Yeast Management

Question 1

Top cropping yeast

Answer 1

Usually much cleaner and of higher quality

Question 2

Bottom cropping yeast

Answer 2

Contamination with non-yeast solids. Sensitive to intensive fermentation practices (eg high gravity brewing). Stress can cause mutation.

Question 3

Pure yeast culture

Answer 3

A collection of cells that is of the same strain and free from any other cell type, both yeast and bacteria

Question 4

Yeast master culture

Answer 4

A single pure strain made up of pure cells with identical genomes

Question 5

Handling yeast cultures: Slope method

Answer 5

-Low cost, easy to manage
-Contamination risk, risk of genetic variants because cells continue to grow in storage phase, requires subculture every ~2 months

Question 6

Handling yeast cultures: -80° storage

Answer 6

-Less frequent subculturing compared with slope method
-More costly than conventional cold storage

Question 7

Handling yeast cultures: Freeze drying

Answer 7

-Stable for at least 1 year, not too expensive
-Stressful for yeast, most cells die, drying process can create genetic variants

Question 8

Handling yeast cultures: Liquid nitrogen

Answer 8

-Gold standard method, infinite storage, minimal viability loss
-Regular supply of liquid nitrogen and bespoke containers, risk of O2 depletion in confined spaces

Question 9

Handling yeast cultures: Active dried yeast

Answer 9

-Bulk supply, shelf life of at least 1 year if vacuum sealed
-Shelf life only a few days if seal is broken, limited choice of strains

Question 10

Laboratory propagation

Answer 10

Day 1: Slope working culture
Day 3: 10mL static culture
Day 6: 100mL shake flask
Day 9: 3L aerated culture with sterile filter
Day 14: 20L aerated culture with sterile filter