Lesson 9 - Yeast Management SAQ

** Your brewery is upgrading its fermentation facilities. Currently you use top-cropping open square fermenters but these are old and are suspected of being a major cause of recent microbiological spoilage problems. It has been decided to replace these with fewer and bigger cylindroconical vessels. At the same time it has been decided to introduce high gravity brewing. You have been asked to write a report which highlights the potential gains of this plan but also to identify potential problems. In addition, you have been asked to indicate what the likely impact on the current yeast handing system will be. What would your answer be?

Response:

1. Impact on fermentation and cropping.

It is presumed that the new vessels will come with an effective CIP system, and this, coupled with their closed nature, should have an immediate positive impact on hygiene.

The new vessels are bigger and will use a high gravity beer, so there will be immediate gains in fermentation productivity.

The new vessels are bottom-cropping, so the immediate concern will be how the current yeast performs. All yeast strains will form bottom-crops eventually, but some work less well than others. The only meaningful way to check this is to carry out proving trials, so a programme of these will need to be developed:

a)    This will identify what sort of crop develops and how much suspended yeast remains in the beer. If the latter changes significantly, how will this affect beer processing?

b)    What is the yeast solids content of the crop and how different is it from current top-crops?

c)    What is the optimum time to take the crop and which fraction should be retained for re-pitching?

d)    What is the viability of the crop, and is it different from current top-crops? If it is lower than current, will this have any undesirable consequences?

e)    How much non-yeast solids are present in the crop? Will this have any impact?

Fermentation control will need to be optimised for the new vessels. They are bigger, will take longer to fill, and the closed design will have an effect. It will be necessary to identify optimum pitching rates and wort oxygenation profiles. Both of these will need to be increased because of the move to high gravity brewing. Flavour-matching trials will be needed to identify a regime that produces acceptable beer.

2. Yeast handling

There will be a significant impact here:

a)    The old top-cropping system will be superfluous and will need to be replaced with one suitable for removing bottom-crops.

b)    The current method of yeast storage is not given, but the new vessels will need storage tanks suitable for cropped yeast slurries.

c)    Higher pitching rates will be needed, so the capacity of the storage vessels will need to be sufficient to meet these needs.

d)    It is highly likely that the acceptable number of fermentation cycles will be smaller than the current number. Therefore, the number will have to be identified; if it is smaller, it will be a factor to consider when deciding on the number of storage vessels required.

e)    If fewer fermentation cycles are necessary, this will have an impact on the propagation plant, which will need to be factored into the planning.

f)     The brewery quality manual will need to be re-written to take into account the process and plant changes.