When making beer, there are four essential ingredients: water, malt, hops, and yeast. There are a few more items that are often added, such as clarifying agents, brewing salts, pH adjusters, among others, but for sake of clarity, let’s focus on the fundamentals.
Previously on this blog, hops were discussed in detail, which you can read about here. Today we’ll be taking an in-depth look at yeast and its importance to beer.
If there’s a technicality that every brewer should note, it’s that we, in this profession, do not make beer; rather, we organize materials and prepare a suitable environment for beer to be made by yeast. Yeast makes beer; brewers make sugar-filled solutions called wort. When done correctly, yeast will metabolize the sugar in the wort and create the primary products of ethanol and carbon dioxide. This is fermentation.
While the process sounds straightforward, it is not as simple and throwing yeast in sugar water and waiting. Fermentation is a complex, natural organic process. Things can often go wrong if the colony of yeast is not healthy, dependent on how the brewer controls the environment for growth.
When the fermentation process begins, yeast uses the energy from the sugar to split itself and multiply. Under anaerobic (oxygen-free respiration) it reproduces using budding, an asexual method where it creates a genetic copy through growth on the cell body before splitting and continuing from there. The result is a higher number of yeast cells consuming sugar and multiplying, bringing the concentration of sugar in solution down and increasing the amount of ethanol in solution.
On a side note, this is how alcohol by volume (ABV) is calculated, by factoring the original, and final sugar levels, and plugging them into a formula. More details on that in a later blog post.
Once there are no remaining sugars, or any residual sugars are too large and/or altered for yeast to consume, the cells will flocculate i.e. group together to form large clumps and sink in solution. The individual yeast cells will then become dormant, a process not unlike a bear undergoing hibernation, where it will wait until conditions are more suitable before becoming active again.
This graph shows the general life cycle of yeast activity throughout fermentation.
Ethanol and CO2 are not the only byproducts of fermentation, and when conditions are not ideal, stressed yeast produces different products that affect the taste of the beer. Acetylaldehyde, diacetyl, hydrogen sulphide, are among many other unfortunate biochemical results.
Yeast stress can be caused by, among other things: under-pitching total cell counts, not aerating the wort with pure oxygen gas during the brewing process, not mashing in properly, etc.. A big wrench in the mechanics of fermentation involves bacterial or foreign yeast interfering with the domesticated brewer’s yeast—often caused by poor sanitary measures. A brewer’s first and foremost job is making sure all the equipment is properly cleaned to avoid infection.
Next time we’ll review different types of yeast and delve into more detail on what flavours can be produced using certain strains. To give you a rough idea, think about the differences between ales and lagers.
Part II: Yeast fermentation