Yeast: An Unsung Hero

Years ago, I took a taxi ride, and the driver asked what I did for a living. When I replied, “My company provides yeast for fermentation,” he enthusiastically asked, “What the #!$@ is yeast?”

Technically, yeast is a fungus, and the species used in fermentation is Saccharomyces cerevisiae, one of more than 500 species of yeast, which can then be subdivided into countless strains. Yeast are single-celled organisms whose main purpose is to produce ethanol and carbon dioxide from glucose molecules derived from complex carbohydrates in grains and other starches. That did not satisfy my driver, but telling him that “yeast make the alcohol and flavor in beer, wine, and spirits” did (mostly).

When considering influences on aroma and flavor, most people consider ingredients such as grist, barrels and aging, or even flavorings. Very few, until recently, considered the art of fermentation an important part of creating beer, cider, and spirits. However, as many professional and hobby brewers know, yeast fermentation plays a critical role in producing unique aroma and flavor compounds.

The minor metabolites produced by yeast during fermentation are crucial to the sensory profile of any fermented beverage. Because yeast is a live, active component of fermentation, many brewers shy away from learning much about it. Yeast can be complicated and can sometimes cause a great deal of head-scratching when they don’t perform as expected. Nonetheless, being well-versed in yeast contribution, aside from ethanol, is one of the most important tools for any brewer.

Yeast Origins

Although fermentation itself has been an age-old documented practice, little was known of the existence of yeast for centuries. Ancient cultures believed that turning grape juice into wine or barley into beer was a mysterious feat of the gods. This belief was carried on until the scientist Louis Pasteur discovered the microorganisms in 1866. Decades later, Emil Christian Hansen of the Carlsberg Laboratory in Denmark isolated the first strains of yeast and began banking them. Since then, we have benefited from using isolated commercial yeast cultures. Aside from ethanol production, yeast strains have been selected over hundreds of years because of the various characteristics they display during fermentation.

As secondary by-products to their metabolic process, yeast produce hundreds of complex flavor- and aroma-active compounds. The types and quantities of these compounds produced vary by strain. Some strains produce high levels of ester compounds, resulting in such aromas as pear, apple, and banana, while other strains produce high levels of phenolics, which can contribute spice, clove, and black pepper characteristics. In fact, most of the aroma characteristics found in fermented beverages are yeast-derived compounds. Today, a diverse collection of strains is available. They vary in fermentation performance and contribution of flavor characteristics.

Metabolism and Those Flavor Compounds

During the formation of ethanol and carbon dioxide in fermentation, yeast cells also produce the by-products that make a major contribution to flavor and aroma. The primary aroma-active compounds of interest are n-propanol (alcoholic, harsh), isoamyl alcohol (alcoholic, vinous), ethyl acetate (solvent, nail polish remover), isoamyl acetate (fruity, banana), acetaldehyde (grassy, green apple), diacetyl (buttery, butterscotch), and sulfur compounds. The levels of compound produced during fermentation are strain-dependent, and many are also controlled by external factors such as temperature, oxygen, and nutrient availability.

Aside from simple ethanol, yeast will also produce higher, or fusel, alcohols as a fermentation by-product. The biosynthesis of amino acids from nitrogen (free amino nitrogen) from barley malt is one of the largest contributors to the formation of these flavor-active compounds. These alcohols contribute to the harsh alcoholic, vinous character and can be typical in many beverages with high sugar concentrations. In large quantities, these compounds can be “hot” or solventy. Reaction of these fusel alcohols with other metabolic intermediates (acids) results in the production of ethyl acetate, isoamyl acetate, and other trace ester compounds. Ethyl acetate is the most common, producing a solvent or nail-polish-remover quality in large amounts.

Isoamyl acetate, when present and detectable, is the larger flavor contributor, giving fermentations and resulting beer and spirits a pronounced fruitiness even when present in small quantities.

Other common by-products are the carbonyls—diacetyl and acetaldehyde. Diacetyl is always produced as an intermediate in amino-acid synthesis. Diacetyl can contribute a buttery or butterscotch character, which is considered an off-flavor in beer but can add complexity to malt-forward spirits.

Acetaldehyde is commonly produced by most strains as well, but typically in lower quantities. It is a very volatile substance and can be desirable in beverages such as ciders, but brewers condition beer to ensure there is no detectable acetaldehyde. In distilling, acetaldehyde can be captured in the distillate if some of the heads are collected, resulting in an apple or nutty character.

Some less detectable flavor compounds are phenolics—vinyl phenols, guaiacol, and eugenol. These typically display spicy, peppery characteristics. In addition, sulfur compounds are produced by yeast during fermentation. The most common sulfur compound is hydrogen sulfide (H₂S), which is usually an undesirable component in any beverage. It is most often described as rotten eggs, and the average person can detect it at a very low level.

Yeast Selection

Today, we have almost overwhelming options when it comes to selecting yeast, and there are many differences among strains: dry or liquid yeast, rate of fermentation, carbohydrate utilization, alcohol tolerance, nutrient requirements, flavor by-product development, and more. With all the choices, it can be difficult to navigate, but if you keep it simple, you can’t go wrong.

Start with your production requirements, such as carbohydrates and type of yeast culture (dry or liquid). For example, are you producing a beverage with only simple sugars? Make sure you select a yeast that is capable of successfully fermenting simple sugars without issues. Almost all strains of yeast will ferment both simple sugars and complex carbohydrates, but some are more proficient than others and will affect the speed and robustness of fermentation.

In the yeast world, you also have choices between active dry yeast and liquid yeast. Dry yeast and liquid yeast cultures each have their benefits, and much of your decision will be based on your needs regarding storage, shelf life, comfort with yeast handling, production needs, and price (in some cases). Both are great options and give you an even wider variety.

From there, you will want to decide on your more artisanal aspects. What alcohol level will you be reaching? What flavor and aroma characteristics are you looking for? Here is where you can really take some artistic license and build upon your base beer or beverage. Specific yeast strains can be used to increase spice and minerality, increase esters (fruity, floral characteristics), minimize negative sulfur compounds, and produce the right amount of acidity or mouthfeel. Various strains will contribute by producing countless aroma compounds, including butterscotch, caramel, nutty, apple, pineapple, banana, spice, and clove. Varying fermentation conditions, such as sugar levels and fermentation temperature, will further affect the intensity and quality of the flavor-active compounds produced by any strain.

All beer, cider, and spirits can benefit from a solid understanding of yeast fermentation and using a carefully selected yeast strain. In brewing, yeast is known to be one of the biggest players in creating beer flavor and aroma, and other beverage producers are beginning to understand the connection between these aromatic compounds and the quality of the final product.

Now that you know what the #!$@ yeast is, make it your hero. Embrace the diverse selection of yeast strains and use them to develop specific flavor profiles, creating a truly complex and unique product.