Yeast works hard but really enjoys itself. This little, single-cell organism, one of the simplest forms of plant life, is responsible for carrying out the fermentation process in beer making, thereby providing one of life’s simplest forms of pleasure (and its production of carbon dioxide is what causes bread dough to rise).
Many brewers consider their yeast to be their most secret ingredient and often guard its identity jealously, calling it a proprietary ingredient.
Yeast is in the fungus family and, because of its cell-splitting capabilities, is self-reproducing. Yeast has a voracious appetite for sweet liquids and produces abundant quantities of alcohol (ethanol) and carbon dioxide in exchange for a good meal.
The vast majority of beer contains between 4 and 6 percent alcohol, but occasionally, brewers make beer with higher alcohol contents. In these beers, after reaching a level of 8 or 10 percent alcohol by volume, the beer yeast falls into a stupor, and fermentation is effectively over. When the brewmaster wants higher alcohol levels, he uses hardy champagne yeast to do the job.
Ale yeast has a lineage that reaches into antiquity — wild, airborne strains did the trick. Yeast wasn’t even considered an ingredient in beer until its role in fermentation was discovered and understood. (This discovery began with the invention of the microscope in the early 1700s and was furthered by Louis Pasteur nearly a century later.) The genetically engineered lager yeast variety was perfected only in the mid-1800s. This factoid isn’t all that important except that before this discovery, brewers couldn’t make what’s now called a lager by plan. They had to brew ale, ferment and store it at cold temperatures, and hope for the best.
Since the late 1800s, numerous pure yeast strains — more than 500 different types — have been isolated, identified, and cultured. Commercial yeast banks inventory these strains in the form of sterile slants (test tubes), and some individual breweries keep their own sterile cultures on hand for future brews.
Yeast can also take credit for the classification of the beer style. Brewmasters pick a yeast according to the recipe or the style of beer they want to make. Yeast is identified as either an ale yeast or a lager yeast.
Ale yeast, which is a top-fermenting strain, works best in warm temperatures (60 to 75 degrees Fahrenheit, 15 to 24 degrees Celsius).
Lager yeast, which is a bottom-fermenting strain, performs best in cooler temperatures (38 to 52 degrees Fahrenheit, 3 to 11 degrees Celsius).
Because of the temperature differential, each yeast strain produces the vastly different flavor and aroma characteristics that, in turn, create the different beer styles you know and love (and drink). Yeast, in combination with different fermentation processes, can also contribute fruitiness and other flavor characteristics to the beer. Brewmasters try to keep these flavors in check, depending on which beer style they’re brewing.
