For thousands of years, little was known about yeast and its ability to transform sugary liquids into something quite different. Since we’ve gained a better understanding of what it is and the role it plays in fermentation, we’ve been able to improve the quality and consistency of the beer we brew. You too can improve the quality and consistency of your brews with a better understanding of what yeast is and the processes it goes through when fermenting beer. With this in mind, we’ve put together a glossary of nine key terms to help you up your yeast game.
Flocculation is a term to describe the tendency of yeast cells to clump together and settle to the bottom of your brew once fermentation is complete.
The degree of flocculation in a particular yeast strain can vary widely and can impact the final appearance and flavour of the beer. Highly flocculent yeast strains will form large, dense flocs that settle out quickly, resulting in a clear, bright beer. Yeast strains that are less flocculent will form smaller, looser flocs that may take longer to settle, resulting in a cloudier beer.
Flocculation can also impact the flavour and aroma of your beer. Yeast strains that are highly flocculent tend to settle out of the beer more quickly, which can result in a cleaner, crisper flavour profile. This is because the yeast cells are no longer present in the beer to contribute additional flavour or aroma compounds.
Conversely, yeast strains that are less flocculent may remain in suspension for longer, which can lead to a more complex and flavourful beer. These strains may also produce esters and other flavour compounds that are more volatile and remain in the beer even after the yeast cells have settled out.
In addition, flocculation can impact the mouthfeel of the beer. Yeast cells that remain in suspension can contribute to a fuller, creamier mouthfeel, while a beer that has been fermented with highly flocculent yeast may have a lighter, crisper mouthfeel.
2. Genetic drift
Genetic drift can have a significant impact on brewing yeast populations over time. Genetic drift is the random fluctuation of gene frequencies in a population due to chance events, such as mutations or genetic bottlenecks. In brewing, genetic drift can occur as a result of changes in fermentation conditions, fluctuations in population size, or selective pressures from the brewing environment.
One potential consequence of genetic drift in brewing yeast populations is a loss of genetic diversity. As gene frequencies shift due to chance events, certain genetic variants may become more or less common in the population. Over time, this can lead to a reduction in the overall genetic diversity of the yeast population, which may make it less adaptable to changes in the brewing environment or less capable of producing desired flavour and aroma compounds.
Genetic drift can also lead to the emergence of new traits in brewing yeast populations. Mutations or other chance events can create new genetic variants that may confer new capabilities or characteristics to the yeast. For example, a yeast strain that undergoes a mutation that improves its ability to ferment complex sugars may have a competitive advantage in certain brewing environments.
3. Yeast viability
Yeast viability refers to the percentage of yeast cells that are alive and able to carry out their biological functions. In other words, it measures the number of live yeast cells in a given population.
Yeast cells can die or become inactive due to a variety of factors, including age, exposure to stressors such as high temperatures or low nutrient levels, and exposure to toxins or other contaminants. As yeast cells die or become inactive, their ability to ferment wort and produce alcohol and carbon dioxide decreases.
Yeast viability is an important factor to consider when brewing, as it can affect the fermentation process and the quality of the finished product. High yeast viability is generally associated with faster and more efficient fermentation, as the yeast cells are better able to convert the sugars in the wort into alcohol and carbon dioxide. Low yeast viability, on the other hand, can result in slower fermentation, off-flavours in the finished beer, and other issues.
Brewers can measure yeast viability using a variety of methods, including staining and counting yeast cells under a microscope and using specialised equipment such as a hemocytometer or flow cytometer. Maintaining high yeast viability requires careful attention to factors such as yeast strain selection, pitching rate, nutrient levels, and fermentation conditions, as well as proper storage and handling of yeast cultures.
4. Yeast Propagation
Yeast propagation typically involves several stages. The first involves preparing a growth medium that provides the necessary nutrients for the yeast to grow and multiply. The yeast cells are then added to the growth medium and allowed to grow and multiply in a controlled environment. As the yeast cells grow and multiply, they consume the nutrients in the growth medium and produce more yeast cells.
Once the yeast cells have reached a sufficient density, they are harvested and prepared for use in brewing. Depending on the desired quantity of yeast needed, the propagation process may be repeated several times to achieve the desired amount of yeast.
5. Lag period
The lag period, also known as the lag phase, is an initial period in the beer brewing process during which the yeast cells are acclimating to their new environment and preparing to start fermentation.
During this phase, the yeast cells absorb oxygen and begin to synthesise the enzymes and other compounds needed for fermentation. The length of the lag period can vary depending on a number of factors, including the strain of yeast, the temperature of the wort, and the overall health of the yeast cells.
In general, the lag phase can last anywhere from a few hours to several days, depending on the specific conditions of the fermentation. Once the yeast cells have acclimated to their new environment and have produced sufficient enzymes and other compounds, they will enter the exponential growth phase, during which they will consume the sugars in the wort and produce alcohol and carbon dioxide.
The lag phase can provide important information about the health and vitality of your yeast. A shorter lag phase and a rapid onset of fermentation are generally considered to be signs of healthy, active yeast cells. A longer lag phase, on the other hand, may indicate that the yeast cells are stressed or unhealthy, which can lead to sluggish fermentation or off-flavours in your finished beer.
6. Growth phase
Following the lag phase, the growth phase is when yeast cells rapidly multiply before the onset of fermentation.
During this phase, yeast cells consume oxygen and nutrients in the wort and begin to rapidly divide and multiply. The yeast cells continue to grow and multiply until they reach a maximum cell density, at which point they begin to consume the sugars in the wort and produce alcohol and carbon dioxide.
The length of the growth phase can vary depending on a number of factors, including the strain of yeast, the temperature of the wort, and the initial yeast cell count. In general, the growth phase can last anywhere from a few hours to several days.
Monitoring the growth phase will provide clues as to how well your yeast is performing. If this phase is longer or shorter than expected it may indicate yeast cells are stressed or unhealthy, or that you may need to adjust the fermentation conditions.
7. Lager yeast
Whereas ale yeast is often referred to as ‘top fermenting’, lager yeasts get down to business at the bottom of your fermenter. They also differ from ale yeasts in their ability to ferment at lower temperatures. Because of this, fermentations can take much longer. Lager yeasts can also struggle to ferment complex sugars. When brewing lagers, more simple sugars are typically used, which, along with the longer fermenting time, leads to a drier finish and crisper taste.
Lager yeast is a species known as Saccharomyces pastorianus, which was originally thought to be a hybrid of two different yeast species, Saccharomyces cerevisiae and Saccharomyces bayanus. However, recent genetic research has shown that lager yeast is actually a single species with complex origins.
8. Ale Yeast
Ale yeast strains belong to the species Saccharomyces cerevisiae and differ from lager yeast in that they are generally top-fermenting and favour temperatures of 18°C to 24°C. Ale yeast is often preferred over lager yeast as it tends to produce more esters, which can contribute more complex and often fruity flavours and aromas. As they ferment at higher temperatures, ale yeast can speed up the brewing process, which can in turn add to the production of desirable esters.
Also known as “Brett”, Brettanomyces is a genus of yeast known to produce complex and sometimes controversial flavours and aromas in beer, including fruity, spicy, barnyard, and earthy notes. It’s most often used in Belgian-style beers, such as lambics and farmhouse ales. In other beers, such as IPAs and pale ales, it is generally considered a contaminant.