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Yeast. Without it, no life. Almost as importantly, without it, no beer! These microorganisms are utterly fascinating, and I love them. Bet you are pretty enamoured with them too. With any luck even more so by the time you get to the end of this blog.

Most brewers will be able to come up with a few good beer fermentation stories. Ones similar to mine.

First day in the labs at Webster’s Brewery and I’m sent across to the brewhouse to collect a sample from the fermentation vessel.  I throw in the can – and the chain slips through my fingers, into the bubbles. I do what anyone who doesn’t know anything would do, and bend down to rescue the chain and get the can back. Not to be recommended. The CO2 hit me like a hurricane and nearly overcame me. I was a cat’s whisker from following the can and chain into the tank.

I’m a weekend duty brewer at Tennent’s Brewery, and I’m off to the sample point outside the building on a platform. The tall, 2,000 barrel vessels tower above me. And soon I understand why the brewers take umbrellas across to collect beer samples even when it’s not raining, because it is. Beer rain.  The yeast has worked too vigorously and the whole thing has erupted.

Fast forward to being a brewer at Belhaven Brewery where they have a beautiful new fermentation hall with lovely clean floors and shiny pipework. It’s a day off.  I’m called by the legendary George Howell to be told, “Carl. Get to the brewery. NOW.” I did, and within minutes I’m waist-high in beer and chest-high in foam. The last action of a disgruntled leaver (don’t even ask) was to go into the hall and open the valves on every vessel. The moral of this is to escort anyone leaving with a bad record off the premises – and change the locks or passcodes.

Records about brewing have been kept since the dawn of civilisation. Back then and indeed until as recently as the 19th century, little was understood about how the sugar turned into alcohol..

In the middle ages when most people drank beer rather than water because it was safer, and brewing was done by women, called Alewives, yeast was called “Godisgood”.

We’ve come a long way since then, and this post will describe the history, science and practical considerations of fermentation.

A Brief History of the Understanding of Beer Fermentation


In 1876, Louis Pasteur published his ground-breaking volume, Études sur la Bière, soon presented in English as Studies On Fermentation. It represented a gaint leap forward in the scientific understanding of the processes involved in beermaking and effectively changed the course of brewing. Brewers around the globe put Pasteur’s findings to work in their breweries, and thus plunged the beer industry headlong into the modern era. The book was re-published in 2005 by if you’d like a copy!

In the same year that Pasteur published his book, Danish mycologist and fermentation physiologist Emil Hansen published a Danish translation of Darwin’s The Voyage of the Beagle and won a gold medal for an essay on fungi.  He was spotted by the people at Carlsberg Brewery in Copenhagen and in 1879 joined them as director of the physiological department in their labs. In 1883 Emil announced his system of pure yeast cultures. He revealed that ‘bad beer’ was not only a result of bacterial infection, as Louis Pasteur had assumed, but of contamination by wild yeast. Until then brewers re-used beer from previous fermentations regardless.

The pioneering work of these two microscope toting scientists paved the way for what we take for granted in today’s brewing world.


Fermentation Overview

Beer Fermentation, History and Process

The general definition of fermentation is as follows;

Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In the context of food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

More specifically for brewing, it’s the process by which brewing yeast metabolises simple sugars and other nutrients from wort into ethanol, carbon dioxide, flavours and aromas to make beer. Out of interest, the study of how beers pair with food is called zythology.

There are three main stages in a brewing fermentation: primary, secondary and maturation.


Primary Fermentation


Yeast is inoculated (‘pitched’ in brewing terms) into cooled oxygenated wort to start fermentation. The yeast begins to reproduce by budding and uses the oxygen in the wort to produce sterols for cell wall production. It uses glucose in the wort (around 10% of the sugar in wort) to gain energy using the following respiratory biochemical pathway;

Aerobic Cellular Respiration Equation | Beer Fermentation

it should be noted that dried brewing yeasts don’t require oxygenated wort as they have sufficient ergosterols for cell wall production.

During this first 24 hours of fermentation, the pH drops from around 5 to 4 due to the absorption of basic amino acids, excretion of organic acids and dissolution of CO2 in the fermenting wort.

Once all the glucose had been used up, the yeast will metabolise the di-saccharide maltose, which accounts for around 50% of the sugar in wort. As this begins to happen, the oxygen in the wort is depleted, and the yeast begins to metabolise anaerobically, gaining energy using this biochemical pathway:

Beer Fermentation | Primary Fermentation Diagram

Throughout primary fermentation, the yeast will be metabolising amino acids and glucose to produce flavour compounds such as esters, fusel alcohols, diacetyl.

Once the maltose in the wort has been used up, the secondary phase of fermentation begins.


Secondary Fermentation


The rate of fermentation slows once all the maltose is used up and the yeast moves on to the tri-saccharide maltotriose. Maltotriose is a carbohydrate comprising three linked glucose molecules and is an important fermentable sugar in wort. Not all yeasts can metabolise this sugar, and these strains can be used in low and no alcohol beer production.

Yeast will reabsorb the diacetyl produced in primary fermentation and reduce it to compounds. Other flavour compounds are also reduced during this phase.

As fermentable sugars and other nutrients are depleted and ethanol content increases, the yeasts become dormant, begin to stick together (flocculate) and come out of solution.

At this point, the desired terminal gravity of the beer should have been achieved, and the fermentation should be cooled to reduce yeast activity and prevent the formation of aldehydic compounds.

Click here to learn more about maturation or ‘conditioning’ beer in our webinar


Factors Affecting Fermentation


Fermentation rate and wort attenuation (the degree of conversion of sugar to alcohol by the yeast) are affected by several factors in the brewery: Wort Composition; Vessel Geometry; Wort Cooling and Oxygenation; and Yeast Strain.

  1. Wort Composition

When the wort is created in the mashing stage of brewing, the levels of fermentable and non-fermentable sugars – along with the types and quantities of soluble nitrogen –  are determined. The temperature and thickness of the mash and the duration of the mashing process  affects the way in which the starch-degrading enzymes digest the amylose and amylopectin in the grains. More information on this can be found in the blog on mashing.

The types of malted and non-malted cereals used will govern the nutritional aspects of the wort. The degree of modification of the base malt will also have a significant effect on the types and quantities of sugars and soluble nitrogen compounds.

Yeasts metabolise sugars in a particular order; glucose is taken up first, generally around 10% of the sugar content of the wort. Maltose is used next, accounting for approximately 50% of the sugars in the wort. Towards the end of the fermentation, maltotriose will be slowly metabolised by most yeasts, although some strains cannot digest this tri-saccharide.

A mashing regime can be designed to create a high proportion of maltotriose. If the wort is then fermented with a yeast that can’t use this, less alcohol will be created and low/no alcohol beers can be made.

Using a well-modified malt will provide sufficient soluble nitrogen for healthy yeast growth. Nitrogen is mainly found in the endosperm of barley, where it surrounds the starch granules. During germination, protease and carboxypeptidase enzymes break down these storage proteins to form the roots and shoot of the new plant. The protease enzymes create peptides – and polypeptides and the carboxypeptidases create amino acids.

10 to 15% of the total soluble nitrogen in the malt is free amino nitrogen (FAN) which yeast rapidly takes up from the wort and stores it in intracellular vacuoles. One of the reasons why brewing yeast predominates in fermentation, is that there’s very little nitrogen left for other micro-organisms.

  1. Vessel Geometry

There are many different types of fermentation vessels that can be used for the brewing of beer. These vary enormously in size, geometry and operation. It is very important that the type of vessel is compatible with the strains of yeast used. During fermentation, the yeast has to be retained in the fermenting wort, while at the end of the fermentation it is vital to be able to remove the yeast from the beer. It is easier and cheaper to remove yeasts from a yeasty head or a sediment than from suspension in the beer., The more yeast that separates naturally, the better, as this makes it easier to get a clear beer.

The type of yeasts that can be used depend on the type of fermentation vessel and vice versa, so it is likely the yeasts and fermenter designs have co-evolved.

In the past, brewers were limited to a considerable extent by the materials available for construction which dictated to some extent the maximum size and the shape of fermentation vessels. Originally, they were made of wood and stone or slate – and vessels were relatively small. They were also difficult to clean and disinfect. The availability of stainless steel makes FV’s easy to clean and reduces contamination problems.

Many modern breweries make much use of cylindro-conical fermenters. They range from as little as 1hl in microbreweries to over 1000 hl in bigger breweries.  They are cylindrical vessels with a domed top and conical bottom in which yeast settles. The large ones are often 80 to 100 feet high, and the weight of fermenting wort is often more than 100 tonnes. The depth is so great that the hydrostatic pressure at the base of the fermenter is high. The high pressure and the increased concentration of dissolved carbon dioxide can damage yeasts and alter metabolism, notably with respect to some flavour compounds.

Cylindro-conical vessels require yeast strains that sediment at the end of fermentation.



‘Burton Unions’

Burton Unions are a complex fermentation system initially developed in Burton-on-Trent. They have a reputation for the production of very high quality beer. The system uses a series of large wooded casks (700 l), each of which is fitted with a ‘swan neck’ pipe. As fermentation progresses, the contents of the casks overflow via the swan neck into a trough which collects from several casks. The yeast and wort separate – and wort and some yeast are returned to the barrels. Cooling is provided by cooling fingers which are fitted into each cask.


Yorkshire stone squares are a type of fermenter that was developed for use with flocculent strains of yeast which adhere to gas bubbles and rise to the surface of the fermentation.

Stone squares have been in operation for about 200 hundred years, and the process was developed and perfected in Yorkshire by Timothy Bentley of Yorkshire Breweries Ltd near Huddersfield. Originally the fermentation vessels were made of slabs of sandstone flagstones (the type used as paving stones). Later, use was made of slate, but even that was heavy and difficult to clean. A few breweries still use squares made of stone, but most vessels are now stainless steel – which is lighter for the same strength and mercifully much easier to clean!

Modern stainless steel flat or dish bottomed fermentation vessels can be square or cylindrical – which makes them easier to clean (something of interest to any of us involved in the day-to-day operations of brewing!).  Top installation also helps with cleaning.

In the old squares, during fermentation the yeast rises to the top of the wort and eventually overflows through the manhole and collects in the ‘tray’ at the top. To assist in the fermentation, liquid is pumped from the bottom of the fermenter and sprayed over the top of the yeasty head. This is done on a regular schedule. It helps mix the fermenting wort and also aerates the culture. The liquid which accumulates in the tray at the top of the vessel can flow back into the lower part via narrow tubes called ‘organ pipes’. Towards the end of the fermentation, the yeast remains in the tray at the top of the fermenter and can easily be removed for re-use.

The newer vessel designs have cooling jackets. The yeast rises to the top after fermentation is complete and this can be carefully and hygienically removed for re-use.

  1. Wort Cooling and Oxygenation

Once the wort has been boiled and late hopped, it needs to be cooled to a temperature suitable for the yeast to grow.

Plate Heat Exchanger used in Beer FermentationThe majority of breweries use a plate heat exchanger to achieve the cooling.

These are designed to have an extensive surface area where cold water is passed through every other gap in the plates, with hot wort on the other side going in the opposite direction. Wort is cooled down and water is heated up, which means that the heat energy from the boiling wort can be recovered if the hot water is put in the hot liquor tank for the next brew. The gaps are quite small between the plates so filtering out solids prior to the device -along with regular cleaning in the forward and reverse direction – will maintain efficiency and speed up transfer times.

If lagers are being produced, the wort needs to be cooled to 10 to 12°C, this can be quite challenging when incoming water temperatures are high in the summer. There are two ways around this:

  • Install more plates on the heat exchanger
  • Cool the water in the cold liquor tank using a jacket or glycol cooled heat exchanger.

You can buy wort heat exchangers with a glycol chilled section built-in, but they are expensive.

Most ale strains will ferment between 18 and 25°C. It’s best to start the fermentation a couple of degrees lower than the core fermentation temperature to allow the yeast to grow. Collecting at the same temperature as intended fermentation temperature can cause the vessel cooling to kick in. Once the wort has been cooled, it becomes much more susceptible to spoilage by wild yeasts and bacteria. The pipework and equipment from the wort kettle/whirlpool/hopback must be cleaned and sterilised prior to casting. This includes the oxygen/air line and associated filter. The fermentation vessel must also be properly cleaned and sterilised, ideally just prior to filling.

It’s important to get the temperature correct as soon as possible, adjusting wort and water flow to get it where it should be. Water pressure can vary a lot in a brewery, so if the cooling system isn’t automated, keep checking the temperature, regularly adjusting valves as necessary.

Wet yeasts need oxygen to multiply at the start of fermentation. Some brewers inject this through a sintered stone pre-cooler to sterilise the air/oxygen being added, whilst others prefer to inject post-cooler from a sterile source. If using oxygen, make sure it’s food or medical grade to ensure purity. It should be noted that pumping in sterile air will achieve only around 8ppm.

Oxygen requirement is variable depending on yeast strain employed; original gravity of wort; and wort trub levels. Some yeast strains have higher oxygen requirements than others.  It is generally safe to assume that you need at least 10ppm of oxygen.  This is an adequate amount of oxygen in most situations.  Over-oxygenation is usually not a concern, as the yeast will use all available oxygen within 3 to 9 hours of pitching – and oxygen will come out of solution during that time as well.  Under-oxygenation is a much bigger concern.

High original gravity (>1.065) wort, in addition to increasing osmotic stress on yeast, can cause problems with achieving adequate levels of dissolved oxygen. As the gravity of wort increases, the solubility of oxygen decreases. Increased temperatures also decrease the solubility of wort.

The unsaturated fatty acids found in wort trub can be utilised by yeast for membrane synthesis.  If wort trub levels are low, yeast will need to synthesise more of these lipids and will therefore require more oxygen.

  1. Yeast

There are many populations of yeast that humans have used for millennia, including those that make wine, sake, lagers, cheese and bread. The genetic history of these yeasts has been guided by humans utilising them for food and drink. These “domesticated” yeasts differ from wild yeasts associated with forests. S. cerevisiae has been used to make wine for 9,000 years and beer for at least 5,000 years. It’s not hard to imagine the significant genetic changes that accumulated as humans selected yeasts that served their purposes best.

Scientists sequenced and compared ale yeast strains with hundreds of yeast genomes from all over the world from different sources (e.g. baking vs brewing vs laboratory strains). They found thirteen distinct populations, four of which were beer-associated, including German ale strains, British ale strains, and a mixture of beer and baking strains. The non-beer strains were grouped by region: laboratory strains, clinical strains, Asia/sake, Europe/wine, Mediterranean/oak, Africa/Philippines, China/Malaysia, and two populations from Japan/North America. Researchers were specifically interested in ale yeast strains because no one knows where they came from – ale strains have not been connected with wild strains from any geographic region.

When the researchers examined genes in the populations more closely, they found that the beer-associated populations all had a mix of genes from Asia/sake and Europe/wine populations. This suggests that humans crossed yeasts from Asia and Europe. Humans mix yeast strains to combine desirable characteristics from each population. If that sounds familiar, it’s a process we use for agricultural crops, farm animals and pets. Yeast could have been exchanged along the Silk Road, an ancient trade network connecting the two continents. Interestingly, the beer-associated populations also had completely unique gene variations not found in any of the other non-beer populations.

To prevent genetic mixing with wild strains of yeast, the ale yeast strains have multiplied their sets of chromosomes. Typically, organisms have two sets of chromosomes, so having three or more sets is unusual. This condition, called polyploidy, reduces or eliminates genetic exchange with other yeast strains, therefore maintaining the brewing characteristics of yeast strains.

Polyploidy is also found in lager and baking yeast strains, along with other genetic adaptations that reduce mixing with other populations. This can lead to the rapid evolution of new yeast strains. Further analyses demonstrated that the clinical, laboratory and beer/baking populations had more recent genetic mixing compared with the lager and two ale populations. This is likely due to polyploidy keeping the lager and ale populations mostly unchanged.

Finally, because many of the beer-specific alleles (a variant form of a given gene) are not shared between four beer-associated populations, there are likely multiple origins from genetic mixing of European and Asian strains, as well as strains that are now extinct or currently undiscovered. The researchers conclude that modern beer strains are the product of a historical melting pot of fermentation technology.

The strains used today will be genetic variants of either Saccharomyces Cerevisiae (ales), or Saccharomyces Uvarum (lager).

Two relatively recent yeasts to the craft beer fold are Saison and Kveik yeasts.

The Saison strains originally hailed from French-Belgian border regions (Wallonia in particular). This simple and thirst-quenching farmhouse beer style was brewed in the winter months and consumed by sated seasonal farm workers (“Saisonniers”) toiling away in the height of the summer. What could be more refreshing?

Traditionally, the recipe composition of these beers has been quite simple. Simple grist and modest flavourings (with hops and/or spices) make way for the centrepiece of the beer; the yeast.

Saison yeasts are revered for their complex aromatic and flavour qualities, offering something unique and complex; citrusy esters balanced with spicy, peppery notes and typical phenols we have come to associate with Belgium in particular. They are top fermenting yeasts that have high temperature tolerance and can be brewed in ranges as extreme as 15-40°C.

As our collective understanding of yeasts has advanced, so too has our appreciation for the darker arts of fermentation. In this regard, Saison yeasts, largely defined by their high attenuation, are classified as Saccharomyces cerevisiae var. diastaticus. They’re found naturally in a wide range of environments and are also used in commercial applications such as brewing and baking. They vary from typical ale strains of S.cerevisiae in that they crucially possess STA (1, 2 or 3) genes. It is these genes that set diastaticus apart and enable the yeast to produce and secrete glucoamylase enzyme.

Kveik is Norwegian for yeast. The strain originated in western Norway where it was used in farmhouse breweries and was passed between farms and down generations for many years in dried form. There’s an interesting collection method;

Yeast Collection Ring | Beer Fermentation

The ring , created to have as many surfaces as possible, was dipped in the yeast head and then dried!

Unlike Saison yeasts it isn’t diastatic. Its unique quality is the ability to ferment at an optimum temperature of 35 to 40°C.

Very fast fermentations are achieved within the optimal temperature range, with full attenuation typically attained within 2-3 days. The flavour profile is consistent across the entire temperature range: neutral with subtle fruity notes of orange and citrus. Flocculation is very high, producing clear beers without filtration or use of process aids.

So, we can see that the strain of yeast has a profound effect on the fermentation of beer. Add to that the fact that the rate at which it’s added (pitched) and how the fermentation temperature is controlled – and the flavour possibilities are incredible.

Yeast can be sourced in a dry, granulated form (Lallemand, Fermentis, Mangrove Jacks). It could be provided in liquid form as a pitchable slurry (usually from another brewery).  It may be a pitchable starter (Murphy and Son, BrewLab, Surebrew, White Labs). Or you may receive it as a slope of a few cells on agar for you to grow up into a pitchable starter yourself.

In the UK we have the National Collection of Yeast Cultures in Norwich. Brewers can study the types of yeast and their characteristics and request a slope of live yeast for propagation into a pitchable quantity.

Dried yeasts are grown in a way that ensures they have sufficient amounts of sterols available to not require the presence of additional oxygen in the wort. High gravity brews should be oxygenated to ensure the wort is fully attenuated. Some of the dried yeasts need to be rehydrated, and this should be done very hygienically to prevent cross-contamination. Dried yeasts can be cropped and re-pitched up to 4 times.

Many craft brewers buy slurry yeast from larger regional brewers who have excess yeast. Few if any of them will guarantee the microbiological integrity or the viability of the yeast – so a quick check under the microscope is definitely recommended!

Other companies keep banks of yeast under liquid nitrogen and can grow up a few cells into sufficient quantity to pitch a brew. Breweries with their own proprietary yeast strains will deposit their yeast with these people and call off a batch of fresh yeast from the mother culture every so often.

Propagating a slope will be more cost-effective than letting someone else do it, but the initial set-up costs are relatively high for the equipment and sterile cabinet necessary to prevent contamination.

Regardless of where the yeast comes from, it needs to be added at an appropriate rate depending on strain and the strength of the wort to be pitched. Dried yeast will have recommended dosing rates and these should be followed. For wet yeast, the solids’ content of the slurry and viability must be determined to guide the correct pitching rate.

Cell Count During Beer FermentationCell counts and viability are done with a haemocytometer, or counting chamber. The yeast is diluted, then stained with a solution of methylene blue to show any dead cells.


 The slurry concentration is calculated by centrifuging a known weight of yeast slurry then decanting off the supernatant and weighing again to get the solids’ concentration.

A microscope and mini-centrifuge can be purchased for a reasonable price online.



Pitching by dry viable weight is the easiest way for ales up to 1.064°. Pitch using the following formula:

Yeast Weight per hl = 0.85 x (40         )  x  (   100         )

(% Solids)      (% viability)

Add 20% more per 10 degrees of gravity over 1.064

For lagers, pitch twice as much as for ales.

If too little yeast is pitched, there will be more diacetyl ester and higher alcohol production. Fermentations will slow or stall, leaving problems with high finishing gravities and the associated mouthfeel; sweetness; and lower-than-target alcohol. There may also be higher levels of sulphur volatiles and an increased risk of infection from other organisms.

If too much yeast is pitched, the fermentation will be too fast and will probably foam over. Ester production will be low, and beers will finish too low in gravity, leading to thin, dry tasting beers. The excess yeast production will also lead to autolysed, meaty aromas.

The key to good brewing is consistency, so monitoring each fermentation is essential. Time, temperature, gravity, pH and yeast count are all essential.

Be sure to make a note of the time the brew is collected, along with the initial gravity and temperature. Temperature control is important and ideally should be done automatically by way of a temperature probe, control box and actuator valve connected to a glycol cooling source.

An Ink Bird controller can actuate a valve on a cooling circuit or kick in an individual chiller unit:

Ink Bird controller | Beer Fermentation Ink Bird controller | Beer Fermentation

Set the controller to the desired core fermentation temperature (usually 20C higher than collection temperature for ales and 20 to 30C higher than the core fermentation temperature for lagers) for 24 hours to promote yeast growth.

Do the next check after 12 hours to make sure the yeast has kicked in and then again at 24 hours. Measure the pH at this point too: it should have come down below 4.5.

Continue to monitor at least every 24 hours until the brew is reaching terminal gravity and then check every few hours until the gravity is reached and cooling can be applied. One or two more checks should be done once cooling is on to make sure the yeast has stopped working. A final pH check is also good practice and should be 4 +/- 2

One of the best ways to record all this information is on a fermentation graph with a typical profile on it;

At the end of fermentation, whether collecting the yeast for re-use or not, it’s good practice to remove it as soon as it’s formed a head or sunk to the cone. The mass of yeast should more or less treble and it can be re-used. Yeast with dry hop material or from high gravity fermentations should not be re-used.

Top fermenting yeasts will form a crust on top of the beer and can be skimmed off with a hygienically designed sterilised scoop into sterile, lidded containers.

Bottom fermenting yeasts will sink into the cone of cylindroconical vessels and can be removed through the filling/outlet valve. Discard the darker yeast that comes off first as this will contain trub and dead cells.

Only half fill the containers so that the lids don’t blow off, and store them in a fridge between 20 and 4°C. Use the yeast within four days and remember to calculate the solids and viability and compensate the pitching rate accordingly.



Final Words


Obviously, our pitches to brewers are usually about our fine UK malts – and they really are key to a successful fermentation – but you can’t overestimate the importance of yeasts either.

There is plenty more to explore, and the science is always developing – so we’ll no doubt revisit the subject in future posts.  In the meantime, give us a shout if you’d like to share some of your experiences, or want a hand with yeast advice or fermentation trouble-shooting. I shouldn’t say this too loudly, but we like a bit of problem-solving. Preferably not the kind when you’re up to your waist in beer and up to your chest in yeast though.

Just give the sales team a shout: 01328 829391 or email

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