Yeast - the underestimated organism

Mankind has been brewing beer for about 10,000 years. At a time when the modern man emerged and moved to settlement and agriculture, cereals were cultivated and fermented. Beer has been part of mankind since the dawn of civilization. Approximately since that time, yeast has accompanied man as one of his oldest domestic animals.

Plinius the Elder already described the use of fermentum in his Naturalis Historia in 77 AD. In medieval breweries, the Hefner was a recognized yeast maker profession, so it is almost surprising that there is no mention of yeast in the Bavarian Purity Law (Reinheitsgebot) of 1516. Although the yeast was known, it was clearly underestimated.

Nowadays, we as brewers do not understand yeast as another ingredient, but as the organism that produces the beer from the wort prepared by the brewer. Just as the farmer must feed and care for his cows well in order to obtain good milk, the brewer does the same with the yeast. He prepares the yeast the suitable food and an optimal ambiance to produce the best beers.

One can only be fascinated by a wonderful organism like yeast.

The baking and brewing yeast is called Saccharomyces cerevisiae in Latin. Literally a sugar fungus for beer. In the Middle Ages, it was simply called Zeug, which means stuff in German. And today in Bavaria, yeast is still called Germ (think of the tasty Germknödel) and in northern German dialects, Gest, which reminds us of English (Yeast), Swedish (Jäst), or Dutch (Gist).

If you think yeast is just a small organism that converts sugar into alcohol and carbon dioxide, you are underestimating the complexity of the brewer's favorite helper.

Let me infect you a little with my enthusiasm for yeast. 

As a so-called eukaryote, i.e. a living being with a cell nucleus, yeast is similar to animals and humans. The only difference is that yeast is so highly developed that it cannot only breathe to gain energy but also survive without oxygen. Respect, we can't do that for ten minutes. This anaerobic energy production is also our subject, fermentation. An amazing ability that speaks for the high adaptability of yeast. Perhaps it was the fascination over this talent of yeast that led humans to decode it completely genetically as the first organism in 1996 and to synthesize its 16 chromosome sets completely for the first time in 2014.

How is yeast structured?

Let's take another excursion into school biology and take a closer look at the cell.

The cell nucleus is the "command center" and contains the DNA with 13 million base pairs, which is the equivalent of 26 megabytes of data. It's like a USB stick, invisible to the human eye at less than 10 micrometers. Billions of generations have stored the complicated life of the cell here.

The mitochondria are the power plants of the cell and, according to science, were independent organisms that merged with other cells and made complex life forms possible in the first place through the division of labor of energy production and use.

At the endoplasmic reticulum, proteins (rough ER, with the help of ribosomes) and lipids (smooth ER) are produced. These metabolic processes keep the cell alive and enable proliferation. The products of these processes influence our later beer flavor, but more on that later. Small vesicles filled with raw materials are received and sent via the Golgi apparatus. The lysosomes recycle cell components that are no longer needed for building new raw materials. The cell is enclosed by a membrane in which the aforementioned organelles float in the cytoplasm.

Many mechanisms are very similar in the thousands of known yeast strains, but, as in other living organisms, there are different specializations. In order to make the processes controllable, today pure culture yeast strains are selected specifically according to desired properties (temperature, turbidity stability, settling behavior, stress tolerance, aroma, etc.) and are used almost everywhere.

The biggest difference between these strains is certainly their adaptation to warm or rather cool environments. As brewers, we essentially distinguish here between the top-fermenting yeast (S. cerevisiae), which is preferably active at room temperature, and the bottom-fermenting variant (S. pastorianus), which works optimally in cold cellars.

The name-defining characteristic, however, was the family behavior of yeast. Yeast reproduces by sprouting, i.e. cell division into mother and daughter cells. This occurs a maximum of 20 times, after which the cell has so many sprouting scars that it cannot reproduce any further. The family-oriented top-fermenting yeast likes to live in company, forming shoot associations. The CO2 excreted from each cell cannot rise to the top without taking the entire association up with it. This foam blanket, called Kräusen, can be easily observed during fermentation and is strongly pronounced in top-fermenting yeasts. The bottom-fermenting colleague is more likely to separate from its fellows and sink to the bottom more quickly. This is a desired characteristic among brewers, as it simplifies the separation of the yeast from the young beer and thus is more pronounced in all modern yeasts. So, for centuries we have been selecting the yeasts that bring preferred characteristics for aroma and technical processing. Probably the oldest intervention in evolution and genetics in human history.

Without technology or selective storage in cellars, bottom-fermenting yeast probably worked more in winter, where it took more time to produce a fine beer, giving rise to the name lager beers. The slower metabolism at lower temperatures gave the yeast time to further process many metabolic products and develop its fine bouquet from long storage. More about this later. The specific selection of yeast for its tasks has shaped various beer styles, such as Märzen, which was last brewed in ice cellars in March and was able to ripen into the summer.

So, what happens during fermentation?

Sure, alcohol is formed and CO2 is dissolved as carbonic acid. In addition, the pH value drops from about 5.5 to 4.5, making beer quite acidic. Only because of the unfermented residual sugar, it doesn't seem that way to us. Fermentation changes the composition of the wort, fermentation by-products are formed, in particular proteins are converted, bitterness compounds and tannins decrease, and it becomes lighter, clearer, and "fresher", i.e. its redox potential increases.

What exactly does the yeast do during fermentation?

After brewing, we add our yeast to the cooled wort. There, the yeast must first become familiar with the ambiance and adjust its metabolic processes. This time is called the adjustment or lag phase. The wort hardly changes, but the brewer need not worry; in the yeast, the processes are working at full speed.

Now our sugar fungus is to ferment the starch, essentially maltose, which has been broken down in the malt house and brewhouse. In the wort of our standard pilsner, we have about 91% carbohydrates, of which 64-67% are normally fermentable. This refers to simple sugars, such as glucose (5-7%) and fructose (1-2%), and in the beer wort, the majority with about 40-45% is maltose, a dual sugar from glucose. In addition, sucrose (household sugar with 3-6%), which is a disaccharide of glucose and fructose. Three glucose molecules are the so-called maltotriose (11-13%). Longer sugar chains are not readily fermentable and are called oligosaccharides or simply dextrins.

Our maltose, produced in the brewhouse by the enzyme β-amylase, is taken to the yeast cell for utilization by transporters (permeases), where it is split into two glucose molecules. With the glucose, the energy metabolism of the yeast, so-called glycolysis, now begins. Here, 38 ATP (adenosine triphosphate, the cell's batteries) are produced during respiration and 2 ATP during fermentation. From this, you can already see that respiration is 19x more effective in gaining energy, even for yeast, and fermentation is really just a kind of emergency metabolism. This is the reason why we aerate the cooled wort before pitching the yeast. We want to boost metabolism so that many new yeast cells are formed. This phase of logarithmic propagation of the yeast is called the log phase. Many young yeast cells improve our beer aroma because they reabsorb the aromatically unpleasant excretory products of the dead generation. If there are more dead than living cells, autolysis sets in, and these unpleasant aromas remain in the beer. The only thing that can help is new yeast to "kick-start" the process. Young yeast cells produce higher alcohols, ester precursors, and sulfur compounds, not all of which can be degraded by storage and we, therefore, want to limit them.

When the oxygen is depleted after a few hours, the actual fermentation begins. Before that, it ferments only a little bit, we call this the Crabtree effect. Now the yeast propagation is at its peak and the stationary phase begins, the actual time of the main fermentation. Two parts of the extract are formed, one part alcohol (ethanol) and just under one part CO2. This dissolved CO2 rises as carbonic acid and provides the foam during fermentation and on the finished beer. The increase also creates more yeast sediment. With this so-called Balling formula (2.0665g extract = 1.0g alc. + 0.9565g CO2 + 0.11g yeast), you can also calculate the expected amount of alcohol and carbon dioxide quite easily from the original gravity.

After about one-third of the extract has been degraded and no more oxygen is present, hardly any new yeast cells are formed and the first yeast cells begin to die. In this lethal phase, fermentation is initially still intense and the rather buttery vicinal diketones (diacetyl, pentandione), organic acids, and rather pungent green apple-smelling aldehydes are formed. In the course of this phase, the supply of nutrients decreases and the metabolism of the cells has to change. In the process, the aromas just mentioned are reduced. This is one reason why the odor increasingly improves in the course of the main and secondary fermentation.

Due to the ever-decreasing supply of nutrients, the metabolic processes shut down, fermentation proceeds more slowly and the yeast settles increasingly. Now the time has come to bung the young beer, i.e., to build up pressure in order to bind carbon dioxide. In the past, this was done by transferring the beer from the open fermentation vat to the pressure barrels. Today, all this takes place in the same vessels. The pressure, just like the increasing cellular toxins alcohol and CO2, reduces the activity of the yeast.

When the brew is finally fermented and the extract is reduced to residual sugar (the aforementioned dextrins), the yeast begins to use up the reserves in the cells. At this point at the latest, and better at the end of the log phase, the yeast should be harvested.

At the end of the main fermentation, the yeast now excretes "superfluous" substances to the substrate. These excreted amino acids, peptides, vitamins, phosphates, glycoproteins, enzymes, etc. have a significant impact on the body of the beer and round out the flavor. This is an important contribution to the maturation of the beer.

So how do you make good beers with yeast?

If you want to make good drinkable beers for a large audience, the yeast bouquet should be subtle and harmoniously integrated. A stressed yeast likes to bring aromas of higher alcohols and distinctive esters. Too little yeast activity also increases the risk of off-flavors, such as butter or green apple. Lack of sanitation, just like yeast with a predominance of dead cells, can bring extremely unpleasant aromas of butyric acid, sewer odor, rotten vegetables, or sweaty odor. Brewers, therefore, learn the requirement of almost sterile working conditions in the fermentation cellar and with good yeast management excludes the latter aromas. So now it is a matter of adding the good cellar work to the already good recipe of malt, hops, and mash work.

This starts with the right amount of yeast.

In the brewery, one works rather reluctantly with specifications such as liters or kilograms of liquid or dry yeast. We want to determine the cell count per milliliter and this should be at least 90% (optimally >97%) alive. This can be done with cell flow analysis or the microscope. The yeast is stained with e.g. methylene blue and counted on a counting chamber, a slide with 1ml capacity, and an engraved grid. The living cells process the colorant and become transparent, thus determining the proportion of active cells and calculating the required dilution as the batch quantity for the brew. For bottom-fermented beers, one million cells per milliliter per percent original wort are added, and up to twice as much for high original wort. For top-fermented beers, we use half as much. However, the amount of yeast has a significant influence on the concentration and composition of the fermentation byproducts and the quality of the harvested yeast. Therefore, the amount of yeast should not be used to control the fermentation rate. Because with the quantity you also influence the yeast propagation rate. It can be seen that although the quantity of newly formed yeast cells is comparable, the ratio of old and new yeast cells differs greatly. If the amount of yeast is lower, the multiplication rate is higher, and more young yeast cells are present. These then form higher alcohols, esters, and sulfur compounds. A larger amount of yeast will result in a more moderate fermentation with more neutral beers and a lower amount of yeast will ferment normally or even faster and will tend to produce more distinctive flavors.

With dry yeast, you have to rely on the manufacturer's instructions and the yeast package must not have been exposed to heat or oxygen for too long, otherwise, the number of active cells will drop and you will have to add more yeast. Shipping is especially critical in this regard during the summer. In a delivery vehicle, it can get hotter than 50°C in direct sunlight, and that's when even the dried yeast denatures. In hot summers, even the 500g packs have arrived dead with me by overnight shipping.

How do we now turn on the yeast correctly?

Of course, the temperature is important. Our yeast should be at the same temperature as our wort. In case of doubt, the yeast may be colder, but not warmer. If the yeast experiences a cold shock, it can quickly cause fermentation problems. Dry yeast can be sprinkled directly on the wort, but a feel-good ambient to "wake up" the yeast is rather a sterile, i.e. boiled, light wort of 5-7°Plato. The stress due to the "surprisingly" high extract of the pitching wort is lower and, unlike water, a light wort contains important nutrients. Water is well suited and should be added to the wort at the latest 30 minutes after pitching so that the yeast receives nutrients again. The cell stores are full when the yeast dries, but if you wait too long to pitch, the yeast will sink, as in the lethal phase.

Top-fermenting yeasts work optimally around 20°C and tend to become more fruit-flavored above 24°C due to more ester production, which can emphasize fruity hopped beer styles. Norwegian Kveik yeasts are usually made up of several yeast strains and like to be fermented at well over 30°C, sometimes 40°C, to enhance the diverse characteristic aromas (orange, tropical fruit, or even caramel) without creating classic off-flavors. The brew is then naturally final fermented in a few hours rather than several days.

Bottom-fermenting yeasts vary widely. Classic strains from Bohemia or Bavaria tend to work at 6-9°C, modern strains at home-brewer-friendly 14 to 22°C. It is worth reading the manufacturer's recommendation. Spontaneous fermentation is therefore more of a decision for the prevailing yeast strain in the air or in the fermenter at the current temperature and takes a long time for the yeast cells to multiply sufficiently for vigorous fermentation. Often, the beer pests also assert themselves first.

The higher the pitching temperature, the more the yeast multiplies and the more yeast metabolites are formed. In addition, the yeast mixes better because fermentation is an exothermic reaction and this heat is transferred to the medium more quickly, creating convection. The pH, protein, and bitter substances decrease and, as mentioned, higher alcohols and esters are formed more frequently. As brewers, we use temperature to control the rate of fermentation, which we measure in the degraded extract within 24 hours.

Certain young bouquet materials, such as the buttery aromas (diacetyl), can be specifically reduced faster by warmer temperatures when half of the fermentable extract is broken down, counteracting some of the coming stress from increasing alcohol and CO2 levels and pressure.

For more neutral bottom-fermented beers, it's worth setting the pitching temperature 1-1.5°C below the target temperature for primary fermentation, so that the yeast can get to the temperature itself and be 0.5°C higher after the brew than before.

If you harvest yeast, then of course it must proceed microbiologically clean. The timing would be ideal when 20-30% apparent extract is fermented. Then the percentage of young cells is the highest. When pitching with yeast from a brew in fermentation (pitching with Kräusen), fermentation should have just reached and been in the high Kräusen phase (full white head). This should be the case for classic bottom-fermenting yeasts after 48-72 hours, top-fermenting or modern bottom-fermenting yeasts after 24-48 hours.

The yeast lies in three layers on the bottom, which, as in the Middle Ages, are divided into top and bottom yeast with the core yeast in the middle. The bottoms are the first to settle and are therefore old cells with little vitality and a tendency to autolysis. The top stuff is usually stuck together with hop resins and would need to be washed. Clean separation of yeast layers is difficult. A rule of thumb for harvesting would be 2.5% harvest yeast per hectoliter of fresh pitched wort.

The yeast should be stored for a short time and as cold as possible, without freezing. Long-term storage is only possible by proper yeast banking. The longer one stores, the more one must propagate the yeast before use, i.e. multiply it again.

How to propagate yeast correctly?

We want to increase the number of active cells in a defined time. Of course, this can also be done during fermentation, e.g. by leaving a brew on top of a half-full tank after one day of fermentation. In "real" propagation, we want to throttle fermentation by deliberately aerating. With a fermentation level of less than 25%, we optimally achieve more than 100 million cells per milliliter, virtually all of which are alive (<1% blue colored cells in cell counting).

Typically, we propagate at room temperature and within 24 hours we manage 8-10 times multiplication of bottom-fermenting cells and 15-20 times multiplication of top-fermenting yeast cells.

Under fermentation conditions, multiplication rates are 6-8-fold in 72 hours at bottom fermentation and room temperature at about 10 million cells per milliliter and 4-6-fold in 72h when it gets colder (11-13°C). Top fermented 10-15 times in 24 hours.

100% propagation yeast brings typical fresh yeast flavors that personally remind me of brioche or yeast plait. Likewise, 100% dry yeast occasionally brings a slight autolysis aroma. So, it makes sense to mix harvest yeast with propagation yeast and to refresh with 20% propagation yeast after every third run.

So how do I control the beer aroma during yeast management?

Higher alcohols are part of the beer bouquet in moderate amounts and can be attenuated by cold fermentation, pressure, and increased yeast addition. It increases especially at high fermentation temperature, low yeast addition, or malnutrition of the yeast with amino acids from the malt (18-20mg/l/°P, see FAN value in malt analysis). Too intensive aeration or "leaving it on" also increases these aroma substances.

The esters from carboxylic acids (e.g. acetic acid) and the alcohols formed, especially during intensive primary fermentation, can increase during long or warm storage. Warm fermentation (>24°C), high base wort (>13.5°P), and thus also a lot of alcohol as well as intensive wort aeration increase ester formation, which leaves behind rather tropical fruit notes, especially with top-fermenting yeasts, and can harmonize well with the hops in fruity ales.

Buttery or sulfurous flavors usually break down the yeast, unless the yeast is malnourished and already dying, in which case the unpleasant compounds increase greatly and odors of cooked or rotten vegetables and sewer increase.

The young beer aroma, like green apple, is acetaldehyde, which comes from the decarboxylation of pyruvate, which we obtained from glycolysis. It is reduced to ethanol over time by the enzyme alcohol dehydrogenase unless there is a zinc deficiency. Likewise, alcohol is broken down by this enzyme in our liver. This is where a considerable part of our hangover comes from, in addition to the higher alcohols, and therefore we should also give the beer time so that the yeast reduces this aldehyde, not only because of the smell.

Incidentally, the yeast, just like our liver, can also recycle the alcohol itself for energy when oxygen is supplied. Again, an evolutionary advantage, first eliminating the competition with alcohol and then using this alcohol itself again as an energy source. Fortunately, we don't let it get that far in the brewery; oxygen levels are controlled in larger breweries because low levels are essential for flavor shelf life.

In addition, protein synthesis with the resulting organic acids has noticeable sensory effects. The balance of acidity with residual sweetness and bitterness is an elementary component of a final drinkability.

Long storage, pressure fermentation, or the aforementioned autolysis can then produce odors from fatty acid synthesis of goat cheese and sweat, which we should avoid at all costs.

Of course, any of the aforesaid aromas can also arise from infection with other microbiological cultures. However, some bring additional capabilities, such as lactic and acetic fermentation with their same-titled aromas, or additional enzymatic capabilities, such as the degradation of dextrins and the formation of more alcohol and carbonic acid, which can lead to foam fountains when opening the bottles.

Therefore, the brewer's top priority is still cleanliness when handling the yeast.

Then the beer also tastes good.

Skål!

Text: Brian Schlede for craftbeer magazine No. 15