Fermentation Primer Lesson 6
From Sugar to Alcohol
Fermentation Primer - Lesson VI
In last month’s lesson we discussed the nutrients needed by yeast and some of the consequences of nutritional deficiencies. In this month’s issue we will look at the standards employed by scientists in order to classify or categorize the different types of yeast found in nature.
Table 1 -The Yeast
Craft distillers often wonder if they already have -or if they can get- wild yeast strains in their fermentation tanks. Many dream of developing or finding a new yeast that would set them apart from their competitors. Identifying yeast is a complex task, so let’s see how the experts do it.
Q: How do scientists classify yeast?
In order to classify or name an unknown yeast it is first necessary to establish the genus to which it belongs. This is often the most difficult part of the exercise, as the genus depends on morphological tests and on spore shape, both of which may be difficult to interpret. The properties of the yeast which are examined to determine the genus are those set out in Table I. For interpretation of the observations, reference should be made to Lodder’s “The Yeast,” where descriptions of the various genera, and keys which aid in distinguishing between them, may be found, and to Kreger van Rij’s “The Yeasts,” in which diagnosis is discussed and which contains many photographs of sporing yeasts.
Once the genus is known, the species can generally be determined by means of a set of standard physiological tests. The nature of these is a matter of history and tradition. The tests normally used are set out in Table II.
TABLE I - Tests for Establishing the Genus of a Yeast
Vegetative Reproduction:
- Shape and size of cells
- Method of budding
- Pellicle formation
- Psoudomycclium formation
- Appearance of colony
- Pigment production
- Production of acid
Sexual Reproduction:
- Ascospore formation
- Ballistosporc formation
- Shape of spores
- Zygote formation
TABLE II - Tests for Establishing the Species of a Yeast
Fermentation or assimilation of:
- Glucose
- Maltose
- Sucrose
- Raffinose
- Lactose
- Galactosc
- Assimilation of nitrate
- Cell size and shape
- Production of starch
- Production of pigments
- Production of acid
- Production of esters
Q: What happens when a potentially-new new yeast is observed?
If a yeast strain is found which is different from any described species in one or more
animation for fermentation
characteristics, it is a matter of individual decision whether it should be described as a new species or as a variant of an existing species. To aid in this process, a system called Numerical Taxonomy (NT) can be applied to yeast identification. The basis of NT is that instead of using a small number of diagnostic tests, a very wide range of tests is applied to each strain and the measure of similarity between the strains is then calculated. NT is unlikely to be adopted for routine identification of yeasts but it is of value for supplying a more objective judgement as to whether two yeasts should be maintained as separate species or whether they should be merged into one.
Join us again next month, as we continue our deep-dive into this fascinating world!
References:
- Kreger van Rij, N. J. W., in The Yeasts, 1969, Vol. I. The Biology of Yeasts, cd. Rose. A. H., & Harrison, J. S., Academic Press London.
- Lodder, J., ed. The Yeasts North-Holland Publishing Company, Amsterdam, 1970.
- Yeast Classification By R. B. Gilliland, St. James’s Gate, Dublin