The Science of Yeast

  • June 19th, 2017
  • By Simon Glassier

Good beer and good science go hand in hand. Each step of the brewing process requires the individual and/or collective manipulation of four ingredients: malt, hops, yeast and water. It is the role of the brewer to meticulously combine water, malt and hops to create wort, then allow the yeast to produce delicious beer. One may call the function of yeast “magic,” but brewers call it science - fun science, of course!

What is Yeast?

Similar to mammals, yeast is a eukaryotic organism, meaning the cells have a nucleus as well as mitochondria, cell walls, endoplasmic reticulum, etc. enclosed within a membrane. The lineage deciphers with the fact that yeast is a unicellular fungus, which belongs to the genus Saccharomyces. The Saccharomyces genus has many different species, but there are two specific types that are of great importance to brewers: ale yeast and lager yeast.

Ale yeast, otherwise known as Saccharomyces cerevisiae, is a top fermenting yeast, which functions optimally at temperatures between 18-25°C. The term top fermenting refers to the fact that this species of yeast rises to the surface during fermentation, and creates a very thick, rich head (foam). Additionally, ale yeasts generally produce floral and fruit-like ester aromas/flavours compared to lager yeast strains. This is because the higher temperatures increase the rate of cell metabolism, and therefore creates more complex and flavourful excretion products including esters, higher alcohols, aldehydes, organic sulfur compounds, etc.

Lager yeast (Saccharomyces pastorianus) is a bottom fermenting yeast that operates optimally at lower temperatures between 8-15°C. This species of yeast is most active when suspended near the bottom of the fermentation tank, hence bottom fermenting. The lower temperatures slow the yeast cells metabolism, which results in more clean and less complex flavours typical of lagered beers.

What is Fermentation?

The process of wort fermentation is generally the same among all species. Put simply, yeast consumes sugar, poops alcohol and farts CO2. Although reading the previous sentence may have put a bad taste in your mouth, the final product is actually very pleasant. In fact, if you are drinking a beer while reading this post, you have experienced first-hand the beautiful by-products of said reaction.

The success of each fermentation depends on many variables, which the brewer must monitor and manipulate to ensure the yeast have an optimum environment for growth. Depending on the wort volume and density of sugar in solution, the brewer will calculate the number of yeast cells that need to be pitched in order to initiate a successful fermentation. In general, upwards of hundreds of billions of yeast cells will be used per batch.

Yeast cells are very sensitive organisms that require an anaerobic (no oxygen) environment with a specific pH, temperature and nitrogen content. If a utopian environment is maintained and the yeast cells are healthy they will be able to successfully consume sugar to excrete alcohol, carbon dioxide, and the plethora of wonderful flavours/aromas that beer provides.

As Benjamin Franklin once said, “Beer is proof that God loves us and wants us to be happy.” And to that, I thank yeast!

Simon Glassier