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Most of the time, adding yeast at bottling time is not necessary since enough healthy yeast remain in suspension to consume the priming sugar. But in cases where the existing yeast might not be sufficient, I was looking for a reference to for the quantity of healthy yeast to pitch at bottling time.

From this site, I have found references to 10%, 25%, and 33% of the original pitch rate, but there were no external references to those recommendations.

The balance seems to be between excess yeast sediment and speed of carbonation. If there is no hurry, it seems like a very, very small bit of fresh yeast would be sufficient. But to complete carbonation faster, more yeast would probably be required.

I am aware that Sierra Nevada bottle conditions their beer and they add yeast at bottling time:

Bottle conditioning is a time honored method for naturally carbonating beer and traditional champagne. We dose back a small amount of fermentable sugar and yeast into the bottle. The fermentation creates the finished carbonation and flavors unique to our beer. https://www.sierranevada.com/faq/beer

Despite adding yeast, I don't recall any dregs in those bottles. So they must add very few yeast cells, but I don't see the specifics.

Are there any sources or references for the minimum effective quantity of fresh yeast pitched at bottling time?

  • I've seen post mentionning 2-3 dry yeast grains per bottle, but I can't find it right now... – Philippe Jul 26 '17 at 20:58
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There actually are a number of references in the literature to this issue. In general it seems that a standard target would be pitching (or to be more accurate, ensuring there are) 1 million healthy cells per milliliter of beer.

I found a few references to actual bottle conditioning, and some to cask conditioning. I can't see any reason they wouldn't be interchangeable, and in fact their recommendations are all fairly similar.

The book Brewing: Science and Practice recommends 1 million cells/ml:

'The range of yeast count at which cask beer can be successfully packaged is from 0.25 to 4 million cells/ml, but nearer to 1 million is to be preferred'

This study, in which Brooklyn Brewery was involved, uses a similar rate:

'Dry yeast was [...] inoculated into beer (referred to as “primed beer”) at a final dosing rate of 1.2 × 10^6 cells/mL'

Another study recommends a slightly lower range, but relies on a sort of hybrid-krausening method (not just priming fully attenuated beer). The in-bottle specification for yeast count is listed as

'0.3–1 × 10^6 cells/mL'

To specifically answer this question:

'Are there any sources or references for the minimum effective quantity of fresh yeast pitched at bottling time?'

The lowest reference I found (first, above) is 0.25 million cells/mL, so I'd suggest that as the minimum. Remember that the choice of target cell count is always going to involve some compromise between speed of carbonation and clarity of the final beer, and is going to be dictated by the health of the yeast, the nature of the beer you are conditioning, the temperature at which you condition and probably a number of other factors.

For practical purposes, assuming a residual cell count of zero and that 1 gram of dry yeast would supply you 20 billion healthy cells, you'd need roughly 0.2 grams of dry yeast per gallon of beer to get the recommended 1 million cells per mL.

  • This answer is consistent with additional research that I've done; according to Lallemand's recommended repitching rate PDF for CBC-1, a 5 gallon batch would need 2.5 grams. lallemandbrewing.com/wp-content/uploads/2017/03/… – Dale Jul 28 '17 at 15:18
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    I find that in most situations there is more than enough yeast left in suspension to correctly carbonate an unfiltered beer. I have never had a failed bottle carbonation using the yeast in the beer but I tend to stick to a small range of yeast types that I know work well for me. – GrainMother Aug 2 '17 at 10:42
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    Random but related comment...A fellow brewer broke a hydrometer and, just to be safe, passed a partially fermented cider through a standard coffee filter. It did NOT attenuate! Even after a long time waiting. – Dale Aug 11 '17 at 22:38

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