Lesson 7 - Yeast Propagation SAQ

1. *Answer all three parts of this question:

a)    Give a definition of yeast propagation.

b)    What criteria are used to judge the methods used for preserving yeast cultures?

c)    What is considered, at present, to be the best method of preserving yeast cultures for long term storage?

2. ***Answer all three parts of this question:

Your brewery is expanding and wants to install a new fermentation tank farm. Currently, your yeast stocks are replenished periodically via a road tanker that supplies bulk pitching yeast from another brewery in your group. This yeast is not of reliable quality, and you have been tasked with designing a propagation plant so that you can manage your own yeast supply.

You will need to propagate three different yeast strains, each of which can only be re-pitched eight times. This will put considerable pressure on the propagation plant.

a)    Using a suitable diagram, show the key features of a propagator which would be suitable for this task.

b)    Describe the yeast supply system you would put in place to support the propagator. Provide details of the conditions you would use for each stage in the propagation: from the recovery of the strain from storage in the laboratory to the final brewery propagation. Indicate the likely cycle times and yields.

c)    Give a list of the criteria you would use to judge the success of the propagation process together with target specifications.

3. **Conventional brewery propagation and the preparation of active dried yeast use very different processes. Discuss how the physiological state of yeast arising from each process differs. Speculate as to what effects these differences might have on the fermentation performance when each yeast type is used.

4. *Answer all three parts of this question:

a)    Discuss what is meant by the term master culture when used in the context of brewing.

b)    If necessary, how would you isolate a new master culture from the brewery?

c)    What methods would you use to confirm that the new master culture was suitable to be used for propagation prior to use in brewing?

5. *Your brewery is required to handle four different yeast strains. All four strains are required for brewing simultaneously. Strains 1,2 and 3 have fermentation cycle times of two weeks, and they can be re-pitched eight times after which a new culture is required. Strain 4 has a three-week fermentation cycle time and can only be re-pitched five times before disposal and replacement.

Your brewery has a single propagation system which requires two weeks in the laboratory and two weeks in the brewery. The brewery propagation uses a two-tank plant at each stage, which takes a week to complete.

Draw up a propagation plan which ensures that pitching yeast is always available for brewing.