Although it is the brewer who makes wort, it is yeast that transforms it into beer. Yeast are unicellular fungi that include several genera including Saccharomyces, the name of which is Latin for “sugar fungus.” And indeed the name is apt—during fermentation Saccharomyces yeasts consume wort sugars and give off alcohol, carbon dioxide, and a range of flavors that we associate with beer. The genus Saccharomyces itself comprises several species, some of them more relevant than others for the beverage industry. The most common species used in the alcohol industry is Saccharomyces cerevisiae, cerevisiae meaning “of beer.” In winemaking, different selected strains can produce many different wines with varied flavor characteristics. The production of distilled spirits and industrial production of ethanol also use specific strains of S. cerevisiae; this species is an ideal candidate because it is able to produce and then tolerate high concentrations of alcohol. It is also utilized in the baking industry for its leavening ability and, of course, for beer brewing purposes. Saccharomyces yeasts are round or oval in shape and reproduce by multilateral budding. Traditional identification of genera revolves around reproduction and morphology. Physiological tests are the norm to differentiate yeast species and include fermentation and assimilation of various carbon sources and growth under different environmental conditions. However, recent technologies involving DNA are widely used these days for genus/species determination. They are based on the detection of DNA sequences that are specific to a particular genus/species of yeast. These methods are mostly used to identify wild yeast contaminants in brewing; large breweries may employ this particular technology as part of their quality assurance program.

Saccharomyces yeasts are mainly composed of carbohydrates, proteins, lipids, minerals, and DNA/RNA; the various proportions of these components will vary depending on growing conditions. Yeast also contains vitamins and spent yeast is commonly used for nutritional supplement. Each cell contains various organelles indispensable to yeast functions.

The nucleus contains the genetic information in the form of chromosomes. A haploid cell contains 16 chromosomes (one single copy of the genome) and in contrast brewer’s yeasts are either polyploid (multiple copies of each chromosome) or aneuploid (different multiples of the various chromosomes). Because genes of importance are present in multiple copies in true brewing yeast strains, they are less susceptible to mutations when compared with a haploid yeast (also called “lab” yeast because of its use in research). This is relevant because brewer’s yeast cultures are typically reused many times and therefore require extra protection against mutations and physiological changes. The yeast genome of a haploid strain of S. cerevisiae was published in 1996 and was the first eukaryote to be sequenced. This major step opened news avenues toward understanding yeast behavior and stress resistance, which is key to improving yeast efficiency in brewing.

Mitochondria are the sites of respiration, where most of the energy is produced when sugar levels are low and oxygen is present. Under anaerobiosis, promitochondria are present (a not fully developed form of the organelle) and they play an important role in fermentation and flavor outcome of the beer. Respiratory-deficient or “petite” mutants have been shown to produce elevated amounts of 4-vinyl guaiacol (clovelike flavors) and to exhibit aberrant flocculation and fermentation profiles. See flocculation and 4-vinyl guaiacol. Mitochondria and the nucleus both contain DNA, the composition of which is unique to each yeast strain. This represents a useful tool when looking at differentiating strains within a culture collection or for taxonomic purposes. Large breweries may use many different yeast strains in production; being able to identify and differentiate them all is an essential step to process control. Many smaller craft breweries also use many different yeast strains, but usually they do not have access to DNA profiling technology and therefore must rely on good practice to avoid cross-contaminations.

Also contained within the cytoplasm, vacuoles are present in various numbers depending on the growth phase and physiological condition of the yeast. They serve to store nutrients and also provide a site for the breakdown of macromolecules including proteins.

The plasma membrane of the yeast cell represents a barrier between the cytoplasm and the environment and regulates the exchange necessary for the cell to survive. It is composed of lipids and proteins; the lipid component of the membrane will be of importance for cell proliferation. Indeed, the concentration of sterols and unsaturated fatty acids will eventually dictate how many times yeast cells can divide. When depleted, oxygen will be needed to replenish the membranes to ensure further division. See aeration. This is the reason chilled wort is oxygenated, so that the yeast can reproduce three or four times during the first hours of fermentation. Changes in lipid composition also regulate membrane fluidity and are triggered in response to changes in environment such as temperature or alcohol concentration. A yeast that has the ability to trigger membrane fluidity changes is more likely to be resistant to alcohol. It is therefore important to select the right strain depending on the type of beer to be produced.

In contrast to the plasma membrane, the cell wall is a rigid structure composed mainly of carbohydrates (glucans and mannans) and proteins (10%–20%), but it is not indispensible to the survival of the cell. Its primary role in brewing is flocculation, which is the result of interactions between cell wall proteins on one cell and carbohydrate residues on the other cell. Calcium is a necessary element for these connections. The process of flocculation is reversible and is an invaluable step for beer clarification and yeast repitching; however, some less flocculent strains can be desired for producing wheat beers, many of which are traditionally served with a yeast haze. Flocculation can also be assisted with the addition of specific process aids to encourage sedimentation and achieve beer clarification. See finings and flocculation.

Finally, the periplasm is the space between the plasma membrane and the cell wall. It is the location of specific enzymes including invertase, which breaks down sucrose to glucose and fructose units, which are then readily assimilated by the yeast. This enzyme is of little use in most brewing fermentations because brewer’s wort is mainly composed of maltose, which is broken down inside the cells.

From a brewing point of view, S. cerevisiae is more familiarly known as ale yeast or top-fermenting yeast. These yeast types have been known for thousands of years, although the nature of microorganisms was a mystery to ancient brewers 3,000 years ago. In contrast, the more recently domesticated lager yeast is a different species known as Saccharomyces pastorianus. It was recently found to be a natural hybrid of S. cerevisiae and Saccharomyces bayanus (a species sometimes used in winemaking). Taxonomists had previously identified the organism as being Saccharomyces carsbengensis, Saccharomyces uvarum, and even S. cerevisiae. Lager yeasts were first used by Bavarian brewers 200 years ago and rapidly made their way into breweries around the world to become by far the most used yeast in the brewing industry. See lager yeast. Because of their relatively recent usage, lager yeasts are not as genetically diverse as ale yeasts and, as a consequence, lager beers tend to have a rather similar flavor profile when compared with the variety of ale yeasts found in breweries across the world. Yeast species other than S. cerevisiae and S. pastorianus have also been identified in most alcohol-producing processes. They are sometimes desired because they contribute to the quality and particularly to the taste and aroma of the final product. Brettanomyces species in particular have been used to produce beers such as sour ales and Belgian lambics. See brettanomyces, lambics, and sour beer. However, in most instances such yeasts are considered “contaminants” because they can affect fermentation performance and generate off-flavors. Contaminating yeast or “wild” yeast can be of either the Saccharomyces or the non- Sacharomyces genera. Among the Saccharomyces wild yeasts, Saccharomyces diastaticus is particularly undesirable because it has the ability to use some of the sugars that brewers yeast leave behind (dextrins), which contribute to the body of the beer. Among non-Saccharomyces wild yeast, those such as Pichia, Rhodotorula, Kluyveromyces, and Candida can negatively affect the quality of beer by causing haze or a film on the surface. Additionally, they may produce off-flavors such as diacetyl or phenolic compounds. See diacetyl, phenolic, and wild yeast. Microscopic examination can sometimes provide clues to the presence of contaminated yeast because they are smaller and/or different shapes than typical brewer’s yeast. Otherwise, they can be detected using specific growth medium or genetic techniques.

Ale and lager yeast are easily differentiated from each other physiologically by their ability to use the disaccharide melibiose (lager yeast does use this sugar, but ale yeast does not), growth at temperature above 37°C (98.6°F; lager does not and ale does), and the ability of the yeast culture to rise (ale) or drop (lager) in the fermenter. The latter quality, however, is these days no longer strictly true because the use of cyclindroconical vessels has encouraged selection of ale yeasts that flocculate to the bottom of the tank, easing their reuse. Beyond the differentiation of ale and lager yeast it is possible to distinguish individual strains based on their unique DNA sequence. Genetic techniques have been developed to obtain DNA profiles for each strain; these are unique and easily differentiated.

Ale and lager yeast can grow aerobically and anaerobically. In the presence of oxygen, cells are encouraged to divide and produce biomass instead of alcohol. However, this is only strictly true if the sugar concentration is kept below a level of 0.2 g/L. Once the level of sugar is higher than 0.2 g/L, yeast will produce alcohol regardless of the presence of oxygen—this has been defined as the Crabtree effect. In breweries propagation (biomass production) is usually conducted in low-gravity worts in the presence of oxygen. In sophisticated yeast propagation systems, oxygen will be introduced and sugars will be continuously fed at very low concentrations. Cell division will occur with low alcohol production and prepare the cells for fermentation conditions. During the first few hours of fermentation, when the yeast cells are under aerobic conditions, they divide and produce ethanol simultaneously. Once the oxygen is exhausted, the yeast enters an anaerobic environment and will keep producing ethanol at a slower rate. Beside ethanol, yeast will produce other by-products that will impact on beer flavor and aroma. Higher alcohols, esters, sulfur compounds, or vicinal diketones are all produced as a result of yeast metabolism and their concentration can be modulated by influencing parameters such as temperature, pitching rate, aeration, or pressure. See esters, fusel alcohols, and vicinal diketones.

When cells grow, they undergo an asymmetric form of cell division called “budding” and go through a cell cycle to generate a new cell. When the conditions are adequate, a mother cell gives rise to a daughter cell (called a “virgin” cell) and becomes itself a generation older. This implies that a yeast culture always contain 50% virgin cells, 25% generation 1 cells, 12.5% generation 2 cells, etc. The average age of a yeast culture is therefore very young. This means that theoretically a yeast culture could be used indefinitely. The reality is quite different; despite maintaining a young age status, yeast cells accumulate stress and are exposed to mutations. To avoid genetic and behavioral changes new yeast is usually reintroduced regularly. The form of aging focusing on cell division is referred to as replicative and is not to be confused with chronological aging, which represents the time-related age of a culture (days, weeks, etc). The age of a culture also refers to the number of times the yeast has been used (repitched) for fermentation.

The division of a yeast cell or cell cycle is genetically programmed and influenced by environmental factors. A culture contains cells of different stages of the cell cycle. The first phase of the cycle is a rest phase called G₁ where no budding occurs. Toward the end of G₁ the keypoint “START” senses that the environment and the cell itself are adequate for division and allow entrance into the reproductive cycle and DNA synthesis. The bud starts to emerge before reaching another rest phase, G₂. Past G₂, mitosis will then take place and nuclear division will occur. The last step is the cytokinesis where the daughter and mother cells physically separate. The separation process leaves a bud scar on the mother cell and a birth scar on the daughter cell. Both types of scars are composed of chitin and can be easily visualized using the fluorescent dyes calcofluor or wheat-germ agglutinin in combination with a fluorescent microscope. A single cell is able to accumulate many bud scars on its surface, each the result of the birth of a daughter. Realistically, under brewing conditions a cell is likely to die of stress before it reaches its genetically determined division potential. When cells are dormant (reversible nondividing state or stationary phase), they enter a G₀ phase until the conditions are again suitable to pass START. Cells can survive for long periods of time in the G₀ state but will deteriorate with time. A yeast culture in storage between brews will contain cells in G₀ phase. When pitched into a new wort for fermentation, the cells will re-enter the cell cycle until a growth-limiting factor will again arrest cell division. When repitching yeast, cells consistently enter and exit the cell cycle; when damages occur as a result of accumulated stress, they may become permanently deactivated and eventually die, hence the need to constantly grow new yeast cultures.

After a determined number of repitchings, new yeast should be used, either obtained in a dry form or propagated by a third party or in-house. See yeast bank. Propagations are typically started from a stock culture. Yeast stocks should be kept at cold temperatures to maintain the integrity of the DNA through time; spontaneous mutations do occur and can affect the characteristics and performance of yeast. To protect yeast strains against mutation for a long period of time, cryopreservation is recommended, with the safest method being storage in the gas phase of liquid nitrogen in a specific container. Working stocks can be maintained frozen at –80°C for long-term storage. Agar slants may be kept at 4°C (39°F); however, this is only for short-term storage because there is a higher risk of mutation and contamination. The number of times a yeast culture can be reused depends on numerous factors; however, it is well documented that cultures should be replaced regularly to ensure fermentation performance and consistency. Although this is the norm, there are exceptions, and some breweries have been reported to have used a single yeast culture for years or even decades without notable mutation of loss of vitality. The genetic stability of the strain used, hygiene process, brewing frequency and schedule, the yeast maintenance program, and type of beer produced will eventually determine how many times a particular yeast culture can be repitched.