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Youthful Yeast

It’s often said that brewers make wort, yeast makes beer! In this article, we explain the importance of beer yeast in the brewing process and some of the commonly used terminology used when talking about yeast. 

The Components of Beer

Beer is the most consumed alcoholic beverage worldwide. It is traditionally made from four key ingredients: Water, Hops, Malted Cereals (Barley, Wheat and/or other) & Yeast.

Each of these ingredients contributes to the final taste and aroma of beer. During fermentation, yeast cells convert cereal-derived sugars into ethanol and CO2. At the same time, hundreds of secondary metabolites that influence the aroma and taste of beer are produced. Variation in these metabolites across different yeast strains is what allows yeast to so uniquely influence beer flavours. 

Although most breweries use pure yeast cultures for fermentation, spontaneous or mixed fermentation is also used for some specialty beers. These fermentation procedures involve a mix of different yeast species and bacteria that contribute to the final product sequentially, giving the beer a high degree of complexity. Commonly, breweries have their own stock of selected yeasts for their specific beers. 

Over 1,500 species of yeast have been identified. These are predominantly single-celled fungal microorganisms able to grow in both the presence and absence of oxygen. Of these, there are basically two major strains of yeast used in brewing: Saccharomyces cerevisiae as the top-fermenting yeast to make ales and Saccharomyces pastorianus which is a bottom-fermenting yeast used in the lager brewing processes.

You may already be familiar with the ale yeast Saccharomyces cerevisiae, in particular if you are a keen baker. This species of yeast is more commonly referred to as baker’s yeast and is used widely in breadmaking and baking. Saccharomyces pastorianus is a yeast used industrially for the production of lager and was named in honour of the French biologist Louis Pasteur (1822–1895).

These different strains of yeast ferment optimally at different temperatures, with ale yeasts preferring temperatures in the range 17–24 °C, and lager yeasts preferring temperatures in the range 8–14°C. The higher temperature for ales results in a faster fermentation, with more flavour compounds being produced. Lagers, however, take longer to ferment and have less flavour from the yeast.


Fermentation is a process whereby yeast converts glucose in the wort to ethyl alcohol and carbon dioxide gas (CO2) to give beer its alcohol content and carbonation. The fermentation process starts when cooled wort is transferred to a fermenting vessel and yeast is added. 

How long is the Fermentation Process?

Many new home brewers wonder what the optimal time frame is for fermenting their beer. The truth is that we have absolutely no control over our fermenting time. Once we pitch the yeast, it’s up to the yeast cells to do all the work,

We can, however, change certain conditions to shorten or lengthen this timeframe, such as controlling fermentation temperature. This will depend on the yeast strain you are using and what your objective is for the beer.

Controlling Fermentation Temperatures

Temperature control is of primary importance during fermentation and can easily make some of the most significant changes in fermentation, both in quality of the finished product and the fermentation time.

As previously mentioned, every yeast strain has a temperature range they work best in. Different temperatures within that range can affect the fermentation ability of different yeasts. Typically the cooler the temperature, the slower the yeast will work, and the warmer the faster. 

The general rule is the hotter the fermentation, especially outside of the yeast's stated temperature range, the more likely you are to get unwanted flavours and attributes in the beer. If using cooler temperatures, below the yeast's stated temperature range, you can sometimes experience stalling, extended time frames or challenges with getting to the normal level of fermentation. Yeast strains like Kveik (a Norwegian yeast strain that can tolerate higher temperature 30-35°C) are the exception along with certain styles like wheat beers and Saisons. 

Temperature Guidelines

It is prudent to aim to maintain a temperature in the lower-middle section of the yeasts fermentation range. For example, if the range is 18-22°C, then it is best to aim for 19-20°C.

This rule can differ, as mentioned above, for styles such as wheat beers and Saisons. These often require more complex fermentation procedures. 

The recent boom in the brewing world is Kveik. This ‘super yeast’ can easily ferment a beer at over 30°C without attracting any of the typical unwanted flavours that you would get with other strains, making it a great yeast for brewing in warmer climates and seasons.

What should beer look like during fermentation?

It’s a bit like Schrodinger’s cat… You can open the top and observe what’s going on inside the fermentation tank, but this may change the result, in many cases for the worse. Clear, transparent fermenters are great for home-brewing use, if you want to view the fermentation progress without having to open the fermenter and exposing the beer to air. 

What the beer looks like during fermentation though, depends on the yeast and the following is a synopsis of the process, once the yeast is introduced to the wort:

Lag Phase Guideline (0-15 Hours) 

In this phase, the yeast cells are being activated and stimulated by oxygen, minerals, and amino acids, in a food rich environment. During this phase, there is no airlock activity and only a small amount of natural convection of the wort in the fermenter due to any remaining temperature stratification. 

Growth Phase Guideline (4 Hours - 4 Days)

The yeast starts replicating and getting to work on the sugars on the wort. The Krausen, a foamy head of yeast proteins and sugars, starts to form and grow. Large amounts of CO2 are starting to be produced, as evidenced via the airlock. Additionally, because the yeast producing alcohol produces heat, the convection currents in the wort start to increase and there is a gentle rolling over of the wort in the fermenter. Most of the alcohol, flavour and aroma compounds are produced during this process.

Stationary Phase Guideline (3 - 10 Days)

At this stage, all the easy sugars have now been eaten and the Krausen starts to turn from creamy white to yellow (from the precipitated malt and hop components) and brown (from the oxidised hop resins).

The yeast starts to absorb many of the compounds we consider off-flavours like higher alcohols, diacetyl, sulphur compounds and esters, reforming them into more alcohol and other ‘nicer’ esters.

At this point, the fermenting wort is called ‘green’ beer and has not reached the proper balance in flavours. The airlock activity and convection start to slow as the yeast settles into the long haul and becomes once again dormant. Eventually dropping out of solution as the little remaining food disappears.

Death Phase Guideline (Several Weeks)

The airlock activity may stop, or have the occasional bubble and convection stops. The yeast is mostly dormant again and at the bottom of the fermentation container, or tank.. The beer starts to clarify and the flavours in the beer mature.

When Does Fermentation Finish?

A mistake that new brewers often make is using the airlock on the fermenter to gauge progress. An airlock is exactly what it describes – it’s a device that ensures nothing gets into the fermenting beer and also lets the built-up CO2 escape.

While many of us are mesmerised by the airlock making that ‘gloop’ sound every few seconds, all this is telling us is that CO2 is escaping from the fermenter. If there is a non-perfect seal on the fermenter, this CO2 could be escaping and the airlock will stop bubbling.

At the end of the day, there is only one way to know if your beer has finished fermenting – by using a hydrometer or refractometer. These devices allow you to check the sugar levels in the wort/beer.

The general advice for knowing when your beer is completed and ready for packaging is to have a stable specific gravity (SG) reading over 2-3 days. This is to ensure fermentation has indeed completed.

Yeast Terminology


This is the percentage of sugar in the wort that is available for the yeast to convert into alcohol. In other words, this is how much work the yeast will do before becoming unviable, or dormant.. 

Cell Count

This is the number of yeast cells per billion (i.e. a count of 150 = 150 billion).

Liquid & Dry Yeast

This is the form of yeast that you purchase as a brewer. Liquid yeast is in a liquid suspension, whereas dry yeast has the liquid removed. Both are effective for fermenting beer.

Note: To calculate the correct amount of yeast, use a yeast pitch calculator on one of the apps available to download.


A starter is a diluted sugar solution that activates the yeast. It is responsible for allowing the yeast to replicate. Starters are mostly used for liquid yeasts but can be used for dry yeasts as well. 


This is the effectiveness of the remaining live yeast cells, how alive and healthy they are. Yeast cells die from the moment they are propagated, which means that the yeast packet becomes less effective over time.

How to Increase the Quantity of Beer Yeast

Purchase & Pitch More Yeast

Using more packets of yeast, both dry and liquid, will immediately up the cell count and provide the yeast with increased capacity for fermentation.

Use a Yeast Starter

Using a yeast starter provides a small amount of yeast the chance to fire up and multiply which, in turn, again provides increased capacity for fermentation.

To explain a yeast starter in a bit more detail, we essentially need to understand what it is… A starter can be as simple as a sanitised bottle containing fermentable wort and yeast to kick off fermentation, or a stir plate, flask and stir bar.

Stir Plate

This is a device used in conjunction with a stir bar and flask to constantly agitate the yeast starter, and keep yeast in suspension whilst promoting gas exchange. It is a common piece of equipment found in chemistry laboratories and very useful for the home brewer.

We hope that you have found this blog informative and useful, but please contact us if you have questions and/or visit us via the brewery experience option, found under the “Services” tab in the main navigation bar at the top of the page. 

If you would like to learn more about craft brewing, read our other blog articles on beer types, grain, hop and water selection.

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