Yeast & Fermentation

Yeast is a single celled organism, which has a huge effect on your beer. Not only does it affect the body and feel of the beer, but they have a large influence on flavour and aroma. Fermentation is the process of transforming sugars, present in the wort, into alcohol, drying the beer out as the specific gravity drops. However, alcohol is not the sole product of fermentation—various other by-products are also formed, such as esters, fusel alcohols, and diacetyl, these and other compounds can generate wanted and unwanted flavours and aromas.

Yeast can be split into two separate groups, lager yeasts and ale yeasts. As you can probably guess, lager yeasts are used to make lagers while ale yeasts are used to create ales. As a rule, ale yeasts are referred to as being top fermenters, because the cells are highly flocculant, this means that they quickly clump together. As they clump together, they trap some carbon dioxide which is being generated and float to the top of the fermentation vessel. Here they form a thick “krausen” (a frothy head which forms on top of the wort during the initial stage of fermentation) which is easy to remove from the fermentation vessel, and can be used later. Alternatively, lager yeast cells usually remain in suspension longer and eventually fall out of the beer towards the bottom of the fermenter. Over the years this division has become more unclear as the number of yeast strains has increased, and it is now possible to find ale yeasts which do not flocculate well and lager yeasts that do.

Yeast Packages

When buying yeast it’s important to purchase fresh samples which have been correctly stored.

The main function of the yeast is to transform sugars into alcohol, but this doesn’t happen in a direct manner. Once the yeast has been pitched it goes through four distinct phases on its way to producing alcohol – the lag phase, growth phase, active fermentation phase, and finally the flocculation/settling phase. Even though we will refer to fermentation being in one phase or another, it is important to understand that all four phases are happening simultaneously. We’re not talking about one specific yeast cell, but rather the population as a whole.

As soon as the yeast has been pitched, the yeast enters the lag phase. It is during this phase where very little activity takes place. During this phase, the yeast is preparing for growth and fermentation by absorbing nutrients and oxygen. Most of the nutrients which the yeast requires are provided by the malt, apart from oxygen, this is provided by aerating the wort at the time of pitching the yeast. In most homebrews oxygen is the limiting factor. If the wort is lacking the necessary oxygen, yeast growth is limited, which causes a flavour change within the beer.

Once the yeast has absorbed the required nutrients and oxygen, it enters the growth phase. Here, the size of each individual yeast cells remains the same, but the population of the yeast significantly increases. Yeast reproduces through a process known as budding, this involves a “daughter cell” growing from a bud on the side of a parent cell, until it is large enough to break off. During this phase, the amount of yeast cells increases at an exponential rate until the required nutrients are exhausted. Throughout both the lag and growth phase, the yeast will produce by-products which have an effect on flavour and aroma.

Once growth has been halted, the yeast enters the fermentation stage. It is here that the yeast transforms the sugars into alcohol. This process starts with the simpler, easier to convert sugars which includes fructose and glucose, the yeast then moves on to the longer chain sugars such as maltose and maltotriose. This conversion of sugar to alcohol is not direct and there are a number of intermediate steps involved, these steps also produce flavour compounds. In addition, the presence of alcohol causes other reactions, such as the synthesis of esters, to take place. Esters provide a considerable amount of flavour and aroma to beer and are associated with fruity, floral notes.

To Secondary or Not to Secondary?

There are numerous discussions within the homebrewing community which cause lively debate; one of these is whether to “secondary” or not. The technique of using secondary fermentation requires racking (transferring) the beer—once active fermentation has finished/when it has significantly slowed—to a separate fermenter to extract the beer from the yeast which will have accumulated at the bottom. It was thought that if the beer was left for too long sat on top of the yeast cake, the yeast cells would die and their contents would be released into the beer, causing development of off-flavours.

There is a discussion around whether or not this actually occurs in homebreweries. It’s believed that that during the early days of homebrewing, when high-quality yeasts had not been developed yet, it was more of an issue. Nowadays the beer has to be sat ontop of the yeast cake for some months, not weeks, in order for it to cause any serious issues.

Even despite this, some brewers argue that a secondary produces cleaner beer, because each time the beer is racked from one vessel to another, a small amount of yeast is pulled back into suspension. While there is some truth to this, the benefits gained from letting the beer sit on the yeast cake far outweigh the clarifying effects. While the beer is in contact with the yeast, the yeast is still at work, precursors and by-products are removed as the yeast matures.


When active fermentation has finished, the yeast moves into the flocculation/settling phase. Here, the yeast begins to starts to settle out of the beer. In yeast strains which are more flocculant, this process will happen faster. It is possible for ale yeast to settle out within a few days, whereas lager yeast usually takes weeks to completely clear. Whilst the yeast has converted the majority of sugars, it continues to break down other compounds which are present, including by-products.

This breakdown of by-products and other compounds lasts as long as the beer is allowed to mature, maturation benefits all beer styles. Diacetyl and acetaldehyde produce flavours which are associated with young beers. As the beer is aged, these compounds are broken down, eliminating the off-flavours.

Here, lagers are traditionally aged in a cold environment, this process is known as lagering. Lager yeasts function at lower temperatures than ale yeasts, maturation also requires notably more time, and due to the fact that lager yeasts don’t flocculate that well, it takes longer for the yeast to settle out of the beer. Chilling the beer results in haze-forming proteins to coagulate and fall out of suspension where they can be removed out.

An Open Fermenter

By tradition, beer was fermented in open fermenters like this one.

There are a huge number of compounds which are derived from yeast which have an effect on the flavour and aroma of the beer; the main ones are discussed below.

Flavour and Aroma Compounds


Esters have the largest impact on flavour out of all of the yeast-derived compounds. Esters provide fruity and floral character and are formed after the yeast begins to produce alcohol. Traditionally, ale strains generate more esters compared to lager strains, key fermentation conditions such as temperature and availability of nutrients change the amount of esters which are generated. Esters are found in all beer styles, but particular styles, such as British ales contain very high ester levels.

Fused Alcohols

While the aim of fermentation is to produce alcohol, ethanol is not the only type of alcohol which is produced; these other types can be as desirable as ethanol, or less desirable. Fusel alcohols are released throughout the lag phase of fermentation, and often create a hot or solvent-like flavour in beer. Generally, large amounts of fusel alcohols are considered to be undesirable, despite this there are some styles where they exist in high levels. Generally, ale strains of yeast release more fused alcohols than lager strains.

Sulphur Compounds

Trace levels of sulphur compounds can have a significant effect on beer flavour. While acceptable in small amounts, they create very undesirable characteristics in large quantities and require increased maturation times to remove. Most sulfur compounds are released during yeast fermentation, and the majority are removed naturally through rising C02 bubbles, which causes a rotten egg smell during fermentation.


Diacetyl presents slickness to the mouth-feel at low levels, however at higher levels it provides a buttery flavour. Diacetyl is formed from the precursors of the adult yeast cells, primarily during the lag and growth phases. Most beers aim for a low level of diacetyl, but as usual, some beer styles aim for high levels. Throughout maturation, diacetyl is broken down into flavourless compounds.


Acetaldehyde is a transitional compound produced by yeast throughout the act of alcoholic fermentation. It presents a flavour, similar to green apples and if given enough time, will be converted into alcohol. Its presence usually signifies that the beer has not been matured for long enough, or that the beer has been exposed to air, as oxygen will carry out the reverse reaction, changing alcohol into acetaldehyde.