Lesson 9 - Yeast Management SAQ

*What is the difference between viability and vitality testing when applied to yeast? What is the value of a vitality test over a simple measure of viability?

Response:

This question focuses how yeast is assessed in the brewery.

Viability is defined as the proportion of live cells in a population. There are many methods to assess this, but within a brewery context, it usually needs to be a quick process; for example, if we are about to pitch a batch of wort and we need to know the viability to work out how much yeast we need. For this reason, microscopic staining methods are most often used, in which a dye stains living and dead cells differently. The most commonly used in brewing is methylene blue, which stains dead cells without an intact membrane, blue. Live cells remain colourless.

Vitality tests try to do more than just viability determinations. The aim is to measure the physiological state of the fraction of the population that is living. Theoretically, this should be much more useful since it tells us something about the state of the living cells, which is likely to be much more important when predicting how well a batch of yeast will perform when it is used.

There are many yeast vitality tests. It can be argued, with justification, that a viability test falls into this category since (if the fraction of dead cells is much higher than expected) it is a reasonable assumption that the viable fraction is severely stressed.

Vitality tests fall into different groups:

• Measures of cell composition; for example, glycogen, sterol, trehalose, and ATP. These are indicative of the general condition, but also give an insight into how the yeast has been handled.

• Rapid tests related to fermentation performance; for example, rates of oxygen uptake, sugar uptake, CO₂ production, pH drop, and heat output. These tests give a direct indication of how well the yeast might perform in real life during fermentation.

• Measures of membrane function; for example, acidification power, response to particular dyes, and Mg release. These methods again give information on the state of the cells with particular reference to the previous handling.

Some of these tests are suitable for the direct replacement of viability tests and are often used in automatic cell analysers. Here, the results do not necessarily give a great deal of information about the yeast’s physiological condition; however, if the result always falls within a given narrow range, it is a reasonable assumption that yeast handling is under control. Similarly, if a very different result is obtained than usually observed, this would prompt an investigation.

Some vitality tests are probably of most use in trouble-shooting investigations.