The Grainfather: Brewer’s yeast

American Poet John Ciardi said: “Fermentation and civilisation are inseparable”.

American Poet John Ciardi said: “Fermentation and civilisation are inseparable”. Moreover, it is believed that early nomadic humans came together to form cities to cultivate and harvest the agricultural products used for fermentation and therefore, without yeast, fermentation and potentially civilisation as we know it would not exist.

The Sumerians of Mesopotamia were the first to discover the art of fermentation, but these ancient brewers had no idea that this tiny microorganism, naturally occurring in the air, soil and on plants, was the critical factor in the fermentation of sugar to produce alcohol. Sweet liquid was often offered up to the shrine of the goddess of beer (Ninkasi) and prayed over for several days until a froth that formed on the surface called ‘Godisgood’ caused this sugary liquid to become the intoxicating beverage.

In the middle ages when brewing became a family affair, brewers started transferring the ‘Godisgood’ from one vessel to another. Brewing implements that were used for this process and produced good beer became family heirlooms because of the dried yeast on the surface. This tradition started the process of domesticating brewer’s yeast to what we use today.

When the Bavarians instigated the Reinheitsgebot beer purity laws in 1516, making it illegal to brew beer with anything other than malted barley, water and hops, they left out yeast because they did not know of its existence. It was more than a century later, in 1680, when Anton van Leeuwenhoek first observed the yeast structure under a microscope. However, it was not realised that yeast was a living organism and the popular explanation was that fermentation was a spontaneous reaction promoted by contact with air, and that yeast was a chemical by-product of the process.

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This theory was due to the fact that scientists of this time had not yet perfected their sterilisation techniques, so a scientist believed that a medium was sterile when if fact it still harboured microorganisms and the reaction still occurred. It was therefore believed that the reaction – the multiplication of the microorganisms to the point where they could be detected – was spontaneous. Moreover, it would take another century until Antoine-Laurent Lavoisier describes the chemical nature of fermentation as converting sugar into alcohol and carbon dioxide, yet scientists still didn’t make the link between yeast and the production of ethanol from sugar.

Along came Louis Pasteur and in the mid-1800s, recognised that yeast was a living organism and analysed its role in fermentation, leading (incidentally) to the birth of biochemistry. Pasteur did not believe the popular theory on fermentation of the time, reasoning that not enough air was present during fermentation to explain the increase in yeast population.

He designed an experiment which would come to be known as ‘Swan Neck’ fermentation, which would put an end to any doubt about yeast’s role in fermentation. He filled a swan neck flask with a sterile medium that would allow air to reach the sugar solution but not the dust which contained yeast on the surface, and therefore fermentation did not occur. It would take another 15 years of experiments to convince the scientific community, but by 1879 his theory was thoroughly affirmed, and beer and wine making went from a mystical art to a specific process, simply by understanding yeast. Pasteur’s research into wine and beer fermentation paved the way for his later works, leading to the development of the first vaccines – an important progression for civilisation as a whole.

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One brewery that adopted these practices very early on was Carlsberg in Denmark. Under the guidance of Emil Christian Hansen, Carlsberg Laboratories first isolated the lager yeast strain Saccharomyces carlsbergensis or Saccharomyces uvarum (now called Saccharomyces pastorianus). Hansen also developed the techniques still used today to isolate and store pure yeast cultures, and started the lager revolution.

At the time, lager quickly overtook the popularity of ales, partly because ales still tended to use mixed cultures of wild yeast, resulting in a shorter shelf life before the beer turned bad. Lagers were also fermented cooler, which suppressed the wild strains, resulting in a cleaner beer that was more shelf stable and could be distributed further. Today, ale yeast cultures are just as pure as lager strains. Some brewers still invite wild yeast into their fermentation by allowing the sugary wort free contact with the environment, particularly in many Belgian styles.

Yeasts are part of the fungus kingdom, along with moulds and mushrooms. However, unlike moulds and mushrooms yeast is a single-celled organism that is round to ovoid in shape and so small that it takes more than ten yeast cells to fit inside the diameter of a human hair. A visible colony of yeast cells on a Petri dish contains more than a million cells.

We find yeast living all over the world, with over 500 known species living on plants, animals, insects and in the air and soil. Brewers, winemakers and distillers use very specific species of yeast for their products. Winemakers most commonly use Saccharomyces cerevisiae or Saccharomyces bayanus. Brewers yeast is the genus Saccharomyces which comes from the Latinised Greek for ‘sugar fungus.’ The two most common species are Saccharomyces cerevisiae (ale yeast) or Saccharomyces pastorianus (lager yeast). Many modern brewers add yeasts from the genus Brettanomyces from the Greek for ‘Brittish fungus.’ which is commonly found on the skin of fruits. This yeast produces what are considered ‘off flavours’ in most styles of beer except for many Belgian and sour styles. 

Yeast creates alcohol through anaerobic respiration to produce energy and materials for cell replication when limited oxygen is available, as it is in the brewing process. Yeast cells replicate rapidly in the presence of oxygen to form a layer on the surface of the sugary liquid. This layer of cells then limits the amount of oxygen available, and the yeast cells change to anaerobic respiration, taking in the sugars, oxygen, nitrogen and some minerals (zinc, potassium and calcium) to produce ethanol, carbon dioxide and some flavour and aroma compounds. The equation for this reaction is:

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Glucose (sugar) + 2ADP + 2 Phosphate --> 2 Ethanol (alcohol) + carbon dioxide

+ 2ATP

The ADP and phosphate on the left-hand side of the equation can be thought of as a depleted rechargeable battery. That is in the yeast cell, and the break down of the sugar to alcohol and carbon dioxide charges this battery into its full form which is the ‘ATP’ on the right-hand side of the equation. In its full form, it is now ready to be used for energy by the yeast cell.

Throughout history, humans in almost every region of the world have developed alcoholic beveages that suit their environmental conditions and available natural resources. Since the presence and workings of yeast are a modern discovery, many alcoholic beverages use a specific strain or couple of strains to produce their particular flavour. Therefore, modern strains available to brewers are characterised by four main characteristics:

 Attenuation - How much of the available sugars turn into alcohol.

 Flocculation - How well the yeast cells clump together and can be removed.

 Flavour profile - What flavours they produce.

 Temperature range – The range in temperature the yeast strain works best at.

Using these characteristics, modern brewers can tailor their beverage to create any flavour profile they desire. So next time you have a glass of your favourite alcoholic beverage in a civilised establishment, raise that glass to the living organism that made that drink possible and is one of the reasons our modern way of life exists.

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