Lesson 8 - Yeast Management
This lesson will explore the methods that are used to measure yeast’s concentration and to assess its health. We will then go on to describe how this knowledge underpins the methods used to store and handle yeast in the brewery.
Yeast Vitality
Yeast vitality tests aim to go further than viability measurements. They assess the physiological state of the living cells within the population.
The concept of vitality
We all know the general definition of vitality; it is something that is not just alive but is full of life! Expressions like ‘full of vigour’ or ‘robust’ might be used in its place.
Like the concept of viability, when it comes to science, things are not quite as simple as they seem. Scientifically, vitality just means living; viability stains are often called vital stains for this reason.
What we actually mean when we talk about vitality, is what is the condition (physiological state) of the viable fraction of the yeast population? Another perhaps more practical definition of vitality is how active are the viable yeast cells? An assessment of viability tells us something about the live cells – but not very much. We can infer that if viability is low, the health of the live cells is probably compromised. This is why we use a minimum viability specification for pitching yeast. Typical specifications are 95% or greater. We must remember that it can never be 100% because cells are already dying naturally, all of the time. However, it should be high.
Yeast vitality tests are designed to go one step further. There are many different tests, and because of this, they can be used for a variety of purposes.
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Some are useful for troubleshooting.
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Vitality tests can be used as a substitute for routine viability tests but in this case looking at the population as a whole. The result confirms that yeast management is working and the yeast condition is within specification. It should be noted that vitality tests are not yet actually used for calculations of pitching rates instead; conventional viability stains are still used for this purpose.
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A preferable outcome would be that the result of the test predicts how the particular yeast batch will perform in fermentation. Even better, if it indicated the appropriate values for pitching rate and wort oxygenation concentration, which gives optimum performance.
This last point is important since it highlights the problem with the term ‘vitality’. Highly vital yeast might be expected to give a very vigorous fermentation. This is not necessarily a good thing if a lot of yeast growth accompanies it. Excessive yeast growth also means a reduced yield of alcohol since more carbon has been pushed into biomass formation.
Vitality tests
Vitality tests are preferably rapid, inexpensive and give reliable and reproducible results.
There are three classes of test:
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Analyses of cellular composition
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Measurement of cellular activities directly related to fermentation performance
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Analysis of cellular activities related to cell composition
Table: Examples of vitality tests
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Test |
Comments |
|
Cell composition |
ATP (Cell health) Glycogen (resistance to starvation) Sterol (membrane integrity) NADH (cellular redox) Trehalose (stress resistance) Bud scars (cell age) |
|
Limited fermentation tests |
Exothermy pH decline Sugar uptake rate CO2 formation rate Alcohol formation rate Oxygen uptake rate Diacetyl reduction rate |
|
Cellular activity related to composition |
Acidification power (spontaneous and glucose induced acidification of media) relates to glycogen and sterol reserves and glycolytic flux
Magnesium (Mg2+) release (relates to predicted rate of fermentation)
Specific oxygen uptake rate (membrane function, sterol content) |