State of Yeast at Pitching

General Considerations

• The state of the yeast depends on its previous history, eg. whether from a culture plant or fermenter, the number of qenerations and the time in storage.

Parameters to be measured are:
Yeast viability and physical appearance, eg signs of autolysis on storage (measure before each pitching)
The presence of microbial contaminants or amorphous matter (measure before each pitching)
The proportions of the component strains for mixed strains (measure weekly).
The percentage dry matter (measure before each pitching).
Flocculation and sedimentation characteristics (measure weekly).
Measurement of yeast glycogen may also provide a useful check on the quality and composition of pitching yeast (see Appendix 1).

Range of Values

• Yeast viability should usually be above 90% with a target of above 95%.
• Microbial contaminants should ideally be absent. Traces of protein and hop debris are common amorphous materials. Acceptable values and/or appearances need to be decided in each case.
• For mixed strains the ideal proprtion should be established and a suitable method of measurement selected. (e.g. WLN agar is suitable for some yeast strains, but not for others.
• Dry matter is typically 20 – 27% for pressed yeast and 10% for slurries
• Cell size should be even with minimal elongation and granulation. Normal cell morphology should be established in each case
• Flocculalion and sedimentation characteristics should be consistent.

Operational Protocols

• Yeast is recycled through a number of sequential fermentations. Where culture propagation plant is available yeast is replaced with a fresh culture at regular intervals or at first signs of deterioration in yeast performance and/or condition. If culture plant is not available, yeast is continuously recycled (Where yeast is composed of mixed strains, topping up of one strain is sometimes necessary )
• Yeast for repitching is stored as described in The Method of Storing Yeast.
• Acid washing of yeast (see Appendix I) removes bacterial contamination, but can lead to preferential survival of wild/secondary yeasts. Alkali washing or proteolytic treatments have been used to remove proteinaceous material, but methods are not well-established.
• Yeast batches may be mixed to compensate for compositional chanqes during storage and re-use of yeast.
• Skimming procedures and times should he altered to adjust compositional balance of multiple component strains.

Measurement Protocols

• Viability is measured by the Institute of Brewing Recommended Method 9.1.3.
• Microbial contaminants are defected by microscopic examination plus various plating and forcing procedures. e g for wild/secondary yeasts use WLN agar (not always suitable), Lysine, crystal violet or actidione based agars, Lins medium or colony morphology etc. (EBC Analytica Microbiologica method 2.4.6 – see Appendix 1)
• Respiratory deficient mutants are detected by the tetrazolium agar overlay method (see Appendix 2)
• Strain composition is examined by microscopy or by various plating procedures eg. WLN agar, giant colony morphology, or flocculation tests on individual colonies
• Percentage dry matter is determined by drying and weighing a representative sample (EBC Analytica Microbiologica methods 3.1.3 and 2 .2.1 – see Appendix 1)
• Flocculation and sedimentation characteristics are measured by IoB Recommended Method 9.l.22 (Note that the Burns test indicates sedimentation ability while the Hough test indicates flocculation/barming )