Note: This is the first in a series of articles that SYD will be posting about yeast and its importance to humanity as a whole.
A key component of any beer is yeast, the fungal microorganism that does the major work of converting the sugary wort in the case of beer, or must in the case of wine to fully ferment into the beverages we know and love. They also serve as the most common agent used to leaven breads. There are a multitude of species of yeast, each of which serve specific purposes and produce different flavors in resultant drinks and breads.
These organisms as a colony metabolize starch as fuel in two ways, respiration and fermentation. They use oxygen in a similar manner to many animals to respirate and reproduce, thereby creating a number of byproduct wastes. In fermentation, the key wastes that brewers and bakers are interested in are alcohol and carbon dioxide, but the esters and chemical flavoring compounds produced have a significant effect on the flavor of the final product. Like in all forms of fuel usage, the cells also produce a small amount of heat.
In fermentation there are different phases of yeast growth and activity. Initially, brewers want to oxygenate the wort they intend to ferment. This oxygenation leads to an explosive growth of yeast cells, but leads to little actual fermentation. This quick growth allows the yeast to fully colonize the wort and prevents other wild yeasts or bacteria from infecting the vessel. Some brewers achieve this process by using an oxygen stone similar to those used in fish tanks connected to an oxygen tank. Other brewers merely shake their wort vigorously to aerate it and hope there’s enough oxygen to achieve the same effect.
Once the yeast has fully colonized — usually after only a few hours, the cells begin to break down and convert the sugars and starches with a more anaerobic process which produces high amounts of alcohol and CO2, but doesn’t really lead to much more yeast growth. Basically, this is the yeast feeding but not really reproducing. This takes the most time in the process, sometimes as little as five days, sometimes as much as several weeks.
The last stage occurs when the concentration of waste products either exceeds the yeast’s tolerance or there is no more sugar to ferment. At this point, the wort tends to cool down as yeast activity decreases and the cells begin to bind to each other and settle to the bottom of the vessel in a process called flocculation. The more flocculant a yeast is, the more clear the final product is once fermentation is complete. Brewers can speed this process up and better clarify their beers or wines by chilling the liquid, a process Coors is happy to refer to as “cold filtering.” There’s actually no filter involved. Industrial brewers sometimes use diatomaceous earth to filter yeasts out of beer in a process similar to how pools and hot tubs are filtered.
Many wheat beers are served with the yeasts remaining in suspension. They require the least effort and time to produce, and are a common stylistic choice for beginning brewers.
Different yeasts have different environmental preferences and react differently to various conditions. Fermentation tends to heppen more quickly when temperatures are higher, but in turn the yeast produces more aggressive flavor compounds and esters. Colder fermentations take longer, but produce a “cleaner” tasting beer, which is less floral and is more of a pure reflection of the malts and hops used. Lager beers are almost always of this character, while ales tend toward the fruitier, more flowery end of the flavor spectrum.
Which will lead us to the next part of this series: Choosing the correct yeast for the job at hand.
Tags: Brewing, Fermentation, Geekdom, Science, Yeast
Where do yeast infections fall into this?
Just because you asked, Yeast Infections – which can also occur in dudes – are largely caused by a completely different species of yeast — Candida Albicans — than brewing yeasts.
[...] the last Release the Yeast we talked about how yeast does the work that eventually gives us alcohol; this time we’re [...]