Tiny Yet Mighty: The Power of Yeast

January 1, 2020  |  By Brian Hatheway 

As you may know, beer consists of four core ingredients – malt, hops, water, and yeast. Each of these ingredients serves an important role in making beer the beverage we know and love. Malted barley brings flavor, color, and fermentable sugars. Hops bring bitterness to balance the beer and add complexity to the flavor and aroma of the beer. Water can be up to 90% of a recipe and adds minerality, vital nutrients for fermentation, and helps create the mouthfeel of a beer. 

But yeast has the most important role – yeast transforms these ingredients into something entirely different. Yeast takes a hop and barley tea and turns it into the most consumed alcoholic beverage in world history. Yeast takes simple ingredients and flavors and turns them into a complex matrix of gastronomical perfection that can be paired with every food known to humankind. 

Yeast makes beer what it is but what is yeast? And more specifically what does yeast do to beer that is so special. The type of yeast used in fermentation comes from a family of fungus known as Saccharomyces. These fungi are single cell organisms used in brewing, baking, and winemaking. 

The most common type of yeast used throughout brewing history is Saccharomyces cerevisiae and is often referred to as ale yeast or brewers’ yeast. Saccharomyces loosely translates to ‘sugar fungus’ and cerevisiae is Latin for ‘of beer.’ So ale yeast is sugar fungus of beer. It is believed that this yeast strain was originally isolated from the skins of grapes. Another yeast commonly used in brewing is Saccharomyces pastorianus, and is also known as lager yeast. It is a hybrid closely related to S. cerevisiae but not unique enough to be its own species.  

The main role of yeast is to produce alcohol, which happens when the yeast consume fermentable sugar and reproduce through a process known as budding. During this process, the yeast cells create ethanol, CO2, and aromatic compounds as byproducts of their reproduction. This process is known as fermentation. In a 5 gallon batch, a homebrewer would add roughly 200 billion cells. In a professional setting, brewers will add trillions of cells to a single batch.

The creation of ethanol is one of the most important factors in the importance of beer to early civilizations. Because of the alcohol present in beer, it was a much more stable beverage than water at the time and therefore served as a crucial source of hydration for many civilizations including the Sumerians and Egyptians.

Remember though, something else is happening during fermentation. While the yeast produce ethanol as one of their byproducts, they also produce flavor compounds of their own. The most prominent types of these yeast or fermentation derived flavors would be esters. Esters give ales their fruity-like flavors and are even present in beers with the most ‘clean’ flavor profiles. They are a combination of an organic acid and alcohol and therefore only get created after the yeast start creating alcohol. Esters take time to form and once formed can lend flavors like apple, pear, and banana. They can also be responsible for some off-flavors in beer, such as solvent. 

This can be seen no better than with a hefeweizen. Hefeweizens are known for their banana and clove-like flavors. And those flavors are entirely derived from the particular strains of yeast used to brew this style of beer. Those strains of yeast can be manipulated into producing more banana flavors and less clove flavors simply by raising the fermentation temperature. And vice versa, lowering the fermentation temperature would produce a beer with more clove-like flavor and less banana. Yeast is truly magical. 

A new-to-brewing type of flavor creation has exploded in popularity and has sparked numerous debates over its effectiveness – biotransformation. This flavor creation is said to happen when hops are added at the beginning of fermentation, or shortly after, and is becoming increasingly popular in the brewing. 

Biotransformation is a process where an organism performs a chemical modification on a chemical compound. In this case our organism, yeast acts on certain chemical compounds in hops and transforms them into new compounds. For example, some hops are known to have floral like aromas. This comes from a compound known as geraniol. Certain yeast strains can biotransform geraniol into beta-citrinol with smells like fresh citrus. 

This process is said to be integral to creating the juicy hop flavors associated with New England IPAs. Brewers use specific yeast strains, mostly those of English origin, which have the ability to perform this chemical modification to hops. 

Certain strains of Brettanomyces, a wild yeast used to create wild ales such as lambics or flanders’ beers, can also perform this modification to a number of compounds. Brettanomyces takes this process to 11 though, most notably transforming butyric acid, which causes horrible cheesy, putrid off-flavors. It has been theorized that brettanomyces can biotransform butyric acid into ethyl-butyrate, which is said to smell and taste like pineapple or mango. 

Yeast does so much to turn some simple ingredients into a complex alcoholic beverage with literally hundreds of different styles and endless flavor profiles. Between creating the ethanol that made beer a safe source of hydration for early humans to creating the wide variety of ales and lagers we now enjoy thousands of years later, yeast is immensely important to the amazingly unique beverage that beer is.