Above: our signature yeast, as it arrives from the lab.

Yeast is a microscopic single-cell organism that is part of the Fungi Kingdom.

It exists in the air around us, and with several hundred different species identified to-date (we’re talking 700+,) it is perhaps one of the world’s most prevalent organisms.

It is a tiny engine, responsible for one of our most important biochemical reactions.

Here’s what it does: it converts simple sugars into alcohol and carbon dioxide (CO2). It also creates a variety of other by-products such as fusel alcohols (hotly debated as a potential hangover cause), aldehydes, volatile sulphur compounds and esters that contribute greatly to the flavour of the finished beer.



Like any other living thing, yeast needs a little care and feeding in order to be healthy. Yeast needs a source of carbohydrates (sugars), as well as amino and fatty acids to live and grow – items that are plentiful in an all-malt wort.

For proper metabolic function, yeast also requires calcium, magnesium, zinc and other vitamins and minerals, which are available in malt and in well-balanced brewing liquor.

Brews that use water that is missing some key components in its chemical makeup, or rely heavily on adjuncts, may require the addition of yeast nutrients to ensure proper fermentation performance.


Above: a simplified drawing of yeast, illustrating the process of budding.

Healthy yeast is able to propagate asexually by budding (or fission) – a process where the cell grows an offshoot or bud, and then divides into two. The mother cell is left with a scar, where the daughter cell originated. A yeast cell can bud many times, propagating as many as 50 daughter cells before the concentration of scars on the cell wall begin to prevent successful budding. Each daughter cell is genetically identical to the mother cell.


Above: a drawing of a yeast cell, with several bud scars present.

Yeast death – called autolysis – can occur spontaneously in old, mutated, or stressed cells. Basically the cell digests itself, releasing a variety of compounds that can contribute unpleasant off-flavours to the finished brew. Off-flavours commonly associated with yeast autolysis are burnt rubber and soapy. Some of the compounds released when a yeast cell ruptures will contribute to a beer haze and may also reduce foam forming proteins.

At Moosehead samples are taken of our yeast cultures, and we can see how many dead yeast cells are present using a laboratory stain. The stain is absorbed by cells where enzymatic activity is present. Dead cells however, remain stained. A yeast population that contains a high number of dead cells won’t ferment the wort as efficiently as it should.

Entire yeast populations can be stressed, or killed, in environments where the temperature or alcohol content is very high, and even when left in the beer for a prolonged period. Rapid warming and/or chilling during fermentation can also contribute to yeast autolysis.



Yeast was first viewed under a microscope by Dutch microbiologist Antonie van Leeuwenhoek (1632 – 1723). However it wasn’t discovered to be a living organism, nor understood to be responsible for fermentation, until 1857 – when French scientist Louis Pasteur came upon the scene.

Prior to these discoveries, yeast still quietly did its work in bread, wine and beer recipes around the world – but folks gave credit for its efforts to God, Mother Nature, and other magical sources. 

The famous Reinheistgebot, or the German Beer Purity Law, instituted in 1516, makes no mention of yeast – limiting the beer recipe to water, hops and barley alone.

Brewers knew that a special process was taking place, and made their recipes in ways that ensured that it would occur. i.e. leaving the wort exposed to the air, adding mature fruit to the recipe, etc.



Although there are many different strains of yeast, two in particular are used as the primary types in brewing. These are ale yeast and lager yeast.

Ale yeast (Saccharomyces cerevisiae) is a top-fermenting yeast – so called because at the end of fermentation the yeast will form loose clumps (flocculation) which are then attached to CO2 bubbles and floated to the top of the fermentation vessel.

Ale yeast is generally a tougher customer than lager yeast. It ferments at higher temperatures, which means it works faster than lager yeast. Depending on the strain, it can also handle higher levels of alcohol without dying off, which means it can be used to create higher alcohol beers.

Lager yeast (Saccharomyces pastorianus, formally S. carlsbergensis and S. uvarum) is a bottom-fermenting yeast – so called because the yeast flocculates to the bottom of the vessel after fermentation.

Lager yeast ferments slower at cooler temperatures than ale yeast. It can also ferment a sugar called melibose – one that ale yeast cannot convert.

Within each of these two categories there are a myriad of yeast options available to the brewer, each with different attenuation capabilities (low, medium, high, very high), different flocculation characteristics, alcohol tolerances and different temperature-related sweet spots.

Esters (fruity notes), keytones (diacetyl – sweet buttered or caramel), phenolics (spicy notes) and volatile sulphur compounds are all by-products of the fermentation process – and each yeast strain may offer these in different combinations. Beer favour development will also vary with environmental factors such as: wort composition, wort density, wort aeration, alcohol level, fermentation temperature and even fermenter design and size. Yeast pitch/growth rates and yeast health will also affect the resulting flavours and aromas.



In addition to having the right nutrient make-up, it is vitally important that the wort be properly aerated when the yeast is pitched, because oxygen is required for yeast growth. While some brewers will use sterile air, others may choose to use pure oxygen.

Specifically, aeration supports the microorganism’s ability to synthesize the necessary sterols and unsaturated fatty acids to build cell walls. Simply put: not enough oxygen means not enough yeast growth. Although recent studies have been conducted using olive oil as a replacement for wort aeration (a source of sterols & fatty acids), this has yet to become an accepted alternative.

Wort must be cooled dramatically before yeast is pitched. After the hot wort comes out of the whirlpool, it is passed through a heat exchanger to quickly lower the temperature. As we’ve mentioned, a high temperature environment will kill the yeast organism. Wort cooling temperatures are generally set a few degrees below that of the desired hold temperature, because the heat generated during the fermentation process will quickly elevate it to the targeted temperature.


The amount of yeast required to create a fully attenuated wort depends on the wort’s original gravity. It just makes sense that the amount of yeast be scaled to fit the amount of sugars present in the wort. A standard pitch rate is defined as 1 million yeast cells per milliliter of wort, per degree Plato – however, it is not uncommon to pitch ales at a slightly lower rate.

It is important to note that yeast can come to the brewer in a variety of forms: dry yeast, liquid yeast, and starters. Each form has a different density of cells – and the brewer must understand the average density of their yeast type prior to use.

A yeast pitch/growth rate that is too low, or too high for the wort can result in higher and lower levels of diacetyl, esters volatile sulphur compounds and other metabolites.  

One should keep in mind that some of the characteristics of a high or low pitch rate may be desirable in the finished brew, and might be a strategy used by the brewmaster to achieve a certain flavour profile.


Fermentations are often broken down into different phases.

The first phase in the fermentation timeline is the lag phase, which is the period of time between wort inoculation and the beginning of fermentation activity. It typically lasts between 12 and 24 hours. Anything longer would indicate there may be a problem with either the wort composition, wort aeration, or the yeast itself.

During the lag phase, the yeast adjusts, and gets comfortable in the wort. The cells begin to absorb the vitamins, minerals and oxygen present in the wort. These items allow the yeast to begin to manufacture the compounds required for yeast growth. To fuel this process of reproduction, the yeast cell will use an energy reserve stored within itself called glycogen. The cell will convert the glycogen into glucose.

When the yeast comes out of the lag phase, it enters the growth phase. During this phase it begins to consume the sugars in the wort, taking up the simplest sugars first. It is here that the yeast cell count climbs dramatically as exponential growth takes place. Yeast growth is usually limited by the depletion of nutrients and oxygen, but also by the rapid accumulation of alcohol and other metabolites at this stage. Acidification begins with the production of carbon dioxide and pH levels drop dramatically.

This leads to the anaerobic phase of fermentation. Simple sugars are now more slowly being converted to alcohol, CO2, and a multitude of metabolites that we know as beer flavour. Heat is also a by-product of fermentation, which is why it is critical to control temperature. High temperature fermentations are most always associated with fusel alcohol production which will give an unpleasant solvent-like flavour and aroma.

As the simple sugars are depleted, and alcohol production slows, the yeast gets ready to complete its journey. The metabolite diacetyl, a product of fermentation, is now scrubbed up by the yeast at the end of fermentation. The brewer must allow time (24-48 hours – or more, depending on the yeast strain and cell concentration) for this to happen. This is referred to as a “diacetyl rest.”

Many large brewers will use diacetyl level as a means to determine the end of fermentation. Once that level is below taste threshold, the beer (yes, it’s now called beer) and yeast will be gradually cooled. Yeast populations will begin to enter a dormant state at lower temperatures.

Thus begins the final phase of the fermentation timeline: sedimentation.  During this phase, the yeast will spontaneously flocculate and form loose clumps that will gradually settle out. Not all yeast will have the same flocculation characteristics. Some strains are very flocculent, while others are not flocculent at all. Non-flucculent yeast will tend to remain in suspension, causing clarification problems. However these yeasts are better at achieving full attenuation and scrubbing up diacetyl. Yeasts that are strongly flocculent will tend to settle too early, causing attenuation issues. Obviously the brewer desires a happy medium and wants a yeast with intermediate qualities. Flocculent characteristics can be related to strain, but there are also a number of environmental factors that will have an impact, including: the concentration of the wort, the presence of calcium ions, and even how a yeast culture is handled and washed.

Once again, the flocculation characteristics of the yeast are a strategic choice the brewmaster makes, knowing the impact it will have on the finished beer.


When the job is done, the brewmaster needs to evaluate how effective the fermentation process was. Attenuation is a word that describes the efficiency of the yeast at converting sugars into alcohol. Yeasts are given an expected attenuation rate, which indicates how much sugar will be converted to alcohol. To measure the exact attenuation, however, the brewmaster must compare the original and final gravity of the brew.

In our next yeast instalment, we’ll look at yeast cropping, washing, storage, mutations, propagations and other brewing yeasts.


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