Attenuation

Journal of the INSTITUTE OF BREWING, Oct. 1922

ATTENUATION

In a recent article (Ann. Brass. et Dist., 1922, 21 [ii], 1—3) our foreign member, Professor A. Fernbach, discusses the question of attenuation and the factors governing it.  There is much divergence of opinion on this subject, and theories are advanced and deductions made which are not justified by our present knowledge.  An authoritative statement of the known facts and the logical deductions therefrom is therefore very opportune.

Following the course of fermentation by observing the apparent gravity of the beer at regular intervals is the readiest and simplest method available to the brewer of assuring himself that fermentation is proceeding regularly and of indicating when it is finished.  Up to quite recent times it has been the practice in some breweries to work to a time-table—that is, a beer would be allowed to ferment for a specified time (fixed by experience) and then racked, regardless almost of whether the primary fermentation was complete or not.  While it is true that, given standard conditions throughout, the time required can be foretold with reasonable accuracy, such empirical methods cannot be expected to be infallible when dealing with fermentation—a process depending on the correct functioning of an extremely delicate organism, and susceptible to large variations from quite small causes, such as a difference of 1° F. in the pitching temperature.  Again, the composition of the wort—a complex and very variable factor—is all important.  This has a direct bearing on the degree of attenuation obtainable, and also on the activity of the yeast, and, consequently, on the time required to reach the limiting value.

Thus, a wort rich in fermentable sugars will give a low final gravity, whereas if—through imperfect saccharification or intentionally—the wort is rich in dextrins and of low sugar-content, then this will be high.  There is also a direct relation between the amount and type of nitrogenous material in the wort and the activity of the yeast.  A highly saccharified mash will contain relatively large amounts of the less complex nitrogenous compounds, and will thus give a low final, both in virtue of its maltose-content and the increased activity of the yeast.  Other things being equal, it may be stated that the composition of the wort and the type of yeast control the degree of attenuation.  The time taken to reach this limiting value depends on many other factors, and cannot be predicted with any great accuracy by rule of thumb methods.

It is often stated that raising the temperature of the fermenting wort or aerating has a beneficial effect when the attenuation is insufficient.  It will be seen from the preceding statements that the degree of attenuation is virtually fixed in the mash tun, subsequent procedure only influencing the time required to reach the limit.  In fact, aerating and raising the temperature may have exactly the reverse effect from that anticipated.  One result of such procedure is to improve the working conditions for the yeast, and thus speed up fermentation; the yeast develops, and in doing so utilises part of the fermentable sugar for cell growth, which would otherwise have been converted into alcohol.  In effect, therefore, the final gravity is raised.

In regard to the best limits for attenuation it is impossible to dogmatise, but it may be stated in general terms that the ideal to be aimed at is the production of a fully fermented beer.  Such a beer is best able to resist harmful influences, and will prove most satisfactory.  Any control of the degree of attenuation should be made during mashing, and not by attempting to adjust matters subsequently.