Beer Styles: The Ingredients (Part Four: Yeast)
While yeast is the smallest component of beer, it is just about the most important because without it, beer wouldn’t exist.
By David Nuttall on Feb. 01, 2018
In this fourth part of this series on beer’s ingredients, we will look at yeast. While yeast is the smallest component of beer, it is just about the most important because without it, beer wouldn’t exist. Even though there are hundreds of varieties of yeast, there is generally only two different strains of the Saccharomyces genus known as brewer’s yeast, either the ale yeast strain (known as Saccharomyces cerevisiae) or the lager yeast strain (known as Saccharomyces pastorianus).
There are several differences between the two yeasts which determine the beer style being made. Most notably is their fermentation temperature. Ale yeasts work best in the 13-24 °C (55-75 °F) temperature range. Lager yeasts’ optimal temperature is between 5-13 °C (41-56 °F). Ale yeasts are called top-fermenting because they cling together near the top of the fermenter, while lager yeasts do not, but instead fall to the bottom of the fermenter, hence are labeled bottom-fermenting.
Yeasts also have different properties or functions. Attenuation is the ability to decrease the wort’s original gravity during fermentation, or the percentage of the sugars the strain will ferment. Ale yeasts range from 69-80%, while lager yeasts vary from 67-77%. Flocculation is how the yeast precipitates out of the beer after fermentation, or how it clumps together to fall to the bottom of the fermenter. Some strains, like Weizen yeast, remain in suspension, creating the haze you see in the glass. Alcohol tolerance is the capability of the yeast to ferment as the alcohol concentration increases, at which point it stops working. Most lager yeast plateau at about 10% ABV, while ale strains can ferment up to 12% ABV, although there are high gravity strains out there that can go higher (but are not easy to play with). The final indicator is sensory profile, which is difficult to quantify and mostly subjective. It essentially creates the character of the beer; its taste, look, and aroma that is dependent on several factors such as temperature, oxygen levels, pitch rates and more. Brewers learn the yeasts qualities as they become experienced with their use.
When yeast is pitched into the wort, it begins the first of several phases called its life cycle. The lag (or latent) phase is the first stage where the yeast adapts to its new environment. It begins to produce the enzymes it will need to grow and ferment the sugars. This leads to the next level called the accelerating phase, where it starts dividing, as much as three times, to reach optimal density for fermentation. In the exponential phase, the conditions are ideal and the yeast is active. It will stay at this level until the growth rate decreases, in the deceleration phase. Ale yeasts will have metabolized most of the sugars at this time, while lager yeasts will continue working until all by-products are metabolized. The final stationary phase is when the yeast begins to flocculate, and drop out of suspension. The time period for all the phases may be as little as 18 hours to as long as several weeks, depending on the yeast used and the conditions. At this time the yeast and trub (other leftover materials) need to be removed to prevent “off” flavours, and the beer moved to a secondary fermenter or conditioning tank. For ales, this period of conditioning is very brief, while lagers need four to six weeks. After this, the beer is ready to drink.
Of the main by-products formed by the yeast to get beer, the major ones are ethanol (alcohol) and carbon dioxide (CO2). However, there are numerous other compounds produced which may or may not be desired, depending on the beer style. These are esters, phenols and fusel alcohols such as acetaldehyde, diacetyl, dimethyl sulfide (DMS), and more, all of which will impact the aroma and flavour of the beer.
Lastly, two other aspects of yeast should be mentioned. There are beers which get exposed to wild yeasts that are then allowed to spontaneously ferment. This is done regularly in the Senne Valley region of Belgium, the birthplace of Lambics, but has been done elsewhere, too. There are also bottle conditioned beers which have sugar and yeast added during bottling, which will activate a secondary fermentation in the bottle.
So beer (as well as wine, sake and spirits) owes its entire existence to this tiny organism. Although it wasn’t really understood until the late 19th century, it has held a magical spell over beer for several millennia. Choosing the correct yeast for each beer style is probably one of the most important jobs the brewer has, and its application and properties determines the final result.
Thirsty for more? Here are the other articles in The Ingredients series: