Lesson 7 - Yeast Propagation SAQ

*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)    How would you isolate a new master culture, if necessary, 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?

Response:

a)    A master culture is defined as a yeast culture that satisfies a pre-determined set of requirements when used to brew beer. Typically, it will be a pure culture of a single yeast strain and a strain that has been used regularly in brewing. Therefore, there will be historical data against which the performance of a new master culture can be compared. It is the culture from which all the yeast used in brewing, is derived.

The key properties are:

• Performance in fermentation, including flocculence behaviour.

• Beer properties – true-to-type for the brand being made.

b)    A new master culture can be made from a sample of yeast taken from the brewery. The source yeast is a culture that is performing in a standard way in fermentation and producing true-to-type beer.

Standard microbiological methods are used for working with the yeast cultures, and great care is needed to ensure that there is no contamination during handing.

The yeast sample is diluted and plated out in Petri dishes using a standard yeast nutrient agar to give single cells. After incubation, the single cells will have grown and produced colonies, each of which consists of clones of the original cell.

If the original sample was a pure culture, all the colonies would have the same appearance. A single representative colony is picked off and transferred aseptically to a liquid nutrient medium. After incubation and growth, this is used to check for purity and identity. If these checks are passed satisfactorily, this is used to make a new culture in a form where it can be preserved and stored.

c)    The key checks made to confirm that the new culture is suitable for use are condition, purity and identity.

• Condition – usually a simple check of viability (> 98%).

• Purity – checked using standard microbiological tests to confirm that the culture is a single pure strain and free from contamination. Various media, including differential and selective types, are used.

• Identity – checked by genetic fingerprinting and compared with an existing fingerprint of the original master culture.