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Beer KnowledgeThe World’s Smallest Beer Factory, Yeast

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25 Oct 2023
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In terms of understanding the beer, or all kinds of liquors, one of the most important things to consider is the yeast. As there is a saying that ‘Humans just make wort and the real beer is made by the yeast.’, talking about the beer without the yeast is totally illogical. Let’s find out what the yeast, struggling day and night to bestow the liquor on humankind, is all about and how it works in the beer.


 

 

What is yeast?

Many people think of a ‘microorganism’ as only bacteria. However, as animals can be divided into a group, known as class, such as fish, birds, reptiles, and mammals, a microorganism can also be divided into different classes such as bacteria, archaea, fungi, algae1. The yeast is one of them. As even mammals such as giraffes, lions, cows, wolves, and humans have totally different appearances and features, the same type of yeasts live in the different habitats and produce different materials if they are different species. Among many yeasts, the microorganism, which we call a yeast in daily life, refers to only few types of yeasts—can be considered as a gift from god—that are non-toxic, especially good at converting sugar into carbon dioxide and alcohol, and also produces tasty beer.


 
 

The reason to emphasize that the yeast is a living organism is that many people consider the yeast as merely a specific material converting sugar into carbon dioxide and alcohol. They even confuse the yeast with enzyme because of the similar pronunciation (‘yeast’ is ‘hyomo’ and ‘enzyme’ is ‘hyoso in Korean). However, they differ from one another. The enzyme is just a protein which helps specific chemical reactions. The yeast, however, is a living organism that can produce these enzymes and use them.


 


Beer Yeast

Among many strains of yeast, only a few of them can be used for the beer. There are three types of beer yeasts: ale yeast, lager yeast, and wild yeasts. Beer can be classified as ale, lager, and other special beer(such as lambic) depending on the types of yeasts.

These three types yeast belong to the different species. The ale yeast is Saccharomyces cerevisiae and the lager yeast2 is Saccharomyces pastorianus. As their name indicates, they are of the same genus(Saccharomyces), but different species. In other words, they are similar, but different in terms of detail.

However, in many cases, wild yeasts belong to different genus. ‘Brett’, which is often used for lambic or American wild ale, is Brettanomyces bruxellensis and as its scientific name indicates, it is quite different from the ale or lager yeast. Therefore, beer brewed with the wild yeast is easily distinguishable since it has a distinct flavor, unlike a subtle distinction between ale and lager beer.

 

  

Ale Yeast and Lager Yeast

As mentioned above, the ale yeast and lager yeast have similar, but different characteristics. One of the distinct features is that they have different optimum fermentation temperatures. The ale yeast is used for relatively warm temperature fermentation while the lager yeast for low temperature fermentation. Sometimes, people think that the ale yeast can grow better at warm temperature and the lager yeast at low temperature, which is not correct. Even the lager yeast can grow well at somewhat warm temperature. It is just that fermenting at warm temperature can cause different flavor and reduce clean taste which is expected from the lager beer.

California common, brewed with the lager yeast at warm temperature, can be a good example of that. It would be better to think that the lager yeast has a higher tolerance to cold compared to the ale yeast.

There is a biological story behind it. Actually the lager yeast is a hybrid between the ale yeast(S.Cerevisiae) and Saccharomyces eubayanus. S. eubayanus was a wild yeast fist found in the forest of Argentina and has a capability of fermenting at a low temperature. Therefore, the lager yeast had become the optimal yeast which is good at alcohol fermentation(The characteristic of the ale yeast) and is fermented well at a low temperature(The characteristic of the S. eubayanus) at the same time. There is no obvious clue how these two yeas strains were hybridized. However, thanks to this product of coincidence, we are able to enjoy this charming lager beer.

Due to the different fermentation temperature, the ale yeast fermentation is fast while the lager yeast fermentation is slow. Thinking of fact that in most chemical reactions, the higher the temperature(of course not too high), the faster they occur will help you understand better.

 

In addition to that, you can’t talk about the beer yeast without mentioning the difference in flavors. Compared to the lager yeast, the ale yeast is excellent at producing flavors such as ester. So, the ale possesses various tastes while the lager has relatively a clean taste. The lager yeast produces various types of sulfides and the problem is that they smell like a rotten egg or sulfur spa. Fortunately, however, these sulfides would be consumed by the yeasts, or disappear during long term aging.

There are other minor differences as well. The lager yeast can produce melibiase, which the ale yeast can’t, so that it is able to break down raffinose completely. However, it doesn’t make a huge difference since wort has very small amounts of raffinose. Also, the difference in flocculation is not that important because even though most of ale yeasts have a high flocculation while the lager yeasts have a low flocculation, there are some exceptions, including a weizen yeast.

 

 



Yeast Flavor

There might be a person who has a doubt about it. Why are there various kinds of yeast flavors even if there is only one type of the ale or lager yeast each? To understand this, think about mankind. Though all mankind shares the same scientific name, Homo sapiens, each individual has its own distinct characteristics, depending on countries or culture. By trial and error, mankind has produced alcohol and these different types of yeasts have been chosen to produce the desired flavors after years of research by each brewery.(They didn’t even know the existence of the yeast at that time). As a result, It was obvious that the flavors from English ale and weizen yeast are clearly different even if both of them are S. cerevisiae.

The different yeast flavors derive from the materials that the yeasts produce. As mentioned above, the yeast is not a merely machine that produces carbon dioxide and alcohol. The yeasts mainly eat the sugar, but they can also eat acetaldehyde or diacetyl created by themselves, eat the yeast cell debris by breaking them down, or take in alcohol in the presence of oxygen. Also, byproducts—various types of esters, phenol, and glycerol, and off-flavor materials—would be different, depending on a growth environment(temperature, nutritive condition) and stains(even the shape of fermenters).

Among them, esters and phenols have a major role in the yeast flavors. The ester is made from an alcohol(refers to not only ‘ethanol’ what we calls alcohol, but also all the chemicals called alcohol) and a carboxylic acid through the esterification—an enzymatic activity occurred inside of the cells. To put it more simply, it is a chemical compound responsible for a fruity flavor. There are many types of esters. For example, iso-amylacetate imparts a banana flavor to weizen and ethyl caproate produces a pear or apple flavor.

On the other hand, phenols provide the beer with spicy or medicine flavor. The production of phenol is suppressed since strong phenolic flavors are normally considered off-flavor. However, phenolic flavors can increase the attraction of some of beers such as Belgian beers. 4-vinyl guaiacol, clove-like aroma in weizen and 4-ethyl phenol, saddle like aroma often found in the beer with wild yeast ‘brett’.
 

Brewers choose yeast strains carefully in order to impart desired flavors to the beers and also provide them a suitable environment. If brewers irritate the yeasts, they might face the worst possible scenario such as beers with anything but off-flavors.


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