Many of us consider the greatest difficulty of brewing to be during the hard labour before fermentation? However, difficulties abound throughout fermentation and, even more so at its end, specifically with the process of attenuation.

What, though, is attenuation? When does it start and, more importantly, when does it stop? A common answer is on the day of delivery since much beer leaves the brewery with only a limited knowledge of how it will perform in trade. Careful attention can, however, indicate whether a beer is likely to continue its fermentation after racking or, more worryingly, remain dormant in cask with little life to show on spiling.

Attenuation is the fall in specific gravity of wort as a result of fermentation converting sugars to alcohol. Such a fall is typically rapid in the first days of
fermentation and slows thereafter although this is dependent on yeast strain and activity. In some cases full fermentation may occur within just three days leaving little fine tuning to occupy the conditioning period in cask.

Not uncommonly attenuation is a fickle target with final gravities staggering their last degrees or sticking for prolonged periods at unreasonably higher levels. Many factors influence the final attenuation point of a fermentation, mashing conditions, oxgyen levels in collected wort, nitrogen, vitamin and mineral levels in collected wort and, of course, temperature. Each of these can be investigated as and when problems arise.

The profiles below show a sitable attenuation in C, a slightly retarded attenuation in B and a very poor attenuation in A. While A is easy to spot early in fermentation B can still cause difficulties in cask but is less obvious and hard to anticipate.

Anticipating your final attenuation is a slightly different issue, however, and not always a easy measurement to make. Final gravities depend most directly on the absence of fermentable sugars. Simple measurements by saccharometer may not be relevant if your yeast is tired, dead or simply inhibited. Gravity levels may still fall if yeasts reactivate.

A full final gravity measurement can, however, be made using the “milk bottle test” where a small sample of a brew is collected in a clean bottle and fermented to dryness in optimum conditions. A 2 litre PET bottle is preferable and should be prepared containing pitched wort directly from the fermenter. Keeping in an airing cupboard or near a heater (but below 30oC) would be ideal to encourage good fermentation. Shake the mini brew every day and check gravities regularly.

With a good yeast pitch final attenuation should be achieved in advance of the main brew and can allow you to anticipate where most of your beer should end up at racking.

Measurements may also be made to compare fermentations by calculating the percentage of the drop in specific gravity dividing it by the original gravity. This factor is termeed the apparent attenuation. For example a beer fermenting from 1.048 to 1.008 has an apparent attenuation of 40 / 48 x 100 which = 83%.

Knowing this figure is valuable for comparisons between brews and for long term analysis of performance. Of more importance is knowing just when to rack and whether to prime or not. Conducting a milk bottle test allows you to chill a beer before the end of fermentation so leaving adequate sugars for secondary fermentation (0.5 to 2 gravity units). Knowing this may save not only poorly conditioned casks but also those with too much conditioning as a result of an excessive gravity at rack. No publican cares for a wet cellar avoid this as well as flat pints can help your impression with any publican and help achieve that often elusive consistency.


Written by Dr Keith Thomas
Brewlab Director

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