The importance of indices related to nitrogen metabolism in fermentation control.

Inoue, T. and Kashihara, T.

Abstract
The principal index by which the progress of the fermentation process is evaluated in most breweries is the attenuation of the wort sugars. However, it is well known that an apparently normal pattern of attenuation does not necessarily indicate that the beer will conform to specification. Investigations are described in which the concentrations of a number of wort constituents and fermentation products were recorded with a view to determining whether any of them might prove more informative in relation to the prediction of beer quality. It was found that the uptake of amino acids and the production of various flavour active constituents (including higher alcohols, diacetyl, acetaldehyde and organic acids) cease when yeast growth stops, but the attenuation of sugars continues. Furthermore, the relationship between amino acid consumption and yeast growth is constant, whereas that between attenuation and yeast growth varies according to fermenting conditions (wort aeration, stirring, temperature). As yeast growth was found to be well correlated with the production levels of those flavour active substances which are only formed while the yeast is growing, its measurement would be useful as an aid to predicting beer flavour, but it is very difficult to measure yeast concentrations directly under industrial conditions with the required accuracy. Accordingly, the authors recommend the determination of constituents known to be closely correlated with yeast growth (e.g. higher alcohols) as an aid to the control of fermentation.
Keywords : analysis beer brewers' yeast composition fermentation flavour growth measurement process control prospect