Posted on February 24th, 2010

Yeast is a key ingredient in beer. It can account for 70% of a beer’s flavor and without it there’d be no alcohol. With these things in mind we thought it was a good idea to take a closer look at the stuff.

As you may know, yeast is a living microorganism, a fungus. The type used in most brewing and baking goes by the name of Saccharomyces cerevisiae.

As we’ve mentioned before, when yeast is introduced to wort (pre-beer) it will eat the sugars for energy and expel CO2 and alcohol as waste products, a process known as anaerobic fermentation. Ale yeast ferments on the top of the beer, while lager yeast usually ferments on the bottom.

However, this is only part of the story of how yeast turns grains and water into beer. In addition to alcohol and CO2, there are other secondary elements which are produced during fermentation and create much of a brew’s flavor. Here’s a quick list of some of the major ones:

Esters – These create fruity flavors, and are frequently found in beers from England and Belgium.

Fusel Alcohols – This is a heavier variation of the standard ethanol alcohol produced during fermentation. They have been linked with hangovers.

Ketones – The most common type of this compound is diacetyl, which gives beer a sweet butter or caramel taste. It’s common in some of the heavier beers of Great Britain, but its tendency to cause stale flavors cause some to regard it as a flaw. Others see it as a benefit, such Samuel Smith Brewery, whose beers all feature strong diacetyls

Phenolics – A type of chemical which can produce spicy notes.

Fatty acids – While these don’t impart a strong flavor on their own, fatty acids can make beer oxidize and grow stale quicker than normal and as a result most brewers try to avoid them

While there is only one main species of yeast used in commercial brewing there are many different varieties, all with different characters and flavors. Most yeast is strained out of beer before it makes it to the bottle as it can produce off flavors. When yeast is allowed in the bottle, as with bottle-conditioned beer it’s still best to avoid pouring it into the glass. That is of course if you’re drinking hefeweizen, or other wheat beers. Consuming the yeast is actually encouraged with style. It all goes to show the tremendous diversity of yeast and beer.