning and not any residual fermentables from the beer. If the desire is to use a more attenuative strain (such as in the next section about Brettanomyces conditioning), be sure to factor any additional gravity points that would be consumed into your desired carbonation.

When accounting for pitch rate, we recommend 1M cells of Saccharomyces/mL of beer (about one homebrew-sized yeast pack for 40 gallons of beer). Higher pitch rates will ultimately result in more sediment and the possibility of off flavors due to autolysis. If using the same bottling strain, it is best to transfer the beer into a blending vessel with the yeast already present to ensure even distribution, much like priming media.

When using Brett as a bottling strain, it can serve two purposes: carbonation, as discussed above, and flavor and aroma contribution. Using Brett when bottling is also a good way to avoid the production of tetrahydropyridine (THP) in your beer, as oxygen exposure is one of the factors that can lead to THP development.

Brett​’s contribution to the final level of carbonation depends on what sugars it can consume that Saccharomyces cannot. Brett attenuation can vary from 70 – 100% depending on the strain, so be judicious in picking the right strain and primary fermentation profile. While most Brett strains will not fully attenuate your beer when used in bottle conditioning, you can reliably expect them to chew off 0.5 – 1°P, or 2 – 4 gravity points, which means as much as 1 additional volume of CO₂. This is somewhat strain dependent (e.g., Brett. anomalus species will not attenuate as much as Brett. bruxellensis species), but until you have a good sense of how much extra carbonation the Brett will yield, it’s always a good idea to bottle these beers in thicker gauge bottles to avoid the possibility of over-pressurizing your packaging and winding up with glass shrapnel.

As for flavor and aroma, inoculation of as little as 50k cells/mL of beer has been shown to be enough to produce above-threshold compounds associated with Brett fermentation. The impact of a higher pitch rate of Brett, similar to Sacch, is a reduction in minimum conditioning time, but at the expense of more bottle sediment.

Ultimately, the way you decide to package and condition your beer depends on the time and equipment available to you, but a delicious, full-flavored beer can be achieved through a wide variety of methods. Take detailed notes, find what method works best for you, and perfect it!