Lesson 6 - Yeast Propagation
This lesson explores yeast propagation, the supply of pitching yeast and its fitness for purpose. We will explore yeast cultures and the protocols for its storage, along with how to propagate new cultures within the laboratory. This knowledge will enable us to demonstrate an understanding of the design features required for a yeast propagation plant.
Future Perspective
Propagation is an area of brewing where developments are happening all the time.
Many brewers, especially in the craft sector, are using active dried yeast. As we have seen, this yeast, grown under de-repressed conditions, is in a different physiological state to conventional pitching yeast. Usually, it is only used once. The quality of the beer made using this yeast is clearly acceptable and so apparent questions are why is it not used by all breweries, and what is the real impact of the physiological state of pitching yeast in fermentation performance and beer quality?
Sadly, we do not have all the answers yet. Yeast recycling has long been used. Brewers are used to it, and generally, it works well, especially as they have the necessary supporting yeast handling plant in place. Brewing is a conservative industry, and changes only happen if there are compelling reasons.
For brewers using very intensive processes, the need for frequent propagation can be testing. There are no current quick and reliable methods, other than a simple viability test, for detecting whether or not any batch of cropped yeast is fit for re-pitching. This means that current limits on generation numbers have to be based on some level of guesswork, and a lot of perfectly usable yeast is probably discarded.
Fed-batch propagators used for active dried yeast are an attractive proposition, but as we have already seen, they would need quite a sophisticated plant and a new growth medium. None of these are impossible, and maybe these would be an attractive proposition. Coupled with the option to use once only, and with a good storage potential of dried yeast, the arguments become even stronger. Perhaps even more important is that active dried yeast has no need for wort oxygenation since it is loaded with sterols and unsaturated fatty acids.
If this is the chosen way forward, then we need to learn a great deal more about the relationships between pitching yeast’s physiological state, the need for wort oxygenation and the fermentation performance.