Dear Mr. Wizard,

I saw the ads for the Coopers carbonation drops and they seem like an easy alternative to the hassle of counter-pressure filling, but here’s the rub: I don’t like that little white layer of yeast in the bottom of the bottle.

I actually like filtration to make my beers fairly clear. Is there a happy middle ground? Is there a beer filter with rougher filtration that would allow just enough yeast through to let the carbonation drops work without leaving the residue in the bottom of the bottle?

Mr. Wizard Responds:

So you’re one of those picky brewers who wants the best of both worlds! Fortunately, there is a way to have bottle-conditioned beer sans yeast cake, but rough filtration is not the method of choice. Most filters are designed to remove particles above a certain size and not all filters are successful in their duty. You are looking for a filter that removes most of the yeast, say 90%, but allows 10% to remain in the beer — a product like this is not on the market. There is, however, an alternate approach. Some brewers producing cloudy beers, such as hefeweizen, want some yeast in the bottle but not the full load suspended in the beer after aging. A common approach to this quest is to filter or centrifuge a portion of the beer and blend the clarified beer with cloudy beer before bottling the mix. This technique is very easy to setup and can be done inline between the fermenter and bottling tank. To do this, install a bi-pass around the filter or centrifuge where the cloudy beer and clarified beer streams are recombined on the way to the bottling tank.

Another method is to filter the entire fermenter and remove all of the yeast remaining from fermentation and then to add fresh yeast prior to packaging. There are two different reasons for using this method. The first is aesthetic. Some yeast strains are very flocculent and settle in the bottom of the bottle into a tight pack of yeast. If the beer coming out of the bottle is supposed to be cloudy, it may not be if the yeast is too flocculent. If the yeast is disturbed the consumer may see chunks of yeast in their glass as opposed to a uniform cloudiness. Some weizen brewers remove the fermenting strain from the beer and replace it with a less flocculent yeast strain. In fact, some bottled hefeweizens actually contain lager yeast in the bottle since they are typically less flocculent than ale strains.

The other reason to remove the fermenting yeast from the beer prior to packaging is consistency. If I want to bottle condition my beer and control the amount of yeast in the bottle, adding fresh yeast to filtered beer is a very good method of accomplishing these goals. Sierra Nevada Brewing Company uses this method for these two different reasons. Like you, Sierra Nevada wants a very faint film of yeast in their bottles that is almost imperceptible to most consumers. They also want to bottle-condition their tasty ales and do not want to trust this final step of brewing to any old yeast hanging about in the fermenter. Instead, they use freshly cropped yeast with the highest viability for this purpose. This is important since the amount of yeast added is just enough to get the job done, hence the freshest yeast is selected. The yeast concentration in a bottle of Sierra Nevada is about 1 million cells per milliliter of beer. This equates to about 1/10 of the yeast added for primary fermentation. Assuming the same cell density in the yeast slurry is 100 million cells per milliliter (typical for a starter) you need to add 3.5 milliliters or 1/10 of an ounce of slurry per bottle — a very small volume indeed.

The easiest way to do this at home is to begin by growing up your yeast starter. Since you will only need about 200 milliliters (~7 ounces) of yeast you can simply buy a liquid starter if that is more convenient. Then, filter your beer and determine the volume of filtered beer by using a calibration strip on your bottling bucket. This step is important because you are going to add the priming sugar based on beer volume. Add the required amount of priming sugar to the bucket along with the yeast. The amount of yeast can be easily estimated by dividing the beer volume by 100. If you have 5 gallons (18.9 liters of beer) you will need 0.05 gallons (0.189 liters) of yeast slurry. Mix up everything and bottle.

One word of caution about beer handling in general is oxygen pick-up and its affect on beer oxidation. Minimizing air pick-up during racking is key and especially during filtration since yeast, which is a good oxygen scavenger, is removed. The small amount of yeast added in the method described above is insufficient to prevent oxidation and care must be taken during the process. Carbon dioxide blanketing and measures taken to prevent splashing are both recommended.

So now you can have your cake and drink it too!

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