The great and glorious mystery of fermentation has only become widely answered in the last 200 years. In the history of beer production, for seven centuries before, the act of fermentation has thought to have been the result of many differing gods, angels, saints, prayers, and rituals. The Vikings of the Netherlands would even shout insults at their beer to awaken its spirit! Since the discovery of yeast by Louis Pasteur in 1857, the art of brewing has advanced, with techniques of yeast culturing, to the point where, currently, there are hundreds of different strains of yeast that are maintained in the brewing world.
The Fermentation process is a simple one. Yeast is the key to fermentation. Yeast devours sugars and spits out both alcohol and CO2. In order to get the best results out of your brewing yeast, control the temperature of your beer as it is fermenting. By keeping your beer in a cool place, you keep the yeast colony from replicating too fast. Yeast thrives in a warm, moist environment. The faster the yeast grows, the faster is eats up all the sugars in the wort. This also means that there is more dead yeast in your beer at the end of the fermentation process. This lends a very yeasty taste to the beer, and is highly discouraged. Keeping your beer in a cellar or basement is perfect for maintaining an ideal temperature for your yeast to ferment at, where your wort (what we call the beer before fermentation) will ferment nicely and evenly.
For an even more extreme anti yeast taste treatment, try lagering your beer, keeping it as cold as 12 degrees C. (or 40 degrees F.)
There are different strains of yeast cultured in different parts of the world. Each one has a slightly different flavor, which it adds to the brew. The main effect the yeast has on the fermentation process is the level of alcohol that the yeast will take your brew. The average ale yeast goes to around 6% alcohol. Wine and Champagne yeasts go much higher, causing the fermentation process to reach as high as 18% alcohol. Different yeasts like different things, but two ingredients cause yeast to go crazy, above all others – malt and hops. Which are, of course, the main ingredients of beer!
Before the advent of yeast culturing, wild yeast fermentation was used (knowingly or not) to attract yeasts (then thought of as spirits) into the wort. The brewers would often leave their brewing vessel in a special hut, uncovered, and say prayers over the brew. When the brew started to foam, they knew the spirit had entered. This wild yeast fermentation process was hit and miss, as sometimes a bad flavor (or spirit) would get into the ale, and it had to be thrown out. Modern home brewers still have to deal with these evil spirits during the yeast fermentation process, but we are better equipped to drive them out than our predecessors were.
Around the 15th century, some of the brewing monks started to catch on to the way the angels (yeasts) were working. They found that if they used the same wooden spoon to stir their cooled wort, the same good spirit resulted. This technique was also used by the latter day Vikings, but they used oak staves carved with runes. The reason why these tools worked their miracles was that yeast fermentation cultures would live in the wood. Even when the spoon or rune was dried, the yeast culture could live dormant in the wood until the next use.
The yeast fermentation process is truly a miracle, and understanding this process is vital to understanding how to make good beer.