Because of the uneven distribution of yeast cells through the fermenting wort, especially in large vessels, accurate direct quantification of the yeast, and therefore measurement of its growth rate, is extremely difficult and in many cases impossible. However, investigations carried out by the authors show that there is a fairly close relationship between the reduction in the "TNBS nitrogen" (assimilable nitrogen as determined by spectrophotometric measurement of the end product of a reaction between assimilable amino acids, peptides or proteins and 2,4,6-trinitrobenzene-1-sulphamic acid (TNBS)) content of the wort during fermentation and the multiplication of the yeast over the same period. It is concluded that this reflects the utilisation of a constant amount of assimilable nitrogen in the formation of each new yeast cell, calculated to average 1.5 micrograms per million cells. For brewery process monitoring purposes, it may be necessary to adjust the formula for calculation to allow for variations in temperature and cell size, which can affect the uptake rate. Since yeast growth rates can be increased or reduced by varying the oxygen or assimilable nitrogen content or the temperature, it should then be possible to correct any excess or inadequacy which is discovered.
Keywords : analysis assimilation brewers' yeast fermentation growth nitrogen content process control wort