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.
- 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.
- 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 )