In the high country of the Andes Mountains in Peru, local tribes continue the centuries-old tradition of brewing beer with available ingredients such as corn. The corn is not malted, but is chewed by the women of the village. Enzymes in the mouth are diastatic and break down carbohydrates to fermentable sugar. The “spittoon” is filled with “mouth mash” and allowed to ferment with indigenous yeast. The yeast is, you might say, very uniquely cultured. I have been told by a very dedicated researcher of indigenous old-world American fermentations that the special yeast culture is derived from the feces of unweaned infants. Tests of the culture have indicated that it was nearly a pure culture of Saccharomyces cerevisiae, or brewer’s ale yeast. A bit startling? Relax. Don’t worry. Have a, ahh, umm, er…homepoo…er…homebrew.
Fermentation opportunities for the American homebrewer are limitless. Most choose to ferment in a more traditional manner with more traditional yeast cultures. Malt (and other fermentables), hops and water are all crucial to the character of a beer’s profile. Yeast is an equal partner with its own unique influences on the qualities of beer. With the accessibility of dozens of yeast strains, the homebrewer can develop even greater flexibility in brewing.
Information, guidelines, techniques and data are extremely helpful in developing one’s brewing skills. The role that experience and experimentation have in helping develop the art of using and choosing yeast is absolutely essential. As with most homebrewing, the information and products available are of high enough quality that it is very unusual for an experiment or new endeavor to yield a brew that is anything less than drinkable.
Technically speaking, all single-celled fungi yeasts are classified by genus, species and strain. The primary genus used by brewers is Saccharomyces, though other genera are used for some specialty beers. Of the Saccharomyces genus, there are two species principally used. They are cerevisiae and uvarum (sometimes called carlsbergensis). Saccharomyces cerevisiae is commonly known as ale yeast. Saccharomyces uvarum is lager yeast.
The strain of brewer’s yeast used is perhaps a most critical consideration for the practical homebrewer. Strains of yeast are designated by any manner of names or numbers and are distinguished from one another by variations in their behavior. Different strains of yeast, whether they are ale yeast or lager yeast, will behave differently in some ways and similarly in others. The behavior of yeasts will affect wort attenuation, ester production, phenol production, fusel alcohols, diacetyl (butterscotch character) levels, flocculation of yeast, alcohol production and mutability. Different strains of yeast are more or less susceptible to the influence of temperature, alcohol levels, mineral content, nutritional balance, atmospheric and osmotic pressure and other factors.
It is quite obvious that choosing your yeast by strain is crucial to the final character of the beer you wish to produce.
Yeasts are available for use by the homebrewer in dried granulated form and various liquid forms.
Dried Yeast
By a long shot, dried packaged yeasts are the simplest to use. When a source of quality dried yeast is found, the brewer is doing well to consistently use it. A quality yeast is one that the brewer is pleased with, giving desired results. Unfortunately only ale yeasts are dependably available in dried form. If the package claims that it is a lager yeast, its behavior will be similar to ale yeasts, and for all intents and purposes, it is an ale yeast.
A mother of all beers! Two Saccharomyces cerevisiae yeast cells imaged by a scanning electron microscope depict the top cell with bud scars. The center scar is about 2 microns in diameter. It would take a string of over 5,000 yeast cells to equal an inch (2.54 cm.).
It is not uncommon for dried yeasts to be contaminated with bacteria and/or wild yeasts. The contaminations usually manifest themselves as undesirable bananalike (esters) and phenolic (characteristic of plastic Band-Aids or clovelike) flavors and aromas. They also produce long, drawn-out fermentations (wild yeasts ferment carbohydrates usually not fermentable by pure strains of common beer yeasts) and sometimes a slight film on the surface of the finished beer (wild yeast or bacteria).
Good beer can be made with dried yeast, but this takes an effort and knowledge of the characters that are the result of poor yeast.
For the hobbyist, dried yeast usually comes in 3.5-, 7- or sometimes 14-gram packages. For a 5-gallon (19 l.) batch, the addition of 7 to 14 grams of active dried yeast is recommended. Prior to pitching, rehydrate in ½ to 1 cup (118 to 237 ml.) sterile water at about 100 degrees F (38 degrees C) for 15 minutes to activate viable yeast cells. The rehydrated slurry can be added directly to the wort at temperatures generally below 75 degrees F (24 C).
Not your beer—you hope! As seen through a laboratory microscope one yeast cell is about 5 to 7 microns in diameter. Saccharomyces cerevisiae yeast cells are depicted here among rod-shaped Lactobacillus bacteria; bacteria you do not want in your beer (Lambic styles excepted).
When the yeast is introduced to the wort, it is crucial to oxygenate the wort to provide the yeast sufficient oxygen for its initial metabolic phases. During this lag phase yeast cells take up the oxygen and nutrients required for later growth and fermentation. When using dried yeast the lag phase is not as crucial because the dried yeast has gone through the lag phase during culturing and so already possesses stores of oxygen and nutrients when it is packaged. This is one reason why dried yeast seems to “take off” within an hour or two when introduced to fresh wort. It jumps right into the growth and fermentation stages much more quickly.
Wet or Liquid Yeast
Using liquid cultures does not in any manner assure or increase the odds that the culture is clean and not contaminated. Actually, liquid yeasts are more susceptible to contamination than dried yeasts once they are handled by the brewer. Care must always be a top priority when using liquid cultures to help assure a maximum degree of purity. Yes, you still may abide by the Golden Rule of Homebrewing, but an elevated degree of respect for the process of handling must be considered—while you are relaxing, not worrying and having a homebrew. Remember, worrying can influence the flavor of beer more than anything else. It can influence the quality of yeast as well.
There are a few essential factors that must be considered when propagating or using liquid yeasts. They require a proper wort environment in order to successfully grow, thrive and ferment. They need proper nutrition. This should never be a problem if the propagating medium or wort is all malt. If using over 30 percent adjuncts such as refined sugar, corn, rice or other unmalted adjuncts, you will need to determine whether the nutrition requirements have been met. An equally important consideration is the requirement of oxygen. Whenever propagating yeast in any amount of sterile wort, it must be adequately aerated. Aeration by vigorous agitation in a sealed sterile container is often adequate. Temperature of propagation is another consideration that influences timely yeast growth and activity. You may find yourself in a situation propagating yeast at slightly higher temperatures than you’d normally expect to have during beer fermentation. This is okay when there is no alternative, but it may influence the behavior of the yeast.
Liquid yeasts can be derived from three principal methods or procedures: from fermentation sediment, from agar slants or from dormant yeast suspended or sedimented in small amounts of beer or fermented wort.
Active yeast can be collected from the sediment from either the primary or secondary fermenter and added directly to fresh wort. Theoretically the best part of the yeast sediment to use is the middle layer of lighter, straw-colored yeast. This sits above trub and other organic material such as hop particulates. It is the most viable.
Four to 8 fluid ounces (125 to 250 ml.) of yeast slurry should be used for pitching into 5 gallons (19 l.) of wort, the upper limit being optimal. The practical homebrewer more often than not has less than ideal circumstances to work with. If the brewer is assured that the finished fermentation is not contaminated, excellent results are usually obtained with a mixed slurry of yeast sediment poured out of a fermenter. Care should be taken to sanitize the mouth of the carboy by swabbing with disinfecting alcohol (preferably grain alcohol or 150-proof cheap vodka) and flaming with a butane cigarette lighter.
Another source of yeast sediment is your friendly microbrewery or pub brewery. While most do not appreciate being bothered at the spur of the moment, many will accommodate the needs of local homebrewers if a regular schedule can be developed and there is one homebrewer contact or homebrew shop contact that will deal with the exchange. Use your imagination as a homebrewer when you reciprocate the favor, but keep in mind a professional brewer is not lacking for free beer.
If a slurry is taken and contained in a sterile jar, it can be kept relatively fresh and active for two to five days in the refrigerator. Don’t seal the container. Fermentation can continue creating gas and dangerous pressure in a sealed container. The yeast’s storability will depend on the strain.
CULTURING DORMANT YEAST IN SUSPENSION
A very practical means for culturing and storing yeast is the method outlined in Beer Recipes of The New Complete Joy of Home Brewing. It is a simple method requiring very little specialized equipment and is accessible to virtually all homebrewers. It essentially cultures active yeast in mini fermenters: beer bottles affixed with a fermentation lock. The yeast is cultured with real wort and is stored as a sediment under fermented wort, conditions most natural to beer yeast. Highly bittered wort is usually made in advance and stored in sterile jars or bottles. The highly bittered wort is an inhibitor of bacterial activity.
Using this method, yeast may be cultured from any source. A yeast bank can be maintained easily. Yeast will remain viable for months at refrigerated temperatures. (I have had experience with a yeast that survived one and a half years “under beer” and was recultured to brew an excellent light lager.) These cultures more quickly grow to suitable amounts for pitching 5 gallons of wort than slants do.
Expose yourself to culture. Culturing yeast in sterile-prepared wort is quite a simple process, requiring only a few precautions. Swabbing pouring surfaces with grain alcohol-soaked cotton swabs helps reduce risk of contamination. Flaming surfaces with butane lighter further minimizes contamination.
Commercial cultures are available to homebrewers as yeast suspended in sterile compartmentalized foil packages. These yeast cultures need to be cultured up to amounts sufficient for pitching into your volume of wort. Instructions are included with the packaging.
Practical homebrewers often find themselves pitching a less than ideal quantity of slurry (1 cup [237 ml.]) for 5 gallons (19 1.). Don’t worry; it will work. You can enhance the results and accelerate the growth of yeast in your pitched wort by holding the temperature of your wort at about 70 to 75 degrees F (21 to 24 C) until yeast activity is observed (usually within 24 hours), then cooling to desired temperatures. The trade-off here is that you may be encouraging some degree of undesirable fruitiness at these warmer temperatures for lager yeasts, but this is reasonable considering the more deleterious effects of extended lag times, which allow bacteria populations to gain a foothold in the wort before the yeast cells sufficiently out-number the bacteria and inhibit them by increasing the acidity of the wort.
For homebrewers able to culture 1 cup (250 ml.) of slurry, yeast can be pitched into more appropriately cooled worts (50 to 55 degrees F [9 to 13 degrees C] for lager) with normal lag times and growth phases.
A sampling of a few types of liquid yeasts available to homebrew shops throughout the United States, Canada and Australia
Ale Yeasts
German ale (Alt-type) yeast
American ale yeast (often referred to as Chico or Sierra Nevada ale yeast)
Irish ale (Guinness Stout-type yeast)
British ale (from Whitbread brewery)
European ale
London ale
Bavarian wheat ale (50% Saccharomyces delbrueckii, 50% Saccharomyces cerevisiae)
Belgian ale
Canadian ale
Australian ale
Lager Yeasts
Pilsen
Danish
Bavarian
Munich
American
Bohemian (Czechoslovakian origin)
Though it requires more equipment, training and practice, culturing yeast from slants is the preferred method of maintaining purity when storing yeast over long periods of time. Essentially, slants are test tubes with a small amount of gelatinized wort in them. Before the gelatinlike substance cools and solidifies, the test tube is tilted to allow the nutrient-rich wort to solidify with a greater surface area. The surface of the culturing medium is slanted relative to the tube. All procedures must be done under extremely sanitary conditions, using pressure cookers and high heat to sanitize everything that comes into contact with active surfaces.
Amounts of yeast invisible to the naked eye can be carefully transferred to the surface of these slants, usually with a sterilized wire that has been in contact with the yeast source. Within days the yeast will multiply under appropriate conditions and grow on the surface of the wort-gelatin, where they will remain dormant or slowly active for many months if kept refrigerated.
There are many excellent resources that explain in detail the process of making slants and similar petri dish mediums for the culturing, purifying and storage of yeast. They are listed at the end of this section.
If you are using liquid yeasts from various sources, the opportunity will inevitably present itself when another brewer will give you a slant culture of a yeast that you’d love to try. The process of culturing from a slant is a lot simpler than making and maintaining slants. Relax. Don’t worry, and have a homebrew AFTER you’ve transferred the yeast from the slant to the initial culture.
Here’s how. You will need a test tube, sterile cotton (from the supermarket), an inoculating wire, butane torch and material you already have for propagating liquid yeast. Sanitize the test tube by boiling in water for 10 minutes. Cool and add about 3 to 5 milliliters (about 0.1 fl. oz.) of sterile, aerated, clear and cooled wort, preferably at a specific gravity between 1.025 and 1.030 (6 and 7.5 B). Temporarily cover with a sanitized piece of aluminum foil and set aside.
Working in a dust-free, draft-free (that’s not draft beer) environment, heat the wire to red-hot, burning off microorganisms. Cool the wire by touching the tip to the surface of slant media containing no yeast or dip into a small cap full of alcohol. (Note: WARNING! Alcohol is very flammable. Do not immerse red-hot wire into a large container of alcohol.) Sanitize the mouth of the test tube slant and test tube of wort by swabbing with an alcohol-soaked Q-tip. If you are using glass test tubes, you can briefly flame the opening with the butane torch. Hold your breath (seriously), open the slant and use the sterilized wire loop to scrape off a bit of yeast from the surface of the slant. Immediately cap the slant, then open the wort-containing test tube and immerse the wire into the wort. Jiggle, remove the wire and temporarily cover the tube with foil. Resume breathing and carefully replace the foil with a wad of sterile cotton. Place the test tube in an environment that is suitable for propagating yeast, upright at 60 to 70 degrees F (16 to 21 degrees C).
Within 48 hours you should notice that the clear wort has a light sediment of yeast. Using similar procedures you will propagate the yeast by doubling the volume of wort in successive steps until you have a pint of fermenting wort, at which time you may consider pitching the sediment or the high kraeusen activity into your awaiting freshly brewed wort.
Step it up. Torched and sterile wire loop is used to scrape yeast cells grown on agar slant. These few yeast cells are introduced to a very small amount of sterile wort (small tube, left foreground) and allowed to ferment for two to four days. Doubling the amount, more sterile wort is introduced to feed the yeast to continue fermentation. Beaker (right) of fermenting wort represents third step of “culturing up” yeast.
When is the appropriate time to pitch the propagation? You may do so at high kraeusen (when the yeast activity has created a foamy surface “kraeusen”) or within 24 hours after the yeast has sedimented. Each method has its advantages and works well. The high kraeusen method seems to result in much quicker activity in the freshly pitched wort. The disadvantage is that you are pitching, along with the active yeast, 1 pint to 1 quart (about 500 to 1000 ml.) of fermenting wort that may not match the carefully designed character of your freshly brewed wort.
For those who use highly bittered worts to propagate their yeast, pitching the sediment is preferred. The relatively clear fermented wort is poured off and only the yeast slurry and a small amount of propagating wort are introduced to the freshly brewed wort.
A note on sterile cotton: Handled with care, a wad of sterile cotton is an effective barrier to contaminating microorganisms. It is useful when actively propagating yeast. However, when storing yeast cultures, cotton is obviously not an effective barrier to evaporation or the oxidizing effects of air. Air locks should be utilized for longterm storage. A small piece of sterile cotton can be placed into the lower end of the lock to help assure that airborne contamination does not enter the stored culture if the water in the air lock inadvertently evaporates.
OTHER REASONS TO PROPAGATE YEAST
Introducing propagated yeast for primary fermentation is but one reason to propagate yeast.
Kraeusening is the process of introducing a measured amount of freshly fermenting wort into finished beer in order to carbonate or condition it. True kraeusening methods rely on an introduction of wort that has just reached the kraeusen stage of metabolism/fermentation. This is usually just after the respiration cycle, when most of the fermentable carbohydrates have not yet been fermented. The yeast has passed through the lag phase, and uptake of energy-providing nutrients and oxygen is complete. At this point the yeast can be introduced into oxygen-free finished beer and be expected to complete the fermentation of the added wort.
Other, simpler methods of kraeusening work but are less than optimal. The addition of a measured amount of yeastless wort or malt extract at packaging time (bottling time) will provide fermentables, but a deficiency of oxygen will result in slow refermentation and long carbonation periods.
If corn sugar is added as a primer, the yeast cells are not sluggish, because the oxygen requirement for metabolizing corn sugar is minimal.
When homebrewers lager their beers for extended times (one to three months), the resulting beer is extremely clear. The reduced yeast count is a disadvantage if bottle-conditioning methods will be used to carbonate the beer. Culturing up a small amount of fresh yeast and adding it to the finished beer just prior to bottling will help carbonate the beer in a more reasonable amount of time.
OUT WITH ONE AND IN WITH ANOTHER
Classic Bavarian-style wheat beer is a great example of a beer brewed with two different yeasts, one of which is introduced at bottling time. Typically the fresh original wort is fermented with a unique ale yeast that does not sediment well. When the fermentation is complete, Bavarian brewmasters and American homebrewers actually filter out the ale yeast, providing clear beer for bottling. Bavarian wheat beer is traditionally bottle-conditioned, and this is why well-flocculating/sedimenting lager yeast is propagated and added to the finished, filtered beer as a kraeusen.
There is quite a bit of printed information on the qualities that various yeast strains contribute to the character of beer. They are at best marginally helpful. It is difficult to generalize, because a particular strain of yeast will behave differently and produce different characters when used with individual brewing systems and different types of wort. For example, if a description of a yeast strain claims that it is well-attenuating, it may be—compared to other yeasts—but it most certainly will not be well-attenuating if proper nutrition hasn’t been provided or a high-temperature mash has been used. Yeast can also differ from its stated description if mineral content varies from one wort to another.
You’re a homebrewer. Take your homebrew in hand. Appreciate and note the variations of each brew. The best behavioral descriptions of yeasts are the ones you have made after using them in your brewery.
After all that, I can say that in general the Irish ale and London ale liquid cultures tend to work best at high fermentation temperatures such as those experienced in warmer climates. Both produce a lesser amount of the esters and phenolic character that usually typify beer fermented at high temperatures. (So can’t I be a bit contrary now and then?)
I’ve already primed you with the legendary account of high-altitude Peruvian diaper beer. Here are a few more interesting anomalies worth noting.
Yeast of the genus Brettanomyces is utilized in the production of certain beer styles, especially in Belgium. Brettanomyces bruxellensis and Brettanomyces lambicus are two yeasts that help produce the very esteryfruity character of Belgian lambics. Alone they do not produce sourness; acetic acid can be produced, but it is very volatile and is not present in any great amounts in the finished beer. These cultures are available to homebrewers through various homebrew computer networks and homebrew club yeast banks. When stored on slants, they must be cultured on wort that contains about 0.5 percent calcium carbonate. The yeasts produce acidity while in storage that will eventually kill them when excessive levels are reached. The calcium carbonate neutralizes the acids.
Homebrewers can obtain very good results with these yeasts if a propagated culture is introduced during or after primary fermentation with normal ale or lager yeast. The Brettanomyces will ferment carbohydrates not normally fermentable by Saccharomyces cerevisiae or uvarum yeasts. They are slow fermenters, but produce a great deal of perceptible esters typifying their character.
Not all yeasts reproduce by budding. There is one unusual yeast called Schizosaccharomyces pombe (S. pombe for short). It is the only yeast in the world that reproduces by fission, rather than budding. This means the yeast cell reproduces by dividing its body into two or more parts, each growing into its own entity. All other yeasts reproduce by producing lots of buds that grow onto themselves.
The sample that I was given was cultured from African millet beer brewed in the bush. It produced a beer from an all-malt recipe that resembled the estery character of beers brewed by some Belgian monasteries. Further discussion with brewing colleagues revealed that this yeast is dry-cultured for some of the wine industry. Evidently it has the ability to make passable wine from less than desirable grapes. The yeast tends to deteriorate (autolyze) rather quickly. If beer or wine made from this yeast is not racked off the primary in a short time, the autolyzed yeast imparts certain characters to the beer. I refrain from saying these characters are undesirable, because my experiment was not racked off and produced a product that resembled certain kinds of Belgian ales that are desirable to many.
An experiment worth trying would be to brew a beer similar in nature to the beers first brewed by the Egyptians and Sumerians 5,000 years ago. They used barley or wheat along with available yeasts. There are heirloom wheats currently available at some specialty natural food stores. They are called spelt, kamut or dinkel and are ancestors of our hybridized modern wheat. Primitive malting and mashing procedures would produce a wort that may be very similar to the worts of 5,000 years past. This brew would not be authentic without heirloom yeast. Even this can be acquired. There is a company called World Sourdoughs from Antiquity (P.O. Box 1440, Cascade, Idaho 83611 U.S.A.), specializing in culturing and packaging sourdough yeast cultures from around the world. These yeast cultures are sure to include unique strains of Lactobacillus bacteria and wild yeast, but this is likely what ancient brewers used. One of the nine varieties offered is Egyptian Red Sea Culture, said to be from one of the oldest ethnic bakeries in Egypt. It was found in the village of Hurghada on the shore of the Red Sea. The bread was actually placed on the village street to rise. It is one of the fastest cultures to rise and has a mild sourdough flavor. Another variety, Egyptian Giza culture, is said to be found in a bakery in the shadow of the Sphinx in the town of Giza. The bakery dates back to antiquity.
Brewing a batch? Use your imagination and drink while still fermenting, as likely did the ancients. The art of brewing will certainly be more meaningful, and if you stand in the sunshine with your second glass of homebrew and you cast a shadow of a sphinx…
I have sometimes noticed with curiosity a dusting of yeast adhering to one or perhaps all sides of a bottle of bottle-conditioned beer. It was a total mystery to me until I came across a summary of some research Belgian brewers had undertaken to unravel this aberrant behavior. They found that yeast is electrically charged the same as glass and is usually repelled. Sometimes if the yeast is deficient in certain nutrients, it will take on a different charge and become attracted to the sides of the bottle. Only the Belgians with their admirably wonderful world of unusual beers would undertake such research. A recent study proposes research dealing with the influence of transport conditions on adhesion of yeast to the bottom of the bottle. They say, “Further studies of these aspects under zero gravity conditions are planned for the next Eurospace mission [1994].” Furthermore they conclude that these considerations of transport conditions “will be required to solve this important national problem.” Right on! Perhaps homebrewers might consider these questions as well. I believe they should.
zymurgy magazine 1989, volume 12, number 4, special “Yeast and Beer” issue. Table of contents includes “Homebrew Starter Cultures,” “Yeast Biology and Beer Fermentation,” “Wild Yeast,” “Yeast Nutrients in Brewing,” “Commercial Production of Dried Yeast,” “Yeast Stock Maintenance and Starter Culture Production,” “A Sterile Transfer Technique for Pure Culturing,” “Collecting Yeast While Traveling,” “Collecting and Reusing Live Brewer’s Yeast,” “Isolation and Culture of Yeast from Bottle-Conditioned Beers,” “Running a Yeast Test,” “Analysis and Evaluation of Commercial Brewer’s Yeast,” “Of Yeasts and Beer Styles,” “Fleischmann’s [aka Budweiser] Yeast,” “Lambic: A Unique Combination of Yeasts and Bacteria,” “The Hybrid Styles [of Beer]: Some Notes on Their Fermentation and Formulation.”
Yeast Technology, by Gerald Reed and Tilak W. Nagodawithana, Van Nostrand Reinhold, 1991. Up-to-date developments in yeast genetics, analytical techniques. Sections on flocculation, nutrition, fermentation, wild yeast, killer yeast and more.
Yeast Culturing for the Homebrewer, by Roger Leistad, G. W. Kent, 1983. How to culture and grow yeast at home in an easy-to-follow format.