Trying to explain yeast and what it does is much more difficult than trying to explain the other components of beer. Because of this, I have broken up my discussion of yeast into several posts. This is the second post in which I describe what yeast does.
In the first post on yeast I described the two types of yeast, ale and lager yeast. To explain the differences, I first want to explain the purpose of yeast in the process of making beer, or any other alcohol for that matter.
Ethanol Fermentation: Ethanol is the type of alcohol found in alcoholic beverages. Yeast produce alcohol and carbon dioxide as a waste product of consuming sugar. Chemically, one part glucose (a simple sugar molecule) is broken up and converted into two parts ethanol and two parts carbon dioxide. The balanced chemical equation looks like this:
C6H12O6 —> 2C2H5OH + 2CO2
In the case of bread making, the carbon dioxide is what causes the dough to rise and the ethanol gets evaporated when baked. In beer making, the ethanol and carbon dioxide stay in the beer with the carbon dioxide being the source of carbonation.
While the above equation shows the fermentation process of a simple glucose molecule, not all sugars are that simple. The above glucose molecule is called a monosaccharide. More complex sugars form compounds called disaccharides or trisaccharides. Whereas all the monosaccharides and disaccharides will be fermented, the trisaccharides and larger polysaccharides are another story.
Prior to the beginning of fermentation, brewers measure the density of the wort. It’s the sugar within the liquid that causes the wort to be denser than pure water. As the yeast consume the sugars, the density drops, though never all the way down since the complex polysaccharides are more than the yeast can handle. The amount that a strain of yeast can decrease the density of the wort is called attenuation. Lager yeast typically has a higher attenuation than ale yeast, meaning that more of the larger polysaccharides get consumed into alcohol and carbon dioxide. The result of this is that lagers are known for their dry, crisp taste and feel whereas ales leave more residual sugar behind giving them a bit more of a complex taste that is typically described as fruity.
As the yeast is actively consuming sugar, they are simultaneously multiplying as well. The amount of yeast within the vessel increases. When using ale yeast strains, the yeast all rise to the top of the wort. Lager yeast, on the other hand, sink to the bottom of the vessel. For this reason, ale yeast is often called top fermenting yeast and lager yeast is called bottom fermenting yeast.
Once fermentation is complete, any yeast suspended in the beer clumps together and sinks to the bottom as it becomes dormant. The amount that the yeast will clump together and sink is called flocculation. Many beers have a relatively high flocculation rate however the yeast strains used in wheat beers have a low flocculation rate, which is why hefeweizens are cloudy looking.
This was a long article containing lots of big words and chemistry. The next yeast article will focus on the factors that affect fermentation, primarily temperature, which is the last main difference between ale and lager yeasts.