In producing beer yeast is an important raw material. The viabilty, e.g. the physiological condition and the activity of the yeast, has great influence on the brewing process and the quality of the end-product - beer. In the context of this research project two parameters influencing the viability of the yeast - the intra-cellular levels of trehalose and glycogen - were analysed. Yeast propagation at different conditions were executed monitoring both levels.

Analysing the intra-cellular levels of trehalose and glycogen of yeast cells in batch propagation the following trend emerges: Levels raise fast at the beginning of propagation, drop to a low level and start to slightly raise again at the end of propagation time. Trehalose and glycogen levels show similar behaviour. The progression of the levels is influence by temperature and oxygen concentration in the wort. Low oxygen (< 0.2 ppm) leads to more defined peak levels, low temperature flattens the progressions especially in the case of glycogen. The changeover between decomposition and formation of glycogen occurs when the original gravity drops below approx. 5 %. Trehalose levels start to raise again in the event of deficiencies in yeast nutrition, for example low FAN values.

Plotting a normalization of the measured values against a normalized growth rate shows the same characteristics. Glycogen and trehalose reach maximum levels at 90 % of maximum growth after reaching the highest growth rate. There is a dependency between growth and intra-cellular levels of trehalose and glycogen in yeast cells.

This information may make it possible to propagate to  optimal intra-cellular glycogen and trehalose levels keeping other important process parameters - like biomass production - well within their normal range. With the use of a existing mathematical yeast growth model a Fuzzy-Logic controller could be developed to achieve this propagation results.