Yeast

See list of yeast strains.

Overview

Yeast are single celled microorganisms responsible for turning fresh wort into finished beer. Yeast consume sugars and produce carbon dioxide and ethanol (the good stuff) when in a low oxygen environment. This process of fermentation transforms malty and sweet wort into alcoholic and carbonated beer. There are many other microorganisms that like to consume sugar, but yeast are comparatively well suited to grow under the conditions found in wort: aqueous, low oxygen, and slightly acidic. Usually brewers intend that fermentation be dominated by a single strain of domesticated yeast. Good sanitation practices and some knowledge of yeast are indispensable for closely controlled fermentation.

Under anaerobic conditions where oxygen is in short supply yeast carry out the following chemical conversion.

C6H12O6	→	2 C2H6O	+	2 CO2
(Glucose)		(Ethanol)		(Carbon Dioxide)

Consuming sugars in this way provides the yeast with energy needed for metabolic activity and reproduction through cell division.

Brewers have domesticated a number of strains of yeast over the centuries. Each cell of a particular strain is genetically identical and shares certain characteristics. The strain of yeast selected for a particular beer can have a substantial impact on the flavor and properties of that beer. Strains differ in the temperatures they prefer for optimal fermentation. Attenuation is a term for the percentage of sugars that are consumed by yeast from the start of fermentation to the end. Strains differ in how quickly they become dormant and in their preferences for consuming different types of sugar. It follows that strains differ in the degree to which they typically attenuate.

Yeast flocculation refers to the tendency of yeast cells to stick together in clumps. Different strains of yeast tend to flocculate to different degrees. This is important because it can affect the course of fermentation. Strains with high flocculation will often ride carbon dioxide bubbles to the top of the fermentor during vigorous fermentation, and then drop to the bottom quickly as fermentation slows. These strains can become dormant while some fermentable sugars still remain. They often result in somewhat maltier beers. On the other hand low flocculating strains will tend to stay in suspension throughout the beer and attenuate well. These strains typically give a cloudy or opaque appearance to finished beers due to suspended yeast cells. Medium floculators exhibit behavior somewhere in between and often produce clean tasting and well-attenuated beers. Medium floculators will generally leave clear looking beer, but require a little more time to settle to the bottom.

Yeast produce minute amounts of other compounds in addition to ethanol as by-products of fermentation. These by-products contribute to the unique flavors of beer even at very low concentrations. The quantity of by-products generated is highly sensitive to fermentation conditions and to the strain of yeast used. In particular higher temperatures during fermentation and higher initial gravity both tend to increase the amount of fermentation by-products. Some by-productions are desirable as a source of flavor and complexity while others are considered undesirable or are a sign of spoilage.

A broad list of fermentation by-products:

1. Esters – A class of fruity-tasting compounds. Ale yeasts typically produce more noticeable levels of esters than lagers. A prominent ester flavor is a distinguishing mark of ales. Specific esters may produce flavors like bananas, apples, or other fruit.
2. Diacetyl – Produces a slick mouthfeel and tastes like butter/butterscotch. Generally considered a fault at levels much above the threshold of detections.
3. Phenols – These chemical compounds are only produced by a few domesticated strains. They are responsible for many of the distinctive flavors associated with Belgian Ales. Phenols can produce a very wide range of flavors from cloves to smoke to medicine.
4. Acetaldehyde – The flavor of green apples. Yeast produce acetaldehyde early in fermentation but then remove it as the beer matures. Hence acetaldehyde flavor is associated with immature or “green” beer.
5. Fusel Alcohols – Alcohols with higher molecular weight than ethanol. They produce an alcohol burn and boozy smell that can work with some beer styles. At high concentration they can resemble solvents or kerosene and cause headaches.
6. Sulfur Compounds – Rotten eggs, cooked corn, burnt match. These compounds can be unpleasant at high concentrations, but faint levels are part of the distinctive profile of some lager yeast strains.

Yeast starters are a miniature fermentation that gets a batch of yeast warmed up and ready to go before pitching into the main wort. Using a starter increases the health of yeast cells and gives them some opportunity to replicate and increase in number. Ideally the starter should have a low gravity so that it provides a low stress environment for the yeast. Healthy and numerous yeast in general will improve the quality of fermentation. Starters reduce the chance that a fermentation will become stuck midway and leave an under attenuated beer. Starters help ensure that yeast cells will vastly outnumber and outcompete any other microorganisms that inadvertently find their way into the wort. This should ensure that the beer has the fermentation character of the desired strain and not the unknown and probably unpleasant fermentation characteristics of the infecting microorganisms. Using a healthy starter will reduce the concentration of many fermentation by-products.

References

1. White and Zainasheff. 2010. Yeast: The Practical Guide to Beer Fermentation. Boulder: Brewer's Publications.