By A. Slator, Ph.D., D.Sc, F.I.C. (Aug. 1941)
A method of estimating the attenuation limit of beers described in this note is of interest in view of the attention now being paid to this figure in the Institute of Brewing Researches. It is a convenient one for routine work, but has not the advantages of newer methods which give rapid results and it may not be applicable to all beers.
The method is briefly as. follows: — After estimation of the sp.gr. of the residue of the beer in O.G. determinations, this residue is transferred to a sterile bottle and seeded with a pure culture of yeast—preferably a secondary yeast. The seeded residue is then kept for some days at about 70° F. until fermentation ceases. The sp.gr. of the fermented residue is then determined. The fall in attenuation gives the fermentable residue left in the beer and the attenuation limit is obtained by subtracting this figure from the attenuation of the original beer.
A convenient bottle (unfortunately now difficult to procure) in which to carry out the estimation is a swing-stoppered one of capacity about 150 ml. The fermenting liquid is examined each day, the sediment disturbed by gentle shaking and the gas released by easing the swing stopper. If desired, the residue can be sterilised before seeding with yeast, but generally this was found not to be necessary.
For example (omitting the thousands): —
Sp.gr. of beer (attenuation) 12.5
Sp.gr. of residue 19.4
Sp.gr. of residue after fermentation 15.0
Fermentable residue 4.4
Final attenuation, or attenuation limit 8.1
The attenuation limit of worts can be estimated by fermenting the sterile wort with a pure culture of primary yeast and then analysing the “beer” for any residual fermentable sugar in the way described above.
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