By John Palmer
The fermentation of malt sugars into beer is a complicated biochemical process. Although the conversion of sugar into alcohol can be regarded as yeast’s primary function, fermentation is much more. Total fermentation is a composite of three phases: the lag (or adaptation) phase, the attenuative (or primary) phase, and the conditioning (or secondary) phase.
Yeast does not end the second phase before beginning the third; rather, the processes occur in parallel. Yeast’s early gusto for wort sugars is gradually phased out as the concentration of remaining easy food (simple sugars) in the wort decreases relative to the amount of complex sugars and yeast by-products. The conditioning process occurs more slowly than the attenuative process, however, which is why beer (and wine) will improve with age, to a degree, as long as they are in contact with the yeast.
The accompanying box below summarizes the key factors required for a good fermentation: Pitch plenty of yeast, make sure the cells have the nutrients they need to multiply, and let them do their thing at the right temperature.
Act I —the adaptation phase:
Immediately after pitching, yeast spends some time adjusting to the wort conditions and indulging in a period of high growth. During this time, yeast takes stock of the sugars, free amino nitrogen (FAN), and other nutrients present in the wort and figures out what enzymes and other attributes it needs to adapt to the environment. This delay between the pitching of the yeast and the formation of the foamy head in the fermentor is referred to as the adaptive phase, or lag time. This initial phase is critical in setting the stage for a good fermentation.
During the adaptation phase, yeast cells use their own glycogen reserves, lipids, and any dissolved oxygen from the wort to synthesize sterols. Sterols are critical for the development of healthy, permeable cell membranes, which makes wort sugars and other wort nutrients available to the yeast. Strong cell membranes will also protect the yeast from premature death due to rising alcohol levels as the fermentation progresses. Yeast can also synthesize sterols under oxygen-poor conditions from the fatty acids found in wort trub, but that method is less efficient and less reliable for the brewer.
With healthy, permeable cell membranes, yeast can start metabolizing its food — free amino nitrogen (FAN) and sugars in the wort. Like every animal, yeast cells live to reproduce, which they do asexually by “budding,” a process in which daughter cells split off from the parent cell. Reproduction takes a lot of energy, and the process works more efficiently when oxygen is present (that is, under aerobic conditions). Thus, an oxygen-rich wort shortens the adaptation phase and allows yeast to quickly reproduce to levels that will ensure a good fermentation.
Once the oxygen is used up, the yeast cells switch metabolic pathways and begin what brewers regard as fermentation: the anaerobic metabolism of sugar to alcohol.
The key to a good fermentation is lots of strong healthy yeast cells that can get the job done before depleted resources, rising alcohol levels, and old age render them dormant. As noted, the rate of reproduction is slower in the absence of oxygen. At some point in the fermentation cycle of the beer, the rate of yeast reproduction will fall behind the rate of yeast dormancy. By providing optimum conditions for yeast growth and reproduction in the wort initially, we can ensure that this rate transition will not occur until after the beer has become fully attenuated.
Worts that are underpitched or poorly aerated will have insufficient quantities of viable yeast, leading to slow or incomplete fermentations. Experienced brewers make a great point about wort aeration and building up a yeast starter because these practices guarantee a large enough population of yeast to do the job well. Under nominal conditions, yeast should proceed through the adaptation phase and begin primary fermentation within 12 hours. If 24 hours pass without apparent activity, then a new batch of yeast should probably be pitched.
Lag time is a common benchmark that brewers use to gauge the health of yeast and the vigor of fermentation. It is possible, however, to overemphasize the significance of lag time, on its own, as a meaningful indicator. A very short lag time, for example, does not necessarily mean that your yeast has gotten off to a phenomenal start and just can’t wait to ferment. It could well mean just the opposite — that a low supply of nutrients and oxygen in the wort gave the yeast no recourse but to begin converting sugar to alcohol in order to survive. The latter stages of fermentation may also appear to finish more quickly when in fact the process was not super-efficient, but rather, incomplete. The point is that speed does not necessarily correlate with quality.
Under optimal conditions, of course, the process will generally be more efficient and thus will take less time. But it’s better to pay attention to getting the process right than to focus on a rigid time schedule.
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Act II — the attenuative phase:
The attenuative, or primary, phase is a time of vigorous, robust fermentation during which the gravity of the beer drops by two-thirds to three-quarters of the original gravity. The primary phase will last anywhere from 2 to 6 days for ales or 4 to 10 days for lagers, depending on the yeast and on the fermentation conditions.
A head of foamy kräusen will form on the beer as the sugar is converted to alcohol and the beer attenuates. The foam will be a light creamy color with islands of green-brown gunk that will tend to collect and adhere to the sides of the fermentor. The gunk is made up of extraneous wort protein, hop resins, and dead yeast. These compounds are very bitter and will contribute harsh aftertastes to the beer if allowed to mix back into the wort. Fortunately these compounds are relatively insoluble and will separate themselves from the wort, sticking to the sides of the fermentor as the kräusen subsides; they can also be removed by racking (transferring) to another, or secondary, vessel. Alternatively, they can be removed using the blow-off method, whereby a tube is attached to the fermentor at one end and submerged in water (makeshift sanitary airlock) at the other. Pressure in the fermentor forces the material on the surface of the wort out of the fermentor through the tube and into the water at the other end.
As this primary phase winds down, a majority of the yeast cells start settling out and the kräusen begins to subside. If you are going to transfer the beer off of the trub and primary yeast cake, this is the proper time to do so. Take care to avoid aerating the beer during the transfer. At this point in the fermentation process, any exposure to oxygen will only contribute to staling reactions in the beer or worse, expose it to contamination.
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Key Factors for a Good Fermentation
Pitch plenty of yeast: The first step to achieving a good fermentation is to pitch enough yeast. Yeast can be grown using yeast starters or it can be harvested from previous fermentations. When recycling yeast, use yeast that has proven itself through good fermentations in the past. Harvest from the middle layer of the primary yeast cake, where you are most likely to find healthy cells from the most active period of the fermentation. This yeast will have the optimum characteristics for repitching: It will have good glycogen reserves and will readily adapt to the new wort. With nominal levels of aeration and nutrients, the yeast will quickly multiply to the levels necessary for an exemplary fermentation. Yeast can also be taken from the secondary, but it may be less inclined to flocculate.
However you obtain your yeast, plan to pitch at least 1/3 cup (75 mL) of yeast slurry for a typical 5-gallon batch of ale, or 2/3 cup (150 mL) of slurry for lagers. For stronger beers (O.G. >1.050 [12.34 °P]), pitch more yeast to ensure optimum fermentations. For very strong beers such as Doppelbocks and barleywines, pitch at least 1 cup (225 mL) of slurry.
Three issues must be considered to ensure that the wort has been properly prepared to support a good fermentation.
Aeration: The first issue is the infusion of oxygen into the wort through aeration. Yeast will use up any available oxygen in the wort during the lag time to synthesize sterols for growth and cell membrane development. Strong cell membranes enable the yeast to absorb nutrients and protect it from rising ethanol levels later in the fermentation.
FAN content: The second issue is the level of amino acid nutrients in the wort, specifically referred to as free amino nitrogen (FAN). Malted barley normally supplies all of the FAN that yeast needs to grow and adapt to the fermentation environment. If the recipe incorporates large amounts of adjuncts (refined sugar, corn, unmalted wheat, or unmalted barley, for example), however, then the wort may not have the minimum levels of nutrients necessary for yeast to build strong cells. In extract brewing, it is always advisable to add some yeast nutrient powder to worts made exclusively from light-colored extracts because these extracts are often thinned with corn sugar.
Avoid refined sugars: Be aware also that in worts that contain a high percentage of refined sugar (about 50% or more), yeast will sometimes lose the ability to secrete the enzymes that allow it to ferment maltose.
Yeast cells are greatly affected by temperature — too cold and they go dormant; too warm (more than 10 °F [6 °C] above the nominal range, which varies from strain to stain) and they indulge in an orgy of fermentation resulting in by-products that often cannot be cleaned up by conditioning.
High temperatures also encourage the production of fusel alcohols — heavier-than-average alcohols that can have harsh solvent-like flavors. Many of these fusels esterify during secondary fermentation, but in large amounts these esters can dominate the beer’s flavor. Excessive “banana” flavors are a common consequence of high-temperature fermentation.
High temperatures can also lead to excessive levels of diacetyl. A common problem among home brewers is to pitch the yeast when the wort is too warm. If the yeast is pitched when the wort is 90 °F (32 °C), for example, and slowly cools to room temperature during primary fermentation, the early stages will produce more diacetyl than the yeast can reabsorb during the secondary stage.
Furthermore, primary fermentation is an exothermic process. Yeast activity can raise the internal temperature of the fermentor as much as 10 °F (6 °C) above ambient conditions, but as long as this higher temperature is within the optimal range for the yeast, the beer will be fine, even if the yeast is warmer than its surroundings.
The point is that if it is midsummer in Florida, you might want to find a way to keep the fermentor cool or just consider waiting until the weather turns cooler. Beer fermented too warm will not be drinkable.
Keep the Yeast Happy
Fermentation is the most important part — the purpose, in fact — of the whole process of brewing. Careful attention to the factors that affect fermentation will help to ensure that your labor of love lives up to your expectations.
Many canned kits and even brewing texts advise bottling the beer after one week or after the kräusen has subsided. Don’t. The beer has not yet gone through the conditioning phase. The flavor of unconditioned beer reveals rough edges such as yeasty, buttery, or green apple flavors that will disappear after a few weeks of conditioning. Take your time — it will be worth the wait!
The final act — the conditioning phase:
The reactions that take place during conditioning are primarily a function of the yeast. The vigorous, primary stage is over, the majority of the wort sugars have been converted to alcohol, and a lot of the yeast cells are going dormant — but some are still active.
Flavor effects. During the earlier phases, the yeast produced many compounds in addition to ethanol and carbon dioxide (acetaldehyde, esters, amino acids, ketones-diacetyl, pentanedione, and dimethyl sulfide, for example). By the time the kräusen has subsided, the yeast has eaten the easy food and now turns its attention toward the heavier sugars such as maltotriose and dextrins, as well as to the reprocessing of its own undesirable by-products.
Diacetyl and pentanedione are two ketones that have buttery and honey-like flavors. These flavors are considered flaws when present in large amounts, and the compounds responsible cause flavor stability problems during storage. The compound acetaldehyde is a specific aldehyde that has a pronounced green apple smell and taste. It is an intermediate compound in the production of ethanol, and is reduced during the later stages of fermentation. Primary fermentation also produces an array of fusel alcohols that often give harsh solvent-like tastes to beer. During secondary fermentation, yeast converts many fusel alcohols to more pleasant-tasting fruity esters.
Natural filtration. Conditioned beer is not only better tasting, but naturally clearer. Toward the end of secondary fermentation, a majority of the suspended yeast cells flocculate (settle out). Tannin and phenol compounds will bind with high molecular weight proteins and also settle out, greatly smoothing the taste of the beer.
This process can be helped along by chilling the beer, very much like one might do in lagering. In reference to ales, this process is referred to as cold conditioning and is a popular practice at most brewpubs and microbreweries. Depending on the yeast strain, cold conditioning for a week will often clear the beer without the use of finings.*
*Fining agents such as isinglass and gelatin are added to the secondary fermentor to help speed the flocculation process and to promote the settling of haze-forming proteins and tannins. While finings are most often used to drop unflocculant yeast strains and to combat chill haze, their real benefit is to improve the taste and stability of the beer.
Conditioning times. How long you choose to condition will depend on your recipe and your preference. Different beer styles benefit from different amounts of conditioning time. Generally, the higher the original gravity, the longer the conditioning time needed for a beer to reach peak flavor. Small beers such as 1.035 O.G. (8.76 °P) pale ales will need less than two weeks. Stronger, more complex ales such as porters may require a month or more. Very strong beers such as Doppelbocks and barleywines can take six months to a year before they condition to their peak flavor.
The downside. Unfortunately, yeast may also consume some of the proteins and other compounds in the trub, the “fermentation” of which can produce any of several off-flavors. In addition, the dormant yeast on the bottom of the fermentor begins excreting more amino and fatty acids. If the postprimary beer is left on the trub and yeast cake for too long (more than about two weeks), soapy off-flavors may become evident. The longer the yeast cells are left in contact with the beer, the more likely it is that they will begin to secrete enzymes that allow them to feed on each other (autolysis), which produces rubbery, sulfury tastes and smells. For these reasons, it can be important to remove the trub and dormant yeast from the beer during the conditioning phase.
Conditioning can take place in either the primary fermentor, a secondary fermentor, or the bottle, but each method produces different results. Debates over the benefits of each abound within the home brewing community.
For and against bottle conditioning: Beer bottled immediately after completion of the primary fermentation will condition in the bottle, and this is the method of choice for many brewers. Many seasoned home brewers declare, for example, that racking to a secondary fermentor offers no real taste benefit and that the dangers of contamination and the cost in additional time are not worth what little benefit may be gained. I agree that for a new brewer’s first low-gravity pale beer, the risks probably outweigh the benefits, and I advise keeping it simple until you have gained some experience with racking and sanitization.
If you are concerned about the risks of racking and want to bottle without using a secondary fermentation phase, you can leave an ale in the primary fermentor for a total of two weeks instead of just one, which will give the conditioning reactions time to improve the final beer. The extra time will also let more sediment settle out before bottling and result in a clearer beer and easier pouring. Just remember not to leave the beer on the yeast for more than two weeks.
For most beer styles, however, conditioning before bottling makes the most sense, partly because the larger yeast mass in the fermentor is more effective at conditioning than the smaller amount of yeast suspended in the bottle.
Priming and bottling right after the primary can create some other unwelcome problems as well. Studies have shown that bottle-conditioned beer actually goes through another mini-primary fermentation, making use of some of the headspace air. Unfortunately, though, only about 30% of that oxygen is used for reproduction; the rest slowly diffuses into the beer as it ages, contributing to staling reactions.
Furthermore, a bottle-conditioned beer is stuck with the job of dealing with all of the usual intermediary compounds and by-products of fermentation, including aldehydes, diacetyl, fusels, and esters, that are produced during this short fermentation process on top of any byproducts left over from primary fermentation. Thus, to avoid these problems and get the most of conditioning, the beer should be given time in a secondary fermentor before priming and bottling. This is as true for homemade beer as it is for commercial “bottle-conditioned” beers. Even if the yeast has flocculated and the beer has cleared at bottling time, enough active yeast will still be in suspension to ferment the priming sugar and carbonate the beer.
Bottle conditioning is not, however, always bad or inefficient. Bottle conditioning simply results in different ester profiles than those that are normally produced in the main fermentor. In fact, in some styles, for instance Belgian strong ale and Hefeweizen, bottle conditioning and the resultant flavors are the cornerstones of the style. These styles cannot be produced with the same hallmark flavors if they are immediately kegged (draft style).
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The case for secondary fermentation: Secondary fermentation is beneficial to all beer styles if you have a good understanding of fermentation processes and can complete the transfer carefully. The key is in mastering some of the simple facts of racking.
Dos and don’ts of racking. Racking from the primary can occur any time after primary fermentation has more or less finished, when the bubbling rate drops off dramatically to about 1–5 per minute. This will be about 2–6 days after pitching for ales, 4–10 days for lagers (though if it has been more than two weeks for ales you may as well just bottle). The kräusen will have started to settle back into the beer. Use a sanitized siphon to rack the beer off the trub into another sanitized fermentor and affix an airlock. The beer should still be fairly cloudy with suspended yeast.
Although oxygen is a desirable commodity very early in the critical adaptation phase, it is absolutely not wanted in the later stages of fermentation. Racking at any time exposes the beer to the potential risks of exposure to oxygen and bacterial contaminants. Also, racking the beer before the primary fermentation phase is completed can result in a stuck or incomplete fermentation.
Most brewers will notice a brief increase in activity after racking which is due not to additional primary fermentation, but simply to dissolved carbon dioxide coming out of solution because of the disturbance.
Fermentation (conditioning) does continue after racking, so just leave it alone for awhile. A minimum useful time in the secondary fermentor is two weeks. Leaving the beer in the secondary for too long (more than six weeks for ales) may require the addition of fresh yeast at bottling time to achieve good carbonation. (Always use the same strain as the original.)
Keep liquid notes. No matter which method you choose for fermenting and packaging your beer, it is always a good idea to set aside a six-pack in the corner of the basement and leave it for a good long time. It can be enlightening to compare a home-brewed beer after three months of bottle conditioning to the batch as it initially tasted.
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Rave Reviews Can Be Yours
Fermenting a beer without allowing time for the conditioning phase is like raising the house lights and leaving the show before the final act has played out. Something will be missing from the total experience, and you deprive yourself of the full value of the show you paid for.
Giving the beer time to condition smoothes and refines the beer’s flavor. Conditioning reduces diacetyl levels, vents dimethyl sulfide, and esterifies fusel alcohols. Yeast is given time to consume the remaining fermentables without also consuming the junk food in the trub. The low concentration of dormant yeast cells also prevents off-flavors caused by fatty acid excretion and autolysis. Finally, secondary fermentation allows time for more yeast and haze to settle out before bottling.
The result is better beer. With a little patience, you can see the process through to the end and discover the full rewards of the grand finale.