Beer Fermentation Brewing Processes Explained
The processes seem pretty simple. Yeast is added to your cooled wort, and they begin consuming oxygen and nutrients and multiplying. They will continue to multiply until all the oxygen is consumed. Then they attack the sugars in the beer like it’s their last meal. Beer fermentation can be so vigorous and so much CO2 can be produced that a foamy head can rise a foot above the top of the beer.
Alcohol, CO2, and other byproducts such as fuel alcohols (or fusel oils) are being produced by yeast fermentation. Fusel oils are organic acids, higher alcohols (propyl, butyl, and amyl), aldehydes, and esters, known collectively as congeners. It is these cogeners that are purported to cause hangovers.
This process continues until all the simple sugars are consumed in the beer. The yeast begins to settle down. Some are going dormant and falling to the bottom, while others continue to eat the more complex sugars and other byproducts in the beer. At some point, the yeast has eaten everything there is to eat. The yeast have nothing to do now but settle to the bottom of the fermenter and wait for more food.
Beer Fermentation-Lagers vs. Ales
This beer fermentation process usually takes about a week for most beers. At higher fermentation temperatures, more byproducts (from fruity esters to solvent-like fusel alcohols) are formed, giving the beer a fruity or phenolic flavor and aroma. The yeast acts much slower at cooler temperatures, and the byproducts are minimal. This is one of the main differences between lagers (fermented cold) and ales (fermented warm).
Lagers taste much cleaner than ales, with the flavors favoring malt and hops. Ales, on the other hand, are much more complex in their aromas and flavors, favoring the fruity, spicy characters. There is a somewhat linear relationship with temperature, though. Ales fermented at colder temperatures will have less “ale character” and more “lager character.” Conversely, lagers fermented at warmer temperatures will show more of the “ale character” with more fruity/spicy ester production.
The other difference, which usually separates the two types of yeast, is that lager yeasts ferment on or near the bottom of the beer while ale yeasts ferment at the top, then fall out. Since yeasts are everywhere, it makes sense that some would be adapted to the colder climates. This is how they were discovered. Beer that fermented in colder climates like Germany was stored in ice caves to keep them from spoiling over the hot summer months. Normal ale yeast wouldn’t be able to continue cleaning up the byproducts of primary fermentation in such a cold environment. Those yeasts that were present in the areas of Germany during the winter were already adapted for cold beer fermentations through evolution. Once the yeast was understood, it became apparent that this was a different type of yeast than the ale yeasts present up until that time. The process of storing the beer in the caves is where we get the word lager, which in German means “to store.”