Although brewery fermentations cannot be precisely quantified the consideration of various phenomena in relation to yeast growth provides a norm of behaviour which directs attention to any divergence and the need to search for some interfering factor. The lag phase, exponential growth and steady state in a fermentation correspond to different metabolic processes each with characteristic formation of volatiles which contribute to the flavour. In practice the yeast population seldom doubles at each generation but still follows an exponential function, this specific growth rate alpha = 2.303/t(log10 N"/N') where t is the time to increase from N' to N". Reduced pitching rate extends the lag phase but in the same medium the specific growth rate does not alter. As shown by Brown & Kirsop (this Journal, 1972, 39) the formation of yeast mass corresponds to the utilization of sugar and to the production of fermentation products. The ratio of these varies with different yeasts and with fermentation temperatures which can direct attention to profitable modifications of the process. A rise of 10 degrees C will double the specific growth rate and the fermentation time could be reduced but the formation of volatile products does not always follow the yeast growth rate and at higher temperatures undesirable flavours may become prominent though it is useful to distinguish if these are specifically due to yeast growth or to the autolysis of cells which have passed beyond the growth stage. The amount of wort nitrogen does not affect the specific growth rate but limits the extent of growth, though this is usually sufficient for fermentation of the available sugar. This is reflected in experiments where wort was diluted with sugar solution, when yeast growth was independent of the rate of sugar admixture but was characteristic of the fermentation temperature. At lower temperatures the lag phase was extended on dilution with sugar.
Keywords: beer control flavour growth kinetics wort yeast