If you’re going to discuss beer fermentation with any degree of authority, there is one and only one definitive statement you can make about it: “It all depends….” That about sums up the whole process in a yeast cell. If you find yourself reading other books about yeast and fermentation, don’t kid yourself into assuming that you’ve read something that will apply to all situations. It just ain’t so. Never, ever and forever. It all depends. Especially for the homebrewer.
A basic introduction to yeast and fermentation is given in The New Complete Joy of Home Brewing. The main points of that discussion are briefly reviewed here, but please read those pages if you aren’t familiar with the basic principles of yeast metabolism and beer fermentation.
Whether you are using ale or lager yeast, the principal factors influencing yeast behavior are:
- The strain of yeast
- Their physical environment
temperature
pH
osmotic pressure (i.e., density of liquid)
head pressure
- Nutrients and food
- Oxygen
- Good initial health
There is no one perfect “schedule” of fermentation. The variables listed above are only some of the many that influence the life cycle of yeast, how yeast behaves, what compounds it consumes and what compounds it creates on its mission of transforming wort to beer.
BEFORE YEAST METABOLISM BEGINS
There are a number of conditions that will evolve in the wort and with the yeast. Brewers have some control over this evolution, but it is the homebrewer’s first task to take a moment or two to have a homebrew and begin to appreciate the essential fact that yeast cells are living organisms and that they are in some ways more evolved than human beings. They can live in beer!
Only after developing this respect for yeast should the homebrewer proceed to consider what can be done to enhance conditions for the happiest and best fermentation.
It all depends…on the beer you’re making and the fermentation conditions you anticipate. Of the hundreds of flavor-related compounds created during yeast metabolism, most are arrangements of carbon, hydrogen and oxygen atoms. Remember, the food the yeast will convert to beer flavor is sugar, itself chains of carbon, hydrogen and oxygen atoms. The yeast metabolize and rearrange these atoms into hundreds of other carbon, hydrogen and oxygen chains, some of which are classified as
- Esters
- Alcohols (including fusel alcohols or fusel “oils”)
- Diketones (including diacetyl)
- Aldehydes
- Organic acids
- Phenols
- Sulfur compounds (not strictly carbon, hydrogen and oxygen atoms)
Every strain of yeast will behave differently and produce varying amounts of these products. And each one is also influenced by the conditions in the wort during fermentations. To highlight only one example out of thousands, studies have shown that the ester level of beer can be influenced by the presence of carbon dioxide during fermentation. The pressure at the bottom of a very tall fermenter is significantly greater than in a small fermenter; the more pressure, the more carbon dioxide remains in solution, thus affecting ester levels. Here’s another gem. Research by brewers at Guinness Brewery in Dublin indicates that prior to fermentation, ester levels can be affected by certain situations during lautering. For example, the amount of ethyl acetate esters can be influenced by the degree of filter bed compaction during lautering. Cutting the bed with rakes serves to loosen the bed, releasing more lipids into the wort and consequently suppressing ester formation during fermentation. On the other hand, long, clear runoffs from a compacted grain filter bed will reduce the amount of lipids (all relatively speaking, remember) making their way to the wort and thus tending to encourage ester production during fermentation. “It depends…” is an understatement.
There is often concern by homebrewers that some of the above-listed compounds are toxic. Aldehydes, some sulfur compounds, some types of alcohols (fusel alcohols) and esters can indeed be toxic. But toxicity is best defined by dose rather than by substance. The amounts of these compounds that can be naturally produced in homebrewed beer are at very low levels of concentration. One would have to drink two or three gallons of beer having high levels of the compounds every day to even have the remotest effect of toxicity. The “clean” alcohol (ethanol) would have more of an unhealthful effect on your body than any other trace compounds. Besides, any beers having perceptible levels of these compounds are most likely NOT going to be something you’ll want to drink a lot of anyway. Much bigger problems can occur with moonshine. The distilling process concentrates these compounds, raising their levels thousands to millions of times higher than what you’d naturally find in homebrewed beer. By the way, high levels of esters, aldehydes and fusel alcohols are the culprits responsible for headaches and hangovers. Some people are more sensitive than others, and we all are certainly sensitive to abuse by overindulgence.
Too little or too much yeast can affect fermentation. The beginning and intermediate homebrewer should consider the negative effects of underpitching, but certainly not worry about the effects of over-pitching. The homebrewer striving for finesse will gain some advantage from knowing a few basics.
If inadequate amounts are used, the respiration cycle will be prolonged, and more reproductive cycles will need to occur to reach optimal yeast population.
If too much yeast is added, there won’t be enough oxygen for all of them and on the average they will all be oxygen-deficient, resulting in some negative side effects.
How much is enough? If introducing liquid live yeast, about 6 fluid ounces (177 ml.) of yeast slurry that is the consistency of a thick cream chowder is ideal for a 5-gallon (19-l.) batch. There is little doubt that quality beer can be made if more or less yeast is pitched. Ask any practical homebrewer. But the possibility of making the best beer is increased if proper amounts of yeast are introduced.
Once the wort has been cooled, it is essential that it be aerated in order to dissolve oxygen. Yeast must have oxygen to carry out the many tasks expected of it. But the amount of oxygen required varies…. It all depends. Generally speaking, though, the required concentration will range somewhere between 4 and 14 mg./l. (ppm). Recall the discussion of trub—you could conclude that if you are removing all of the trub or most of it, then your oxygen requirement will be on the high end of this range. If your trub is carried over into fermentation, then fatty acid/lipids will provide the yeast with compounds that it can synthesize into the energy it would have received from oxygen. Thus your oxygen requirement will be less. These are only two variables to consider. Oxygen requirements will also vary with yeast strain. If insufficient oxygen is provided, a sluggish fermentation may be observed.
There is a downside to adding too much oxygen. Yeast tends to remain in the respiration cycle for a longer period and continues to reproduce, using more of the fermentable carbohydrates. There may be more undesirable flavor compounds that are produced during the respiration cycle in the finished beer. If the addition of oxygen is continued while the yeast is in the wort, there will be a tendency for the yeast to remain in the respiration stage (or even revert to the respiration stage if oxygen is added during fermentation), reproducing, and producing no alcohol. The flavor compounds resulting from this unnatural process are less than desirable. Some yeast strains do better at this than others, but so far, none have been found that will produce a good-tasting nonalcoholic beer using this process—yet.
When saturated with air at about 60 degrees F (16 C) by shaking, splashing or spraying, wort will pick up enough air to saturate to 8 mg./l. of oxygen. This is usually adequate, especially considering that most homebrewers will not remove every bit of trub. If additional oxygen is desired, then sterile pure oxygen can be introduced to the wort to reach a maximum saturation of 40 mg./l. of oxygen. (This will vary somewhat with wort density. Remember, it depends.) This amount is far in excess of proper levels. It is interesting to note that water at 60 degrees F (16 C) will hold more saturated air than wort at the same temperature, so it is possible to boost the levels of oxygen to some degree (but certainly not to an excessive degree) by adding cold water to the wort. This would be primarily a factor for malt extract brewers to consider. But don’t get swell-headed and think you’ve found salvation. Just have a homebrew and appreciate it for what little it’s worth.
YEAST METABOLISM AND FERMENTATION
Once yeast is added to the prepared wort, metabolic activity begins. There are dozens of different cycles that yeast can enter during their residency in wort and beer. There are several excellent references that go into much greater detail than this discussion does.7 Remember all of these activities and flavor compounds that are produced are influenced by the type and strain of yeast as well as the condition of the wort and beer before and during the fermentation process. The general phases of yeast metabolism are:
- 1) Lag Time—A period of a few hours when yeast cells take up oxygen and nitrogen-based nutrients (protein amino acids) from the wort. This period is sometimes considered part of the respiration cycle.
- 2) Respiration—The period when reproduction occurs. Carbon dioxide, water and flavor-related compounds are produced. No alcohol is produced.
- 3) Fermentation—The population of yeast is optimal and the metabolic cycles begin, whereby carbohydrates are converted to heat, alcohols, carbon dioxide and other flavor compounds.
- 4) Sedimentation—Most of the conversion of fermentable carbohydrates is complete. Yeast enter a dormant life-preserving metabolic cycle and fall as sediment.
- 5) Aging—While not an actual yeast cycle, the aging process is a period in which flavor compounds produced by yeast can transform into other compounds, many considered favorable.
SINGLE-STAGE FERMENTATION OR TWO-STAGE FERMENTATION: WHICH IS BEST? AGAIN, IT ALL DEPENDS
Even though one-stage fermentation is the simplest, it should be considered only when the yeast strain you use does not autolyze quickly and impart yeast-bite flavors. Note that cooler fermentation temperatures can reduce the autolyzation process. Choose your yeast strains and fermentation conditions carefully if using single-stage fermentation. Generally if beer can be lagered or aged for short periods (less than three weeks) at below 60 degrees F (16 C), the single-stage homebrewer may be able to brew excellent beer without removing the beer from the primary fermentation yeast sediment. However, if warmer temperatures, longer aging times or sensitive yeast strains are used, two-stage fermentation is needed to optimize the quality of your homebrew.
The two-stage process removes the beer from prolonged contact with the primary fermentation sediment. Long, cold lagering and postfermentation aging require that the beer be moved off any sediment for optimal quality development.
“My beer stopped fermenting before it was supposed to.” This is a frantic statement best dealt with by having a homebrew and relaxing. Reading a recipe and assuming things are “supposed to” happen a certain way is not really homebrewing. Homebrewing is what you do and how your beer behaves. And it usually behaves the way it is supposed to, not the way a recipe concludes it should. Remember all of the variables. It all depends…. Most of the time when a fermentation is apparently stuck, it is the result of a wort that is high in unfermentable carbohydrates. If you don’t want a beer with such a high finishing gravity, you will have to look at all the variables you can change—the next time.
Sometimes a lack of oxygen or nutrients will result in a stuck, or more likely very slow, fermentation. Starting a new culture of yeast, waiting until it reaches high kraeusen (that is the phase when all the oxygen and nutrients have been taken up, the yeast is energized and the wort is frothy with foam), then pitching it into the stuck fermentation will help, if anything can.
Be careful of foam-over, which occurs if the fermentation is cool and there is quite a bit of dissolved carbon dioxide in the stuck beer. Adding sugar or new fermenting beer with agitation may cause a sudden and messy release of carbon dioxide. Be prepared and cautious.
Some homebrewers will create conditions whereby complete fermentation will occur within 24 hours. It’s possible, and you may be fooled into thinking that it prematurely stopped. A hydrometer reading will indicate the status of your brew.
A period of cold storage often referred to as lagering enhances the qualities of bottom-fermented lager beers. Cold storage of ales is not as critical a factor in flavor development.
Lagering and Cold Storage
A period of two weeks to several months can improve the quality of lager beers. The temperatures at which beers should be lagered will vary with different styles of beer and with different strains of yeast, though in general the temperature should begin at about 45 degrees F (7 C) and be slowly reduced to 32 degrees F (0 C). During this time several things happen.
- 1) Remaining yeast cells continue to break down fermentable carbohydrates.
- 2) The evolution of carbon dioxide purges (scrubs out) volatile undesirable flavor compounds such as hydrogen sulfide (H2S) and dimethyl sulfide (DMS).
- 3) Yeast activity can reduce diacetyl levels.
- 4) Esterification (i.e., more esters are produced) occurs by the combination of organic acids and alcohols. Esterification during lagering is activated by enzymes produced by yeast. (Remember, “it depends”—different strains of yeast will produce different enzymes in various amounts.)
- 5) Oxygen-reduction reactions occur, affecting the flavor of the beer.
- 6) Yeast continues to settle out, clarifying the beer.
- 7) Chill haze is formed at very low temperatures. When beer is lagered for long periods, much of the haze will precipitate and sediment out. Commercial brewers have 10- to 50-foot-high tanks of cold beer for haze to fall out of. They can’t wait that long, so many of these brewers filter. A 5-gallon fermenter of homebrew at near freezing temperatures can drop out its chill haze in a reaonable time, resulting in clear beer.
An alternative to cold storage for long periods of time would be to bubble carbon dioxide gas through “green” beer for a period. The bubbling will purge many of the green flavors from the beer by scrubbing out the undesirable volatiles quickly. The disadvantage here is not being able to naturally minimize chill haze by allowing adequate time for precipitation and sedimentation.
Ales and Settling Time
Cold lagering is not as necessary with ales. The warm-temperature fermentation will accelerate the evolution and dissipation of undesirable volatiles from the beer. Chill haze is not as serious a consideration if ales are served at their traditional temperatures (about 55 to 60 degrees F [13–16 C]).
Traditional English ale-brewing practices accelerate yeast sedimentation with the addition of isinglass finings. The beer is often served at its best within two weeks from brew day.