Status of the yeast propagation process and some aspects of propagation for re-fermentation

Yeast propagation is an old and well established process in breweries. Yet, development is constantly going on and some questions still remain unanswered. The demand for good brewers yeast can be summarized in the following words: A non-stressed, highly vital and viable yeast that is free from infecting organisms. The road to this goes over a carefully designed sanitary propagation plant with an aeration aggregate that is able to supply a sufficient and adequate amount of oxygen to all cells in the propagator without causing mechanical stress to the yeast cells, and an appropriate propagation wort with the right nutrient composition. No matter how much these parameters are optimized it is, however, still only possible to obtain relatively low cell numbers of around 100–200 million cells per millilitre equivalent to approximately 2.5–5 g dry matter per litre. This cell number can be obtained by consuming only a few degrees Plato. In the case of propagation yeast for re-fermentation not only the cell number is essential, but also the fact that the fermentable sugar in the wort is consumed so that an overpressure is not created in the bottle. In order to avoid loosing time while waiting for the yeast to consume all the sugar another process must be used. The only way to do this, is by learning from the baking yeast industry and perform the propagation in a fed batch reactor whereby the sugar concentration is kept constantly low – though not too low in order to avoid that the yeast grows purely aerobic and therefore may loose some of its fermentation characteristics – during propagation. Therefore, a kind of hybrid process between the traditional brewery propagation and a purely aerobic yeast propagation process seems to be the right solution. The feasibility of this process is discussed.