Lesson 2 - Yeast Morphology and Evaluation
This lesson explores what yeast cells are, how they grow and multiply, what internal organelles they contain, and the functions that these organelles perform. This knowledge will equip us well, so that we can examine some of the practical methods that we use to characterise and evaluate yeast in the brewery.
Point of Reflection – Reconvene:
Welcome back. So, what did you think?
At some point historically, domesticated strains must have been isolated from the wild population. You would expect that an environment such as a brewery would have a population of microorganisms capable of maximising their own ability to grow on media such as wort.
Of course, with some beer styles, fermentation relies on accidental inoculation by the natural microbial population found in the brewery.
In these spontaneous fermentations, a handful of microorganisms tend to become dominant. These are the ones that can grow most easily, and out-compete others.
In other words, these conditions are self-selecting. In the case of yeasts, these would be the ancestors of the strains now used commercially in pure cultures.
A big advantage for both the organism and the brewer is that the strain is very stable, particularly if it has multiple copies of genes. When chromosomes are copied, mutations can always occur. Some of these will be detrimental to the cell (or to the brewer). If you have more than one copy of a gene, you are less likely to have a problem with single mutations in one of them.
Recent studies have revealed that the true picture is not so rosy. Most industrial yeast strains do not possess exact numbers of copies of each gene. This is called aneuploidy. In one piece of work (Codon, et al., (1998) Chromosome polymorphism and adaption to specific industrial environments of Saccharomyces strains. Applied Microbiology and Biotechnology, 49, 154 – 163) it was shown that for 16 industrial strains, the ploidy (where haploid = 1n) was:
Table: Ploidy of industrial strains of S. cerevisiae
|
Strain type |
Ploidy (n) |
|
Baking |
1.3-3.0 |
|
Wine |
1.9 |
|
Brewing |
1.6-2.2 |
|
Distilling |
1.3-1.6 |
The true situation is even more complex than this. Multiple copies of one chromosome are not necessarily all the same; they often vary in length, which is called chromosome polymorphism. This is important when considering lager strains.
There are fundamental differences between the genetics of ale and lager yeasts. These differences explain why the two types have differing properties. In evolutionary terms, ale strains (members of S. cerevisiae) are much older. They have had a long time to mutate and are thus a more diverse group when compared with lager strains.
Lager strains are hybrids, formed from two parental types: an ale strain and a strain similar to a related species called S. eubayanus. Cells of these two different species fused and formed a hybrid of the two. This hybrid contained genetic material from each parent and was able to reproduce by budding. The hybrids contain chromosomes derived from either of the two parental types, or mosaics of both. This hybridisation is very modern in evolutionary terms. It probably took place in the Middle Ages, in Central Europe – or possibly in Bavaria. Traditional lager yeast strains can ferment at lower temperatures than traditional ale types; the naturally cold climate in this region was a factor that encouraged the formation of this hybrid. The S. eubayanus parental type naturally adapted to cooler temperatures; this trait was passed on to the hybrid.
Remarkably, recent genetic analyses of a yeast isolated from the wild in wooded regions of Patagonia, show a great deal of similarity with the non-cerevisiae component of the lager yeast genome. This may be at least one of the original parents. It is difficult to explain how it came to Europe to take part in the hybridisation event!
In fact, it is now thought likely that these hybridisation events have happened several times. The ancestor of European lager strains is probably living wild somewhere in Europe, we have just not found it yet!