After last week's post about Yeast, I received a couple of questions asking for a bit of clarification. I thought it would be a good idea to post the questions and my response here.
In the article you wrote:
'which is the classic bakers or brewers yeast'
With that statement, it took it to mean that they are one and the same yeast but having read further it would seem not to be the case. Have I read it properly?
Sorry if I made it a bit confusing. The traditional general purpose brewers yeast is indeed the same as bakers yeast. The other 2 yeasts I mention are a bit more specialist.
Thanks Mike. I wonder how they isolate specific yeasts, as in the pombe one?
One way is to put a dilute solution of growing culture on an agar plate, which is a kind of jelly made using the growth medium, and leave the yeast to grow for a few days. After that you pick out individual colonies, which will usually have grown from single yeast cells. There is likely to be 100s of colonies of various types depending on what is present.
You can then do whatever tests are required to identify them, which these days will often involve some kind of genetic test. If you are wanting to brew wine or beer you can make trial runs with the different colonies to see how they affect the flavour. This is probably how they ended up with the different wine yeasts where you can buy yeast 'optimized' for different types of red or white wine.
This is the first in a short series of science-related posts where I will be explaining a bit of science behind some things which are of particular interest or relevence to me. I will be starting with fermentation since it is important in both baking and wine- and beer-making.
Types of Yeast
I apologise in advance for the use of the scientific names for the yeast but I need to be precise about which types of yeast I am talking about.
There are over 1,500 different types of yeast but the one which most people will be familiar with is called Saccharomyces cerevisiae, which is the classic bakers or brewers yeast. This has been used for centuries for this purpose. This is known as a 'top fermenting' yeast since, when making beer or wine, the live yeast forms a thick layer on top of the liquid.
Another yeast used in fermentation is Saccharomyces pastorianus (sometimes called S. carlsbergensis). This is a hybrid between S. cerevisiae and a naturally occurring yeast, S. eubayanus, which has been recently isolated from populations in South America.
A third type of yeast is called Schizosaccharomyces pombe. It gets its name from the Swahili word for beer and was isolated from strains used in East Africa to brew beer from millet. Along with S. cerevisiae it is used in research as a 'model organism' to investigate things such as signalling or communication between cells.
Yeast are typically most efficient when using simple sugars as a food. The common everyday sugar (known as sucrose) is a 'disaccharide' which means it is composed of two simpler sugar molecules (called monosaccharides) joined together.
In the presence of oxygen, the yeast grow and multiply rapidly which leads to the rapidly forming froth on top of fermenting liquids. After the oxygen has been used up, the yeast can then get on with their primary job which is fermentation:
Glucose → 2 molecules of ethanol and 2 molecules of Carbon dioxide. In brewing it is the ethanol we want, whereas in baking it is the carbon dioxide which is needed to make bread rise (Of course if you are making champagne, cider or any other 'fizzy' drink, you also want some of the carbon dioxide too).
Another way of writing this is:
C6H12O6 → 2C2H6O + 2 CO2
Brewing yeast can normally only directly consume the simpler monosaccharides so sucrose needs to be broken down in a process called 'Inverting' (this is what the 'Inverted Sugar Syrup' means on ingredient lists on manufactured foods).
Sucrose + Water → Glucose + Fructose
The above reaction is known as hydrolysis because it involves adding water to chemically change something. It can be done by boiling a sugar syrup with citric acid, which is a technique sometimes used to speed up brewing (at least in some homebrew circles).
As a chemical reaction, this can be written as:
C12H22O11 + H2O → C6H12O6 + C6H12O6
The two C6H12O6 on the right hand side look the same but they are actually Glucose and Fructose. They contain the same atoms but arranged differently.
(Based on images from Wikipedia)
The yeast do not actually do this themselves but they produce an enzyme called Invertase which they release into their surroundings, where this reaction actually takes place.
Future posts will cover such subjects as Astronomy, Chemistry and Biology in various combinations. Feel free to leave any comments, including requests for future articles.