Hops, Malt, Water and… Yeast. It is the yeast that actually makes the beer, all the brewer does is make this possible.
Photo credit: The Journal of Undergraduate Biological Studies (J. Ugrad. Biol. S). (2010) Retrieved from https://www.ppdictionary.com/fungi.htm
In the beginning, all fermentations were spontaneous; no one was really sure how the wort turned into beer. That all changed when Louis Pasteur showed in the 1850s that a micro-organism, brewer's yeast, performed this amazing feat.
Yeast in the wild.
Yeast is a type of single celled fungus which is found all throughout the environment. There are two species of yeast which are important in brewing beer namely ale yeast, Saccharomyces cerevisiae, which is typically found on the surface of fruit, and lager yeast, Saccharomyces carlsbergensis, which is a species that is relatively new and originated as a hybrid between Saccharomyces cerevisiae and a wild yeast from South America known as Saccharomyces eubayanus.
The cold conditioned Bavarian ‘lager’ was a hit and German brewers selected and propagated the strain. All this happened in the 1600s and the rest, as they say, is history. At the microbiological level, lager yeast is distinct from ale yeast in its ability to digest the carbohydrate melibiose.
How it works.
The short story goes:
Sugary Water + Yeast = Alcohol + Carbon Dioxide + More Yeast
That's useful for a start. Like anything, however, once you start scraping the surface off, things start to get complicated rather quickly.
One of the hallmarks of brewer's yeast is its ability to survive in the absence of oxygen which is known as being facultatively anaerobic. For successful yeast handling, understanding the different processes that occur in yeast with and without oxygen is important.
In the presence of oxygen, yeast will break down dextrose to carbon dioxide and water in order to release energy to power the rest of the cell. This is a process known as respiration and is identical to that which occurs in you and me.
While respiring, the yeast is able to produce a fatty compound (a lipid) known as oleic acid, which happens to be a major constituent of the lipids in olive oil. It uses this to grow its cell wall in order to divide in two by a process known as binary fission.
Each time a yeast cell divides, a budding scar remains on both the mother and daughter cells. These budding scars are a region of the cell wall which is high in a carbohydrate known as chitin. When referring to the age of a yeast culture, it is referring to the average number of budding scars in the culture.
The average brewing yeast cell will be able to replicate by division up to 26 times and cells with fewer budding scars have more replicative capacity than ones covered in these. Once the culture becomes covered in scars and ‘old’, it can no longer replicate by binary fission and it will undergo sexual reproduction.
The daughter cells produced through this may not be genetically identical to their parents, which will change the behavior of the strain. It is for this reason that breweries must continuously culture large volumes of yeast from preserved slants.
In contrast to respiration, in the absence of oxygen the yeast cells produce carbon dioxide and alcohol (ethanol) from dextrose in order to power their cellular machinery in a process known as fermentation.
Perhaps fortunately for us we produce lactic acid instead of ethanol while exerting our selves as our muscle tissue runs out of oxygen.
Life cycle of a yeast culture.
The cell growth during a fermentation process, be it beer, wine, yoghurt or some fancy pharmaceutical, is described by four distinct phases:
- Lag phase: During this initial phase, the yeast is adapting to the new environment by producing the required enzymes enzymes to digest its new food source. If the yeast is cultured in a medium similar to what it will ferment, most of this work has already occurred and the duration of the lag period will be significantly reduced.
- Exponential phase: During this phase, the number of cells in the culture will grow exponentially, doubling their number every 80 – 100 minutes. In a fermenter full of wort, this will occur until the oxygen (and therefore the oleic acid it is used to produce) is consumed.
- Static phase: During this phase, the cell count remains static and the rate of cell divisions is said to be equal to that of cell death. It is during this phase that the majority of sugars are consumed. For beer brewing, this is the start of the fermentative process.
- Death phase: At the end of a fermentation, with all the sugars consumed, the yeast will go dormant, sink to the bottom of the fermenter and hibernate. If you don’t harvest your yeast, given enough time the cells will begin to die in increasing numbers; however, you should have hopefully racked your green beer off the yeast cake long before this stage.
Yeast comes in 32 flavours.
Although it’s been known for over 150 years that yeast is responsible for beer production, the last thirty years has seen significant technological developments to help brewers for the better.
Not least amongst the achievements in the last 150 years of microbiology has been the development of brewing yeasts consisting of a single strain. Until the start of the 20th century, a mixture of yeasts and other microbes (a microbial zoo) was the standard.
The use of Brettanomyces strains such as bruxellensis and lambicus is currently a very trendy thing for brewers to do. The use of bacterial genera such Lactobacillus and Pediococcus for sour beers is also becoming increasingly popular. These organisms are traditionally considered spoilage organisms but modern techniques are allowing their use to achieve repeatable and controlled results.
Making a Starter Culture.
Making a starter culture is a pain free process and all you require is a conical flask or other suitable container, some light malt extract and some yeast.
Due to the wide base of the conical flask, there is a large air to liquid surface which helps to dissolve oxygen out of the atmosphere and into the broth which makes it a favorite when growing a starter culture.
- Into your flask, add 100 g of light malt extract per L of water, aiming to produce a ‘wort’ with a gravity of 1.040.
- Microwave for several minutes to dissolve everything and sterilize the mixture. Alternatively, you can boil the mixture for 10 minutes and add this to the sterilized flask. If you are using a screw top bottle in the microwave, ensure the lid is loose.
- Cool this by placing in a sink of cool water. If you are using a screw-lid bottle, make sure the lid is still loose. Loosely cover the top of your flask with a layer of tin-foil.
- Once the mixture is cool, swirl it several times to help re-oxygenate the broth.
- Pitch your yeast and incubate them if available; a stable temperature is the key to not stressing the yeast cells.
- Try to gently swirl your culture whenever you can as this will help oxygenate the broth. After several days, the job will be complete. Storing the culture in the fridge will settle out the cells, which will allow you to tip off the majority of the fermented wort at the top.
- Let your starter gradually come up to pitching temperature and then add to your brew.
Why we do, while we brew.
The purpose of making a yeast starter is to increase the number of yeast cells to a level where they can comfortably complete the fermentation. With too low a pitching volume, not enough yeast cells can be created by binary fission, which will force the yeast to undergo sexual reproduction, which may alter the properties of the strain.
Oxygen is the essential ingredient for growing new yeast cells, and this is why it is important to oxygenate the wort before pitching and to swirl your starter cultures to ensure they have a ready supply of oxygen.
Good yeast handling skills will greatly enhance your brewing and understanding the underlying principles will expand your technical horizons as a brewer.