Yeast

Aha! Biology! Something that I do (kind of) at last! No, I don’t actually work on yeasts or any sort of microorganisms and no, I don’t work on metabolisms at all. If there are specialists out there reading this, please do correct me if you spot any mistakes. 

Back in the days I was at school (gosh, just over 4 years ago), I was only told of the 5 kingdoms, followed by the phylum, class, order, family, genus and lastly, species. Of course, you always find out that whatever you learned at school is not entirely the whole story and before long, you get the addition of the 3 domains on top of the 5 kingdoms. We won’t concern ourselves with these hugely complicated evolutionary genetics, but I am going to describe the lineage of baker’s yeast, which belongs to the fungi kingdom under the eucaryota domain. 

Fig 1

Fig 1 Phylogeny of baker’s yeast S.cerevisiae.

Yeast is a type of unicellular fungus and baker’s yeast is almost entirely formed by the species, Saccharomyces cerevisiae. The “Saccharo” part means sugar and “myces” refers to fungus. The species name, cerevisiae, is derived from the name Ceres, the Roman goddess of agriculture. Don’t worry, I won’t dwell on the history of yeast at all, as I don’t even know them myself.

Yeast is not only involved in bread making, it is also used in the production of alcohol, cheese and various recombinant human proteins including antibiotics. This very useful organism ferments sugar, or fermentable sugar to form ethanol and carbon dioxide (CO₂) (Fig 2).

Fig 2

Fig 2 Aerobiosis and anaerobiosis following glycolysis. All species under the animalia kingdom can carry out the aerobiosis reaction following glycolysis into the kreb’s cycle and the electron transport chain to generate ATP to provide energy. However, unlike mammals, yeasts are capable of alcoholic fermentation, using the pyruvate generated to form ethanol and CO₂. Diagram taken from Alba-Lois and Segal-Kischinevzky (2010).  

From figure 2, we see that yeast is capable of producing CO₂, which is trapped in the matrix of bread dough formed by gluten, causing it to rise. The reaction requires glucose, but glucose is not readily available in the mixture of flour, water, salt and yeast. Flour contains starch, which is a large complex polysaccharide, containing many monosaccharides joined together. Glucose and fructose are released from the breakdown of starch; glucose enters glycolysis (Fig 2) directly, whilst fructose must be converted to glucose before it can enter the pathway.

Although S. cerevisiae is the dominant species used in both baking and brewing, there are different strains that control which yeast is used. Strain is used to distinguish microorganisms of the same species, which have very similar genetic make up, but may have specific mutations within their genome to adapt to a specific environment and subsequent changes in their phenotype of functionality. For example, it has been described that a species is a collection of different strains that show at least 70% cross-hybridisation, i.e. when the 2 strands of DNA are separated from two different organisms, at least 70% of the DNA from one organism will anneal or bind to the DNA of the other organism (Wayne et al, 1987) and this concept is generally applicable to the bacterial domain (Brenner et al, 2000).

Brewers’ yeast and bakers’ yeast behave very differently and they certainly belong to different strains of S.cerevisiae. Bakers’ yeast can be divided into three main categories: active dry yeast, instant dry yeast and compressed (cake) yeast.

Compressed yeast is made by a series of dehydration and literally compression processes after fermentation to concentrate the active yeast into a block. This yeast is alive and active and therefore, is much more prone to “death” and needs to be kept refrigerated in many cases. Even then, the yeasts don’t tend to last more than a few weeks. However, it has better leavening capacity compared to the other two yeast types and generates a weaker fermented flavor.

Active and Instant dry yeast are very similar. They are both dried yeasts, which mean they can be stored at room temperature and for a much longer period of time before they lose their leavening properties. These yeasts are not active immediately; they are in their dormant form. Many fungus become dormant when their surrounding environments become unfavorable, this process allows them to protect themselves from germinating or reproducing in undesirable conditions and conserve energy until time for growth. Active dry yeasts are in granules and needs to be soaked in water to activate before using in a dough. Instant yeast on the other hand, is elongated and therefore, has a larger surface area for water uptake and can be incorporated into the dough directly, but the two yeasts act in very similar ways if not identically once activated.

Yeasts are living organisms, and they need to be kept alive if you want it to do their job. Therefore, temperature and osmolarity of their surroundings are very important in maintaining their efficiency and liveliness. Yeasts work best at around 40 degrees C, but allowing it to ferment at lower temperature at about 30 degrees C will allow the dough to rise slower but develop the flavor over a longer period of time, some even put the dough in the fridge to retard the efficiency of yeast action and proof for a much longer period of time. However, do bear in mind temperature at about 60 degrees C will kill most yeast, so when adding warm water to the mixture, make sure the temperature is well below 60 degrees. High level (concentration) of salt will also retard yeast actions and even kill them. I actually don’t know why this occurs, but I speculate it is due to the osmolarity of the surrounding and draws water out of yeast and dehydrate them and hence “kills” the yeast preventing them from fermenting.

Leaving the yeast to work and proof the dough over a longer period allows flavor to develop in the dough. This is a result of breaking down the starch of the flour. Yeast when they ferment and multiply as well as metabolize, it releases various metabolites include aldehydes and ketones that affect the flavor of bread.

That’s it for now. Maybe I’ll come back to this and add more to it soon.

References

Alba-Lois, L. & Segal-Kischinevzky, C (2010).  Yeast Fermentation and the Making of Beer and Wine. Nature education. 3(9): 17

Brenner D, Staley J, Krieg N (2000). Classification of prokaryotic organisms and the concept of Bacterial speciation. Springer; New York, NY: Bergey's manual of systematic bacteriology

Wayne L.G, et al (1987). Report of the ad hoc committee on reconciliation of approaches to bacterial systematics. Int. J. Syst. Bacteriol.37: 463–464