6

Yeast and Fermentation

How Yeast Work

Defining Fermentation

Building a Better Fermentation

Open versus Closed Fermentation

A good fermentation of a bad recipe is going to make a better beer than the bad fermentation of a good recipe.

There was a time when the role of yeast in brewing was unknown. The German Beer Purity Law of 1516—the Reinheitsgebot—listed the only permissible materials for brewing as malt, hops, and water. Everyone talks about how Louis Pasteur discovered yeast in the 1850s, but he didn’t. What he did was the conduct the experiments and publish the papers that determined yeast were responsible for the fermentation process and defined how it worked. Everyone knew about yeast, in fact, yeast slurry was packaged and sold by the Dutch in 1780, with a compressed cake form entering the market in 1825. Brewers and bakers knew the importance of transferring yeast from batch to batch long before Pasteur; they just didn’t understand what it was. They referred to it as the “mother” or “God is Good,” among others. The word “yeast” is derived from words that mean “foam” or “to rise.” Other scientists at the time ascribed fermentation to a purely chemical reaction catalyzed by air. The idea that a living organism would be responsible was too biologic—it was too old-fashioned for modern thinking. Scientists considered yeast to be a byproduct of fermentation rather than the cause.

Beer would not be beer without yeast and fermentation. If you recall my “Top Five Priorities for Brewing Great Beer” from chapter 1, fermentation temperature and yeast management are second and third, respectively. Why is fermentation temperature placed ahead of yeast itself? All else being equal, the yeast quality, quantity, and activity level (a function of temperature) are all important; but, when you ask expert brewers their opinion, fermentation temperature control is always their first priority. However, before we get into that, let’s discuss what yeast is and how it works.

Figure 6.1. Aerial view of yeast ranch. Magnification 300×.

Figure 6.2. Budding yeast cells. Magnification 1000×.

How Yeast Work

Brewer’s yeast (Saccharomyces cerevisiae) is a type of fungus, and the species name literally translates to “sugar fungus beer.” It reproduces asexually by budding, that is, splitting off little daughter cells (fig. 6.1 and 6.2). Yeast is unusual in that it can live and grow both with or without oxygen. Most microorganisms can only do one or the other. Yeast can live without oxygen by a process that we refer to as fermentation. The yeast cells take in simple sugars, like glucose and maltose, and produce carbon dioxide and alcohol as waste products. Yeast cells reproduce during fermentation by cloning themselves, budding off little daughter cells that can grow and create daughter cells of their own.

Table 6.1 lists normal ranges of the different sugars (technically known as saccharides) found in a typical beer wort. The main constituent is maltose, followed by assorted dextrins, maltotriose, glucose, sucrose, and fructose. Maltose is a glucose disaccharide, which means that it is made up of two glucose molecules. Maltotriose is a trisaccharide consisting of three glucose molecules. Sucrose (commonly known as table sugar) is a disaccharide that is made of one glucose and one fructose, and occurs naturally in plants (e.g., in sugar cane, beets, maple sap, and nectar). Dextrins (a.k.a. oligosaccharides) are larger sugars consisting of more than three monosaccharide groups.

Yeast consumes sugars in the wort methodically, starting with the monosaccharides (simple sugars) and the disaccharide, sucrose; then moving on to the main wort constituent, maltose; and finally finishing up with the trisaccharide, maltotriose. Interestingly, yeast seems to work on sucrose first, breaking down this disaccharide into glucose and fructose, then consuming all of the glucose and fructose before moving on to maltose. The last fermentable sugar, maltotriose, typically accounts for 15%–20% of the total sugars in wort, but is usually not fermented completely, although this depends on the yeast strain. For example, lager yeast ferments maltotriose better than strains of ale yeast.

Table 6.1—Typical Sugar Spectrum of Wort

Sugar	Typical range
Glucose	10%–15%
Fructose	1%–2%
Sucrose	1%–2%
Maltose	50%–60%
Maltotriose	15%–20%
Dextrins	20%–30%

Note: Dextrins are not fermented by brewer’s yeast.

The sugar composition and yeast strain are what determine the fermentability of the wort. If the wort was mashed to have high fermentability (methods of mashing are described in chapter 17), the percentage of unfermentable dextrin would be about 20%, meaning that 80% of the wort was fermentable. As the yeast consumes the malt sugars there is a change in the specific gravity of the wort, or beer, which is the degree of attenuation. In a wort that is 80% fermentable, you might expect the attenuation to be 80%. However, attenuation will only get near 80%. This is because the yeast strain also determines the fermentability of the wort, and most yeast strains attenuate in the range of 67%–77%. For example, a yeast strain having an attenuation of 75% would ferment a beer from 1.040 to 1.010, or from 1.060 to 1.015. If the yeast strain is a low attenuator, then it may not ferment very much of the maltotriose, resulting in an overall attenuation of about 67%. If the yeast strain was a high attenuator, it will ferment most of the maltotriose, resulting in an overall attenuation of about 77%. In addition, wort with a high percentage of monosaccharides (e.g., 30% rather than the normal 15%) can inhibit the production of the enzymes that yeast uses to ferment maltose, resulting in a “stuck” fermentation. This aspect is discussed more in chapter 24, “Developing Your Own Recipes.”

Brewer’s yeast doesn’t actually respire oxygen during any stage of the brewing process. The yeast cells use oxygen, but not in the same “breathe in, breathe out, burn food” sort of respiration that other cells do. Instead, yeast cells use oxygen to chemically synthesize unsaturated fatty acids and sterols they need to build and maintain their cellular membranes.

What Is a Cell Membrane?

If you compare a yeast cell to the human body, the cell wall is your skin (the epidermis), and the cell membrane is the living tissue underneath. Of course, this analogy is a gross mangling of the facts, but it serves the purpose. The cell wall is the structural container of the yeast cell and the cell membrane directly beneath it is the semi-permeable boundary layer through which the yeast cell interacts with its environment. The cell membrane allows sugars to be taken into the cell and waste products (alcohol and carbon dioxide) to be excreted. A fresh, healthy yeast cell has a pliable membrane; an old yeast cell has a tougher, less permeable membrane that makes it harder for the cell to take in food and excrete waste.

Brewer’s yeast produces many other compounds in addition to ethanol (ethyl alcohol) and carbon dioxide. Some are desirable, such as esters and phenols, and some are not, such as the precursors for diacetyl and the production of fusel alcohols. Esters are responsible for the fruity notes in beer, and phenols cause the spicy notes. Both are generally desirable, depending on style. But, you can easily have too much of a good thing. A good example of this principle is diacetyl, a vicinal diketone compound that can be beneficial in very small amounts. Diacetyl gives a buttery note to the flavor of a beer and helps round out the malt flavor in some ale styles, but it is very easy to end up with too much diacetyl in your finished beer. Another undesirable product are fusel alcohols, which are heavier molecular weight alcohols that are readily apparent as “solventlike” notes.

After fermentation, when all the food is used up and their cell membranes are too old to function well, the yeast cells go into hibernation mode. They build up their glycogen and trehalose reserves (i.e., storage carbohydrates), clump together (i.e., flocculate) and settle to the bottom of the fermentor. Different yeast strains flocculate differently and will settle faster or slower as a result. Some yeast practically paint themselves to the bottom of the fermentor, while others are ready to swirl up if you so much as sneeze. Highly flocculent yeasts can sometimes settle out before the fermentation is finished, leaving residual sugars and unacceptable levels of acetaldehyde and diacetyl, which would otherwise be cleaned up by the yeast during the maturation phase of fermentation.

Defining Fermentation

The Three Phases of Fermentation

Figure 6.3. Three phases of fermentation

The fermentation of malt sugars into beer is a complicated biochemical process. From a brewer’s point of view, there are three phases of fermentation after pitching the yeast (fig. 6.3):

1. 1. Adaptation, or lag, phase before activity is seen in the fermentor.
2. 2. High-growth, or high-kräusen, phase when most of the attenuation occurs.
3. 3. Maturation, or conditioning, phase when the yeast becomes quiescent, the kräusen drops, the beer clears, and the flavor improves.

We will discuss each of these phases in more detail in a minute. However, it is worth noting that, from the yeast’s point of view, there are also three phases, but a slightly different three:

1. 1. Adaptation phase, a relatively short period before growth.
2. 2. High-growth phase, when eating and reproduction take place in earnest.
3. 3. Stationary phase, when the yeast essentially hibernates.

You may notice that the maturation phase seems to be missing from the yeast’s list of priorities. Yes, it is, and it is up to the brewer to understand this and create the proper conditions for it to occur.

Yeast cells do not behave like a synchronized school of fish. They are more like a large group of people—while many are doing the same general activity, some will be more active than others, and some will be less active. In other words, the yeast cells don’t transition from one phase to another as a group; instead, the phase is determined by what the majority of them seem to be doing at the time. In the wild, any particular yeast cell within a group could randomly be at any point in its life cycle (fig. 6.4). As brewers, we can get the yeast cells to become more organized by pitching them to yeast starters and controlling the temperature. That’s how we can get them all to march in step and ferment more efficiently.

Figure 6.4. Diagram of the life cycle of a yeast cell. The yeast cell starts at the top center of the diagram in the stationary phase. When pitched into wort the cell enters the adaptation phase, where it takes in essential nutrients and physically increases in size. When it has sufficient nutrients and is an appropriate size, the yeast cell begins the reproduction process by producing a bud on its cell wall. The nucleus of the cell, containing the DNA, replicates and divides as well. The vacuole is like a combination storage shed and workshop for the cell, containing nutrients, enzymes, and waste products. The vacuole resources are shared with the budding cell and it is these resources that ultimately limit how many times the cell can bud. The budding cell separates, leaving a bud scar on the cell wall of the mother cell. The new cell—the daughter cell—is smaller than the mother cell and will need to grow before it too can start budding daughter cells of its own. At the end of the growth phase, when nutrients have become limiting, the yeast cell enters the stationary phase and lives off its reserves until pitched to new wort.

Adaptation (Lag) Phase

Immediately after pitching into wort, the yeast cells start adjusting to the wort conditions, changing their metabolism in response to the sugars and other nutrients present, and regulating enzyme production and other attributes according to their needs in the new environment. At this stage, yeast cells use their own glycogen reserves together with lipids and dissolved oxygen in the wort to synthesize sterols and fatty acids. These substances are critical to maintain cell membrane integrity and function, enabling the cell membrane to be permeable to wort sugars and other wort nutrients. Long-chain fatty acids are used to build structural lipids, while short-chain fatty acids are esterified and excreted. Under oxygen-poor conditions, yeast can produce these sterols and fatty acids using lipids found in wort trub, but that metabolic pathway is much less efficient.

Once the cell membranes are permeable, the yeast cells can start metabolizing the sugars and free amino nitrogen (FAN) in the wort for food and growth. An oxygen-rich wort shortens the adaptation phase, and allows the yeast to quickly revitalize and move on to the next phase of fermentation, the high-growth and attenuation phase.

Under normal conditions, the yeast should work through the adaptation phase and begin the high-growth phase within 12–24 hours. In other words, most, if not all, of the first day after pitching will be taken up with the adaptation phase, with the next phase beginning on day 2, or thereabouts. If 48 hours pass without apparent activity (foamy, yeasty mass on top of the wort and rapid bubbling in the airlock), then a new batch of yeast should probably be pitched.

In the past, a short lag phase has been overemphasized as a benchmark indicator of a good fermentation. It was regarded as “the shorter, the better” because it meant that the yeast were strong and ready to work. While it is a notable indicator, a short lag phase does not guarantee an exemplary fermentation and outstanding beer. A short lag phase only means that the adaptation time was short; for instance, it could be because there was not much oxygen to take up and it was time for the yeast to eat or die. However, these cautions aside, a short lag phase (6–12 hours) and rapid bubbling in the airlock generally means that the yeast are healthy, happy, and able, though 12 hours is probably more the average.

Of course, wort temperature will have a large effect on yeast activity. A common mistake that many homebrewers make is pitching the yeast when the wort has not been chilled enough and is still warmer than 80°F (>27°C), thinking that it will cool down by the time the high-growth phase starts. This is risky—it is very likely that more metabolites (byproducts) will be produced at this early stage than the yeast will reabsorb during the maturation stage, which will affect how clean-tasting the final beer will be.

Managing temperature is a critical part of the fermentation process, and it starts as soon as your yeast is pitched and the adaptation phase begins. If you want a cleaner-tasting beer (i.e., less esters, less fermentation character), the yeast should be pitched into the wort when the wort temperature is at or slightly below the intended fermentation temperature. For example, if the recommended temperature range for California ale yeast is 68–73°F (20–23°C), then pitching the yeast at a wort temperature of 65–68°F (18–20°C) will reduce the amount of byproducts produced early in the high-growth phase, and so moderate the overall amount through your fermentation. If you want more fermentation character in your beer, then raise the wort pitching temperature to the middle of the recommended range. I would not recommend pitching at the top of the range unless you have experience with the yeast and the style, and know that you like the resulting flavors. Also be aware that pitching too cold can stress the yeast, resulting in lots of fermentation character.

Bottom line: a warmer wort temperature at pitching will shorten the adaptation (lag) phase, but assuming your sanitation up to this point has been good, there is no benefit to speeding up this phase to outcompete bacteria or other microbes. The pitching temperature sets the stage for the activity in the high-growth phase. The warmer the wort, the faster the yeast will grow and the more byproducts they will produce.

High-Growth Phase

The visible churning of the wort and rapid bubbling in the airlock heralds the high-growth phase of fermentation. Once a yeast cell has adapted to its surroundings, it starts to eat, and once it has started to eat, it starts reproducing. The high-growth phase is a period of vigorous fermentation and can last anywhere from 1–3 days for ales, or 2–5 days for lagers, depending on the original gravity and pitching rate. The majority of the attenuation (e.g., 98% of the attenuation) should occur during the high-growth phase.

A head of foamy yeast kräusen will form on top of the beer (fig. 6.5). The kräusen consists of yeast cells and wort proteins, and is a light cream color with islands of green-brown gunk. The gunk is a residue composed of extraneous wort protein, hop resins, polyphenols, and dead yeast. Many brewers skim this residue off the kräusen, especially if they plan on harvesting some of the yeast foam for another batch. (Harvesting from your own fermentation is described in chapter 7.) Fortunately, the residue is relatively insoluble and typically sticks to the sides of the fermentor as the kräusen subsides, so removal is not strictly necessary.

High-attenuation yeast strains tend to be low or poorly flocculent, whereas low-attenuation strains tend to be highly flocculent. Flocculation is the propensity of yeast cells to clump together when they go dormant, which causes them to settle to the bottom of the fermentor at the end of fermentation. These traits will be discussed in more detail in chapter 7.

Most of the esters and other flavor compounds are produced during the high-growth phase. The best way to control these flavors in your beer is to control the growth, and you do that by controlling the temperature. Yeast activity is highly dependent on temperature—too cold and they go dormant, too hot (more than 10°F [5°C] above the recommended range) and they will indulge in an orgy of fermentation that often ruins the flavor of the beer.

Furthermore, the fermentation process during the high-growth phase produces heat. The internal temperature of the fermentor can be as much as 10°F (5°C) above ambient conditions, just due to yeast activity. This is a good reason to keep the fermentation environment well within the recommended temperature range, so that you will get a normal vigorous fermentation where the beer turns out as intended, even if it was warmer than the surroundings.

Many off-flavors, such as acetaldehyde and diacetyl, can be cleaned up by the yeast, but others cannot. High temperatures promote the production of fusel alcohols, leading to solventlike flavors. High temperatures can also promote an excess of esters, leading to banana or bubblegum-flavored beers. Once created, these flavors cannot be reduced by maturation. Temperature control is the key to controlling fermentation and fermentation character.

The high-growth phase will last as long as the yeast’s sterol and lipid reserves hold out. When those reserves are exhausted, the yeast cells no longer have the resources to share and they stop producing daughter cells. The yeast cell membranes will become old and less able to transfer food and waste, so general activity slows down as well. This decrease in activity means less carbon dioxide is created, which causes the foamy kräusen to settle back into the beer.

Wort Temperature or Room Temperature Control?

There are two schools of thought about using electronic temperature controllers to manage fermentation temperature. Some advocate controlling the temperature of the wort, or beer, while others advocate controlling the temperature of the room or refrigerator. Both methods can be misused depending on how and when the control is applied. The key is understanding the phases of fermentation and knowing when the temperature of the wort should be controlled, that is:

• • Fermentation byproducts are generated during the high-growth phase.
• • The temperature of the wort at the beginning of the high-growth phase should be at the lower end of the recommended range to moderate the rate of growth in order to limit the amount of byproducts produced.
• • The temperature of the wort or beer should be raised, or allowed to rise, after the high-growth phase to increase the activity of the yeast during the maturation phase.
• • The temperature of the fermentation should never decrease during fermentation. The beer temperature should only decrease after maturation is finished. This applies to all beers, even lagers.

Therefore, when you pitch your yeast to the fermentor, the temperature of the wort must not be higher than the set point of whatever controller you are using. In other words, don’t pitch until your wort temperature reaches the set point. If the wort is warmer, the yeast will experience high growth (and produce a lot of byproducts) for a short period of time and then lose activity due to the decreasing temperature. You don’t want the yeast to slow down halfway through.

If you are controlling the temperature of the wort or beer, verify that the controller does not overchill by more than 2°F (1°C) below the set point. You will need to monitor the gravity or fermentation activity (i.e., bubbling in the airlock) in order to raise the set point for the conditioning phase.

If you are controlling the temperature of the refrigerator or room instead of the beer, the temperature of the fermentation will rise several degrees above the room temperature. This is okay, provided that the fermentation temperature does not exceed the recommended range. What you will find, in fact, is that the fermentation often meets the higher temperature guideline for good maturation.

This type of temperature control has worked for thousands of years. But be aware that, as yeast activity slows, less heat will be generated—if the room is cold, the yeast may flocculate and enter the stationary phase too early. Be prepared to raise the room temperature set point accordingly.

Maturation Phase

The start of the maturation phase doesn’t mean that fermentation is over. There are now a lot of fermentation byproducts in the beer, including compounds that contribute off-flavors, such as acetaldehyde (green apple or raw pumpkin flavors) and diacetyl (stale butter or milk flavor). The beer is considered to be “green” at this point. Before it can be considered ready for consumption, the beer needs a maturation period to allow the yeast time to clean up these compounds.

The key to maturation is active yeast. At the end of the high-growth phase the yeast cells are worn out and ready to call it a day and go to sleep. The beer isn’t finished yet, but the yeast cells are too old and tired to do anything about it. The cells flocculate and settle to the bottom (fig. 6.6).

Figure 6.5. A healthy, creamy, kräusen of yeast floats on top during the high-growth stage of fermentation. This is a good time to skim off some yeast to save for a future batch.

Figure 6.6. As the high-growth stage winds down and the maturation phase takes over, the yeast starts to flocculate and the kräusen settles back into the beer.

Physically rousing the yeast (swirling the fermentor) will sometimes help to keep the cells in suspension and working on the beer. However, this doesn’t change the fact that the yeast cells are old and tired, and it can take a long time for them to eat the remaining sugars and consume undesirable byproducts like acetaldehyde and diacetyl. How do we keep the yeast from becoming old and tired?

As the brewer, you must balance the pitching rate and wort resources so that the majority of the yeast cells will still be relatively fresh and have not reached their reproduction limit by the time the majority of the fermentable sugars have been consumed. This will leave lots of active yeast cells in suspension that will then look for alternative food sources and clean up the beer.

Maturation consists of the reduction of acetaldehyde and other aldehydes into alcohols, the enzymatic breakdown of diacetyl, and the adsorption of other off-flavors onto the surface of the yeast cells as they settle to the bottom of the fermentor. The mechanisms of maturation were poorly understood for many years, but research over the last fifty or sixty years has improved our understanding of yeast metabolism and the maturation process. We now know how to achieve full maturation in a shorter time. The key is managing the fermentation using pitching rate and temperature.

The details of pitching rate will be discussed more in chapter 7, but for now think of it as managing the number of yeast relative to the amount of fermentables, so that the fermentables run out before the yeast gets tired. Yeast activity is directly affected by temperature, so raising the temperature toward the end of the high-growth phase will greatly increase activity and allow the yeast to clean up the byproducts faster. This technique for raising the temperature towards the end of fermentation is generally known as a “diacetyl rest.” A diacetyl rest is most typically used for lager beers, although it is applicable to all yeast strains and will help with the cleanup of other byproducts, such as acetaldehyde, as well.

Therefore, as you see the bubbling in the airlock start to slow down toward the end of the high-growth phase, approximately day 3 for ales and day 4 for lagers, raise the fermentation temperature by 9°F (5°C) for a diacetyl rest. A more precise indicator is when the beer is 2–5 specific gravity points from the final gravity. This rest is more typically used for lager fermentations, but the same principle can apply to ales as well (see table 6.2 for both lager and ale guidelines).

A higher pitching rate will mean a greater proportion of active yeast cells remain toward the end of attenuation, so a smaller temperature increase can achieve sufficient maturation activity. As you might expect, a lower pitching rate will mean fewer active yeast cells remain, so a larger temperature increase may be necessary to fully clean up the beer. Of course, temperature adjustments can only be taken so far. The key is having the right amount of yeast.

A good rule of thumb is that the maturation phase should be as long as the high-growth phase. If you choose not to raise the temperature for maturation then you should probably allow twice as long for the maturation phase, just to be on the safe side. For example, looking at the guidelines for lagers in table 6.2, the suggestion is to raise the temperature of the fermentation by 14–18°F (8–10°C) around day 4, assuming that the high-growth phase (steady bubbling in the airlock) is going to last until day 6. You would then raise the temperature for the diacetyl rest (maturation phase) and leave it there for at least six days. Same idea for ales. The point is to give the yeast all the time and help it needs to clean up the beer. Your yeast may be done sooner, but the extra time doesn’t hurt the quality of the beer. Patience is a virtue.

Table 6.2—Maturation Rest Guidelines

Schedule	Lager	Ale
When (approx. time)	day 4 of 6	~day 3 of 4
When (specific gravity)	2–5 points from FG	2–5 points from FG
Raise temperature by	14–18°F (8–10°C)	5–10°F (3–6°C)
Leave for	minimum 6 days (up to 12)	minimum 4 days (up to 8)

Cold Conditioning (Lagering)

Traditionally, lagering was viewed as a long, slow maturation process, and this view has persisted in the literature right through the latter half of the twentieth century. But it is important to separate the two types of maturation that need to occur in “green” beer. The first type is the reduction of fermentation byproducts by the yeast, as described in the section above. The second type is physical clarification of the beer, that is, clearing it of excess yeast and haze, and this is what we call cold conditioning.

New beer is hazy beer, and hazy beer doesn’t taste the same as clarified beer. Haze due to suspended yeast can give the beer a yeastlike, brothy flavor, but these flavors should decrease with time as the yeast flocculates and settles to the bottom. Haze due to protein and polyphenols are harder to get rid of. The protein-polyphenols that cause haze are chemically the same compounds that create astringency in the mouth, and they can promote staling reactions in the beer. A hazy beer can be a good beer, but a clearer beer is often a better tasting beer. There are, however, some styles where haze is an important part of the beer’s appearance. Haze and clarifiers are discussed in more detail in appendix C.

Cold conditioning is a process of slowly cooling the beer down by 2°F (1°C) per day to about 9–15°F (5–8°C) below the fermentation temperature to promote the flocculation of the yeast and the coalescence of the protein-polyphenol complexes that cause haze. The protein-polyphenol complexes in beer are held together by hydrogen bonds, which are stronger at colder temperatures. Hence, cold temperatures help these complexes to become large enough to settle out. The cooling rate for settling protein-polyphenol haze does not matter as much as the final temperature, colder being better. The point of slow cooling is to prevent thermal shock of the yeast cells and subsequent excretion of fatty acids and other lipids. These lipids can interfere with head retention and will readily oxidize, creating stale flavors. Thermal shock at any time can cause the yeast cells to release protein signals that cause other yeast cells to shut down to protect against the cold, potentially leading to premature flocculation and underattenuation.

Cold conditioning and lagering are useful tools for clarifying the beer, but it must be understood that it needs to happen after the yeast have completely fermented the beer and cleaned up the byproducts. Otherwise, you risk high levels of diacetyl and aldehydes, and underattenuation. You should only have to cold condition for a week or two to settle most of the haze. Fining agents, such as silica gel, gelatin, and isinglass, can help greatly and will usually clear the beer in a couple of days instead of weeks. Usually, the beer can be left on the yeast for several weeks without causing any flavor problems, but you should be aware that there are risks to leaving your beer on the yeast and trub for too long after fermentation.

As they lie on the bottom of the fermentor, dormant yeast cells can excrete undesirable amino acids, short-chain fatty acids, lipids, and enzymes. As a result, leaving the beer on the trub and yeast cake for too long (more than a month, for example) can lead to soapy, waxy, fatty, and other oxidized flavors. Even worse, after very long times the yeast cells begin to die and break down (i.e., undergo autolysis), which produces meaty, brothy, and soy sauce tastes and odors.

Building a Better Fermentation

Step 1.	Cool the wort to the pitching temperature.
Step 2.	Aerate to add oxygen to the wort.
Step 3.	Pitch the yeast.
Step 4.	Add yeast nutrients if necessary.

Yeast cannot live on sugar alone. Yeast also need minerals, nitrogen, amino acids, and fatty acids to enable them to live and grow. The primary source for these building blocks is the malted barley. Refined sugars like table sugar, corn sugar, and honey do not contain any of these nutrients. However, let’s talk about oxygen and aeration first.

Oxygen and Aeration

The role of oxygen in yeast fermentation has already been discussed at several points in this chapter, but not how to get it into your wort. Commercial breweries generally do it by inline injection of pure oxygen into the wort as it is pumped to the fermentor. At the homebrewing scale it is easier and less complicated to cap and shake the fermentor, or to use an aquarium airstone with either an air pump or small oxygen tank. Another option, one that has worked for thousands of years, is open fermentation, which will be discussed at the end of this chapter.

Depending on the strain, yeast typically needs 8–12 ppm of oxygen for a good fermentation. Without aeration, fermentations tend to be underattenuated. Higher-gravity worts need more yeast cells (i.e., higher pitching rates), and thus need more oxygen, but the higher gravity makes it more difficult to dissolve oxygen into the wort in the first place. Boiling the wort drives out the dissolved oxygen normally present, so aeration of some sort is needed prior to fermentation. For the homebrewer, proper aeration of the wort can be accomplished in the following ways:

• • pouring the wort with splashing or spraying (gives about 4 ppm);
• • shaking the container (only applicable to small volumes, gives about 8 ppm);
• • using a stainless steel airstone with an aquarium air pump to bubble air into the fermentor for 5–10 min. (gives about 8 ppm);
• • or using an airstone with an oxygen tank to bubble oxygen into the fermentor for about 1 min. (gives about 10 ppm).

Supporting data is given in table 6.3, but it is apparent that there is a lot of variation in results with pouring and shaking. Airstones are generally more effective and consistent, but cause the wort to foam, which will often ooze out of the fermentor as a result. Anti-foaming agents can be used to minimize this problem.

Table 6.3—Oxygenation Data for 5 gal. (19 L) 1.040–1.080 Wort

Method	Time	Dissolved oxygen level
Siphon with sprayer attachment	(siphoning time)	4 ppma
Shaking with air (small volume)	1 min.	8 ppma
Shaking with air (large volume)	5 min.	2.7 ppmb
Airstone with aquarium air pump	5 min.	8 ppma
Airstone with oxygen, 1 L/min.	30 sec. 1 min. 2 min.	5.1 ppmb 9.2 ppmb 14.1 ppmb
Airstone with oxygen, rate not specified	1 min.	12 ppma

a Greg Doss, David Logsdon, and Company, “The Meaning of Life According to Yeast,” Wyeast Labs, http://www.bjcp.org/cep/WyeastYeastLife.pdf.

b White and Zainasheff (2010, p. 79).

For the beginning homebrewer using rehydrated dry yeast, I recommend the simplest aeration methods of shaking the starter and pouring the wort. Pouring is also effective if you are doing a partial boil and adding water to the fermentor to make up the total volume. Instead of pouring the wort, you can just pour the water back and forth several times to aerate the water. Dry yeast typically requires less oxygen, because the yeast is grown with high glycogen and lipid reserves before being dehydrated and therefore generally needs less oxygen in the adaptation phase.

If you are using liquid yeast cultures, including yeast harvested from yeast starters or previous batches, I recommend aerating with an airstone using an aquarium pump or oxygen tank. Otherwise, be prepared to form a vortex and stir your arms off. Using a high-speed electric mixer is not recommended, because the shearing action can damage the yeast and denature some of the foaming proteins for head retention.

Using an air pump and airstone to bubble air into the fermentor is effective and saves you from lifting a heavy fermentor to pour or shake the wort. The saturation point of oxygen from the air in chilled wort is about 9 ppm. Most yeast strains require 8–12 ppm of oxygen for adequate growth and activity, but that requirement also depends on the pitching rate and the volume of the wort. The yeast will take up the oxygen quickly, generally in less than an hour. An air pump and airstone should reach an oxygen concentration of 8 ppm for 5 gal. after about 5 min., but I recommend aerating for longer, about 10–15 min., to make sure that your yeast has the time to get all it needs.

The only precaution you need to take, other than sanitizing the airstone and hose, is to be sure that the air going into the fermentor is not carrying any mold spores or dust-borne bacteria. An in-line filter is recommended to prevent airborne contaminants from reaching the wort. One type is a sterile medical syringe filter (fig. 6.7), which can be purchased at hospital pharmacies or at your local brewshop. An alternative, build-it-yourself bacterial filter can be made by filling a tube with moist cotton balls. The cotton should be changed after each use.

Figure 6.7. Here is an example of an aquarium air pump with an airstone and microbial filter. The filter is a HEPA medical syringe filter. An alternative is to make an inline filter from a plastic tube, cotton, and rubber stoppers. The pre-moistened cotton provides the filtering action and should be discarded after each use.

You can also buy small oxygen tanks used for soldering and brazing at the hardware store or welding shop. Pure oxygen has a wort saturation point of 40 ppm, so you only need a relatively short time compared to pumping ordinary air. However, using pure oxygen can result in overoxygenation and off-flavors if done excessively. In addition, oxygenation should only be done before pitching, because pure oxygen is toxic to the yeast. Frankly, these reasons are why I tend to recommend using an air pump rather than an oxygen tank for homebrewing, because it is nearly impossible to over-oxygenate with plain air. Pure oxygen is necessary and practical at the commercial level, where brewers deal with very large worts and have the equipment to monitor oxygen concentrations.

Free Amino Nitrogen

The second nutrient the yeast need from the wort is nitrogen. Nitrogen is available in the form of amino acids and small peptides, specifically referred to as free amino nitrogen (FAN). Nitrogen is a universal component of amino acids, peptides, and proteins, and is therefore vital for all metabolic processes in yeast (and all living things, for that matter). Malted barley normally supplies all of the FAN the yeast need to grow, sometimes too much, in fact. This is one reason that beers brewed using adjuncts can have cleaner flavors than all-malt beers. However, if the recipe uses too much low-protein adjunct (e.g., corn, rice, honey, or refined sugars), then the wort may not have enough FAN. Both of these conditions can affect beer flavor. High FAN levels, consistent with a highly-modified all-malt wort, would tend to produce more fruity esters. Low FAN levels, consistent with a high-adjunct wort, would tend to have a more floral character. Most worts probably fall between these two extremes.

FAN, Fermentation, and Diacetyl

The level of FAN in the wort can affect the amount of diacetyl in the beer.

Diacetyl is a vicinal diketone, as is 2,3-pentanedione. While “vicinal diketone” sounds very ominous, it simply means that the molecule consists of two ketone groups side by side, formed from neighboring carbon atoms. Diacetyl can also be called 2,3-butanedione, so you can see how these two compounds are very similar, butane having four carbon atoms in its carbon chain and pentane having five. Diacetyl has a buttery flavor and aroma commonly associated with microwave popcorn (because diacetyl is used as the principle artificial flavor ingredient), while 2,3-pentanedione has a sweeter buttery flavor and aroma that is more like honey-butter. Together, these compounds are the main off-flavors in new beer.

Brewer’s yeast is very effective at removing diacetyl and 2,3-pentanedione during the maturation phase, but diacetyl will often appear again later after packaging. Why? The reason is the presence of acetohydroxy acids in the fermentation. Acetohydroxy acids are a metabolic byproduct of the yeast, but the yeast cannot break them down further. Instead, acetohydroxy acids have to undergo oxidation to diacetyl (and 2,3-pentanedione) before the yeast can do their work. Therefore, this oxidation step is the limiting factor.

A strong vigorous fermentation at cool temperatures can seem clean after conditioning, but still have lots of acetohydroxy acid in solution that is simply waiting for a little bit more oxygen or higher temperatures to oxidize. This is another reason that the conditioning phase should take place at a warmer temperature than the high-growth phase, to promote conversion of all the acetohydroxy acid to diacetyl and 2,3-pentanedione so that it can be cleaned up by the yeast.

Essential Minerals

Minerals present in the brewing water and malt are vital for the yeast. These include both macronutrients, such as phosphorus, and other nutrients used in smaller quantities as enzyme cofactors and catalysts for yeast metabolism, such as magnesium and zinc. Magnesium in particular plays a vital role in cellular metabolism, and its function can be inhibited by excessive calcium additions to the water. Brewers adding calcium salts to adjust their water chemistry should include magnesium salts as part of the addition if they experience fermentation problems.

Calcium stimulates yeast growth but is not growth limiting like magnesium. Calcium plays an important role in membrane structure and function and is necessary for yeast flocculation. Only small amounts are required for metabolic function and it is usually completely supplied by the malt. However, calcium reacts with malt phosphates and other compounds, such as oxalates, throughout the brewing process, resulting in insufficient calcium for flocculation and hazy beer.

Sulfur is used by yeast for synthesizing certain amino acids (methionine and cysteine), enzymes, and vitamins. The preferred source for sulfur is the catabolism of the amino acid methionine, although the yeast can use inorganic sulfate from the wort and water when organic sources (i.e., amino acids) are exhausted. Excess sulfur can be stored in the yeast cell vacuole in the form of glutathione, a peptide that can serve as an anti-oxidant.

Additions of zinc can greatly improve the cell count and vigor of the starter. Unlike the other essential minerals, zinc is often deficient in the wort, or trapped in a form that the yeast cannot assimilate. However, adding too much zinc will cause the yeast to produce excessive byproducts and cause off-flavors. Zinc acts as a catalyst and tends to carry over into succeeding generations, therefore, it is probably better to add zinc to either the starter or the main wort, but not both. For best performance, zinc levels should be between 0.1–0.3 mg/L, with 0.5 mg/L being the maximum. If you experience stuck fermentations or low attenuation, and you have eliminated obvious factors, such as temperature, low pitching rate, poor aeration, or low FAN, then low zinc may be a cause.

Nutritional Supplements

Finally we come to nutritional supplements: vitamins, minerals, and energizers for the yeast. Nutritionally speaking, the wort should supply everything the yeast need, but some high-gravity styles or styles that have a high proportion of refined sugar may need supplements to get the job done. There are many different products but only two main types: fertilizer and dead yeast. Fertilizer-type supplements are simply refined chemicals, such as diammonium phosphate, which can supply much needed nitrogen and phosphorus. In any case, yeast seem to prefer more organic sources for their nutrients, and that’s where dead yeast cells come in. Yeast extract (a nicer way of saying dead yeast) is easy for yeast to assimilate. Modern yeast nutrient supplements are usually a combination of essential vitamins, minerals (including zinc), amino acids, and yeast extract, all of which yeast can readily absorb.

All of these aspects of the wort—oxygen, nitrogen, minerals, and other nutrients, are vital for ensuring good yeast health and a good fermentation. These are the internal forces that determine the quality of the beer. But what about the external forces? We have already discussed temperature, so it is time to discuss the fermentors themselves.

Figure 6.8. Open fermentation is still fairly common, with each batch protected by its layer of yeast kräusen. Skimming and harvesting yeast is as easy as sanitizing a clean shovel and transferring the yeast to a waiting bucket. The fermentor is usually loosely covered with a lid. (Photo courtesy of Austmann Bryggeri, Trondheim, Norway.)

Open versus Closed Fermentation

There are two types of fermentation: open and closed. An open fermentor is open to the air, and a closed fermentor is not. An open fermentor can be covered with a lid, but as long as the lid is not sealed to the outside air it is still considered to be open. A closed fermentor usually has an airlock that allows carbon dioxide to vent from the fermentation but doesn’t allow outside air in. Closed fermentation using airlocks helps prevent contamination and oxidation of the batch.

The difference between open and closed fermentation is essentially the use of an airlock, but the geometry of the fermentor is usually different as well. Closed fermentors, such as a tank, carboy, or a bucket with a lid, are typically taller than they are wide. Open fermentors are generally broader and shallow, with a 1:1 height to width ratio or less (for the homebrewer, this usually just means using a bucket or bin). This geometry has advantages and disadvantages. The broader shape means open fermentors shed heat more effectively than closed fermentors, and they provide more oxygen to the yeast. Yeast harvesting is easier from open fermentors (fig. 6.8) and the yeast health is usually better. But, open fermentors typically hold less volume and take up more floor space than taller, narrower closed fermentors. In addition, open fermentors usually require racking the beer to a secondary fermentor for maturation, while closed fermentors do not.

The transition from open to closed fermentation happened relatively recently during the twentieth century. It is safe to say that before 1900 most fermentations (both commercial and home) were open, whereas after 2000 most fermentations were closed, although open fermentation is not uncommon even today. The reason for the change? Stainless steel tanks, which didn’t come into general use until the 1950s. Prior to this, fermentation tanks were made from wood, brick, or stone and lined with pitch, wax, fiberglass, or epoxy.

Many of the standard guidelines and procedures that we take for granted as being the best practices for fermentation are dictated by the equipment we use, and it is important to understand the reasons why. Let’s look at some examples.

Primary and secondary fermentation. The distinction between attenuation and maturation, and the racking from a primary to a secondary fermentor comes directly from open fermentation systems where the beer was casked away from oxygen to mature before serving.

Dropping the beer and rousing the yeast. It used to be common practice to aerate the beer a couple of times during fermentation by pumping it between open fermentors, or agitating the yeast to get it back in suspension. This was because the role of oxygen was not well understood. Now we have airstones and inline oxygenation to fully aerate the wort before fermentation.

Racking the beer from the yeast. It is recommended not to leave the beer on the yeast for too long, because tall, cylindro-conical fermentors place a high hydrostatic pressure on the yeast that increases the likelihood of autolysis after flocculation. This pressure doesn’t occur in most shallow open fermentors, or in a small homebrewing cylindro-conical fermentor.

Long cold lagering. Historically, packaging the beer in wooden casks to age the beer tended to result in souring, especially if the beer was not kept cold. But cooling the beer slows down the maturation process by the yeast, and thus long cold lagering became the standard practice out of necessity. With modern equipment and controls, maturation can be accomplished much faster, and you don’t have to move the beer as much. It can all be done in a single vessel.

Basic Procedure for a Closed Fermentation

Most homebrewing is done with a closed fermentor, usually in the form of a food-grade plastic bucket with a tight-sealing lid, or a food-grade plastic or glass carboy. Either way, an airlock or blowoff hose is used. Closed fermentation is generally recommended for new brewers because it is simpler—pitch the yeast, affix the airlock, and walk away for two weeks. Open fermentation requires the brewer to pay more attention to the fermentation activity.

The basic procedure for a closed fermentation is the same as described in chapter 1:

1. 1. Pour the chilled wort into the fermentor.
2. 2. Aerate the wort with an airstone, or by rocking or shaking the fermentor.
3. 3. Pitch the yeast, seal the lid, and affix an airlock.
4. 4. The fermentor should be placed in a cool room with a consistent temperature within the fermentation temperature range for the yeast.
5. 5. The fermentation should start in 12–24 hours and will bubble in the airlock for several days. Leave it alone for at least two weeks (after pitching) before bottling.

Basic Procedure for Open Fermentation

There are many short videos on the Internet that can show you the open fermentation process and give you a good idea of what to expect. Good sanitation is always important. Here is a basic procedure:

1. 1. Chill the wort to the intended fermentation temperature and pour it into the bin with turbulence to get some aeration. For best results, the wort should be aerated with an airstone for at least 5 min.
2. 2. Pitch the yeast into the wort. Drape a clean towel over the fermentor.
3. 3. A kräusen should appear within 24 hours. Skim off the brown crud that floats on top. The yeast is creamy white. Once the kräusen is clean, you can scoop some of the fresh yeast from the kräusen into a closed container and store it in the refrigerator for your next batch.
4. 4. The best time to harvest yeast for your next batch is when the attenuation is three-quarters of the way to final gravity. For example, if the OG is 1.050, and the expected FG is 1.010, you would harvest at about 1.020 specific gravity. For a typical ale fermentation, this is around day 3 after pitching. This is also a good point during the fermentation to raise the temperature 5–10°F (3–5°C). Top cropping of yeast from open fermentations helps adapt the yeast to the method. Every professional brewer will tell you that batch-to-batch repitching of the yeast for the same recipe definitely improves the fermentation performance and the beer flavor after three generations or so.
5. 5. On day 4 (assuming a typical ale fermentation) the beer will need to be covered with a lid and an airlock affixed, or transferred to a secondary fermentor, to allow it to mature away from oxygen. This timing assumes the beer will be fully attenuated by the end of day 5. Your fermentation may vary. Maturation can take anywhere from two days to two weeks depending on the temperature and health of the yeast.
6. 6. If you are transferring the beer to a secondary fermentor for maturation, use a sanitized siphon. Fill the siphon with sanitizing solution, and cover the outlet with your finger until you place the racking cane in the primary fermentor. Allow the sanitizer to drain into another bucket, and then move the outlet to your secondary fermentor as the beer starts to flow. Place the outlet hose on the bottom so that it is quickly covered and doesn’t splash. Watch out for any leaks in the siphon that will bubble air into the green beer. The beer should still be fairly cloudy with suspended yeast. The maturation temperature should be the same, or a few degrees higher, as previously discussed.

You would think that open fermentation would have a problem with contamination from the air, but the dense mat of yeast on top of the beer protects it pretty well. Any debris that falls into the fermentor is usually carried to the sides by the yeast mass. An easy way to prevent debris falling in is to drape a clean towel over the bucket or tub. Air can still easily pass through the towel, but it keeps the dust and pet hair out. A lid that comes down over the sides of the fermentor but leaves an air gap is also a good solution. Airborne bacteria and mold spores cannot fall up to get inside, and there is usually so much carbon dioxide coming off the fermentation that insects won’t come near it.

An open fermentor for home brewing can be a standard fermentation bucket, a standard brewing kettle, or a large food-grade plastic storage bin. The greater air exposure is part of what gives these beers their greater fermentation character, but it is not necessary to go overboard with it. Before refrigeration, wort would be run into wide shallow coolships to help dissipate the heat of the boil, and fermentors were often wider and shallower than what I have described here in order to dissipate the heat of fermentation, but those are solutions to problems we don’t usually have nowadays. A roughly cubic volume is a good starting point. A larger surface area will result in more esters, perhaps too much, depending on your taste. A common HDPE plastic storage bin (fig. 6.9), such as those used for storing clothes and blankets, works well.

Figure 6.9. Open fermentors used to be the norm; any food-grade bucket or bin will work. Cover it loosely with an overhanging lid or drape a clean towel over it to keep debris out. If you are using the lid, there should be an air gap for venting.

Open fermentation usually requires the use of a secondary fermentor for maturation, because oxygen exposure after the high growth phase will oxidize the beer and shorten its shelf life. If the open fermentor is a standard bucket, then sealing with the lid and airlock is sufficient and the beer can mature without being transferred.

Racking or transferring the beer to another fermentor is always an oxidation and contamination risk. Glass or plastic carboys make good secondary fermentors because of the smaller head space. A smaller headspace is better to minimize the oxygen exposure and make it easier for the headspace to be purged by the still-fermenting beer. Do not dilute your beer to increase the volume; even boiled water contains about 1 ppm oxygen, which can cause staling. The head space will be purged fairly quickly anyway.

The maturation time should be 1–2 weeks and then the beer can be bottled or kegged. If the beer was cold conditioned for a long period of time between maturation and packaging, additional yeast may be necessary for priming and carbonating.

Hopefully this chapter has helped you understand the big picture of what fermentation is and how it works. The next chapter will get into the nuts and bolts of yeast management.