One of the biggest advantages commercial brewers have over homebrewers is an ample and ready supply of yeast (Saccharomyces cerevisiae). They routinely “harvest” yeast from a recent batch and pitch it onto a new batch of beer. It’s common practice to do this multiple times and then reculture the yeast from a pure stock in order to minimize the possibility of contamination and mutations that can cause the properties of a yeast strain to “drift” and change the character of the beer.
As a homebrewer, the easiest way to reuse yeast is to time your brewing schedule so that you are brewing a new batch at about the same time you are racking the beer from a previous batch. This can be either the transfer to a secondary fermenter or for bottling or kegging. At that point, you can harvest the yeast and repitch it into the fresh wort. Furthermore, yeast sediment can be stored under beer or distilled water and refrigerated, to be revived anywhere from a day to a year later, depending on the storage technique and the health of the yeast itself. With reasonable sanitation, these methods will allow reuse of yeast at least several times before it needs to be discarded.
If you can’t reuse your yeast in a timely fashion, there are a number of ways to store it, then grow it up to pitchable amounts later.
It’s seldom worth the effort to reuse dry yeast, which is relatively inexpensive and convenient and the packages can be stored in the refrigerator for a very long time. Commercially available “liquid” yeasts have a shorter storage life, but are still relatively cheap. Conversely, if you have cultured yeast from a bottle of your favorite beer or otherwise obtained a yeast strain that is not commercially available, knowing how to store and propagate this yeast can be very valuable.
Long-term maintenance of your stock of yeast strains demands more stringent quality control and greater involvement in the process. If you want to continue to reuse yeast over an extended period and through repeated pitchings, you need to become more scientific about it.
Current procedures for yeast culturing are adapted from the biomedical and microbiology fields, which ironically have their origins in brewing science. (Early microbiological pioneers Hansen and Pasteur studied beer and wine.)
When using these techniques, sanitation is more critical than with ordinary brewing. It’s important to have a clean space in which to work, one that is relatively free of airborne contaminants. Commercial labs employ laminar flow hoods and partial vacuums. This is not necessary for the homebrewer — but, in general, avoid areas such as kitchens and basements that may have a high level of bacteria or humidity. Close nearby windows, especially during warm weather.
A very clean bench or table surface washed and rinsed with sanitizer before being allowed to dry is desirable. A flame source for sterilizing is also a good idea. This can be a small alcohol lamp, or alternately a butane lighter for a gas grill or fireplace. You can also use small propane bottles with fan-style burners. A spray bottle of alcohol or sanitizer is handy for quick tasks. Obviously, keep any alcohol sanitizer away from the flame. Some people wear surgical gloves, which may be overly compulsive, but at least wash your hands well with an antibacterial soap.
When using yeast from long-term storage, the yeast population is relatively small to begin with and increases many-fold as it grows and multiplies. Any other living microorganisms contaminating your culture — which can include wild yeast, molds and various bacteria — will multiply along with your yeast, and sometimes more quickly. It’s not unusual to pitch a total yeast population into your wort that is hundreds or thousands of times greater than that of the initial stock. You certainly want to minimize the presence of any organisms other than pure brewing yeast.
Some of the required materials you likely already have or can improvise, but in other cases you will need laboratory equipment and supplies. (See the “materials” sidebar for a list of useful equipment.) If you’re lucky, you may have contacts at a university, medical or biotech lab. If not, there are several scientific suppliers that continue to sell to individuals; among them are Fisher Scientific and Cynmar. It’s a little more difficult since September 11, 2001, though, and you may be asked to explain the purpose of your order. Also check the yellow pages and Internet search engines for “laboratory equipment and supplies.” Another source for certain items is your local full-service pharmacy.
A comment is in order about agar, one of the supplies you will need. This is available from scientific supply houses, but also at Asian grocery stores. It comes in small sticks or sheets. An alternative is unflavored gelatin, but gelatin begins to melt at a temperature of about 78 °F (25 °C), while agar will not do so below 122 °F (50 °C).
Going to the source
The first step in yeast culturing is to start with a relatively pure source of the yeast itself. For most homebrewers this is a vial, tube or smack pack of liquid yeast, but it may also be the yeast sediment from a bottle-conditioned beer or a container of yeast from your local brewpub. The objective is to “borrow” a small amount of this yeast for growth, storage and later use. Whether you are making a starter (recommended for larger batches and moderate to high gravity beers) a couple of days before a brewing session, or pitching the yeast directly from the package into your chilled wort, this is also the time to make a yeast culture. Save a small amount of the yeast and do the culturing very soon after pitching the rest into the starter or your batch of beer.
At this point you have two options. You can culture the yeast on agar plates for refrigerated storage for a few months, or prepare the yeast for freezing and store it for a year or longer. Plates are also preferred if your yeast source is sediment from a previous batch or a bottle-conditioned beer, commercial or homebrew. This will allow you later to isolate, select and propagate from a single yeast colony, virtually ensuring that you have an uncontaminated form of the strain.
For culturing on plates, you will have to prepare a growth medium. Start by heating 1 cup (a little less than 250 mL) of tap water in a Pyrex flask or saucepan on the burner. Dissolve one-quarter cup (20 g) of dried malt extract into the hot water and bring this wort to a boil for about 15 minutes (be careful about boilovers). Then turn down the heat and stir in one-half teaspoon of agar (or unflavored gelatin powder) until it is completely dissolved. Again bring to a boil, watching carefully so that it doesn’t boil over, for another 15 minutes. Remove the flask from the burner and allow the flask or pan to cool in the air rather than in a cold water bath. The mixture will thicken — but not solidify — as it cools below 122 °F (50 °C).
Sterilize from three to six Petri dishes, vials or clean baby food jars and lids by steaming them in a pressure cooker for 10-12 minutes. As a less sanitized alternative, they can be immersed in boiling water for 30 minutes. Sterile, plastic Petri dishes are also available, although obviously these are single use items. When the Petri dishes (or other containers) are cool enough to touch, sterilize the mouth of the flask or lip of the saucepan with the flame source and pour the medium into each, filling it to about one-fourth of its capacity. Put the lids on the Petri dishes, or cover the container with plastic wrap, and let them cool a little longer, perhaps 30 minutes. Eventually the medium will solidify to the point where the color lightens somewhat and the plate can be tilted without running.
To save time, the covered plates can be prepared ahead of time and stored in sanitized plastic bags. (Sterile Petri dishes come in plastic sleeves.) It’s best to store the plates upside down. Otherwise, condensation may form on the lids and drip into the agar. You can store poured plates in a cool, dry place for up to several weeks. If the medium turns hard and brittle it has been stored for too long and dried out. To be useful, it should remain somewhat soft and pliable.
The next step in culturing is to inoculate the agar plate with yeast. Sterilize the inoculation loop by heating it in a flame until it glows red. Then, cool the loop by dunking it in a shallow dish of alcohol. As an alternative, you can wipe it with a paper towel or cotton ball moistened in sanitizer or alcohol. Take a deep breath and draw the loop through the yeast sediment, collecting some of it on the surface. (You don’t need — and in fact don’t want — a visible amount of yeast on the loop. Just touch the yeast lightly and the loop will have enough yeast on it.)
While holding the loop in one hand, remove the cover from one of the agar plates with the other hand. Quickly streak the plate by lightly drawing the loop across the agar surface of the plate. Quickly close the cover when you are done and once again turn the plate upside down. Resterilize the loop and repeat the process for however many plates you plan to streak. The purpose of inoculating multiple plates is to avoid problems with infection or failure of the yeast to grow on one or more of them. It also provides more than a single yeast source for later reculturing.
Keep the plates covered, upside down and in a somewhat warm (70–80 °F/21–27 °C) undisturbed location. Within several days, the yeast should multiply and grow. A milky layer will develop on the surface of the medium, and you may notice trails of small “dots,” which are individual yeast colonies. Contamination by molds, which can occur, will be obvious by the appearance of “fuzz” or “balls.” Discard any such plates.
You now have successfully cultured the yeast on agar plates. Seal the covers or lids of the plates with electrical tape (in labs, they use the shrink wrap Parafilm), label them with masking tape and a permanent marker, and store them in a sealed plastic bag in the refrigerator. They will survive for several months or a little longer.
Baby, it’s cold inside
The other method of serious yeast storage is in the freezer. Merely freezing the yeast in water, beer or wort will rupture the cells and kill them. However, if glycerin is added to the yeast in the proper proportion, it will inhibit the formation of ice crystals and minimize damage during freezing. When yeast is frozen in glycerin, a large amount of yeast is stored (relative to the amount present on the surface of a Petri dish). As a consequence, the potential for contamination is higher when yeast is stored this way. It is recommended to first prepare a plate from the frozen yeast if there have been more than a few repitchings since the last culture was performed.
Careful sanitation of the work area and all utensils, tools and materials for freezing is just as important as when preparing agar plates. The first step is to treat the yeast sediment. If you are using sediment from a previous batch or a bottle of commercial beer, it is a very good idea to wash the yeast. This is accomplished by stirring the yeast into boiled and cooled distilled water in a sterilized container, covering it with a sanitized lid or plastic wrap and letting the sediment settle before pouring off the liquid. In some cases it may be desirable to do this more than once. It is not necessary to wash the yeast from a new package.
Next prepare a 30% solution of glycerin and distilled water. Use a graduated cylinder or beaker to measure 250 mL (8.5 fl. oz.) of distilled water and 100 mL (3.5 fl. oz.) of glycerin. Stir until mixed well, then boil for about 10 minutes. Cover with sanitized plastic wrap and cool to room temperature. Pour the cooled glycerin/water solution into sterilized test tubes or small vials about one-third full. Just as when culturing on agar plates, it’s best to use several tubes or vials as insurance against contamination or non-survival during storage. Then carefully add the yeast slurry, again about one-third of the total volume of each tube or vial, using a sterilized pipette or eyedropper. Screw the sterilized caps or lids on tightly, shake well to distribute the yeast and mark each one with a masking tape label. Once prepared in this manner, the yeast is ready to be frozen and stored.
The problem with most home freezers is that they are frost-free. A heater periodically warms the refrigerant lines to melt frost on the freezer walls. This has a minimal effect on frozen foods, but will greatly shorten the storage life of yeast. There are two methods of preventing this from occurring. The first is to find a way to disable the defrost cycle, which requires some knowledge of refrigeration and electrical expertise. The other method is to place the test tubes or vials of yeast inside a small, covered Styrofoam cooler between frozen packs of “blue ice.” Store the cooler in the freezer. This will greatly minimize the temperature changes that occur during the defrost cycle and prevent damage to the frozen yeast. Set the freezer to its lowest temperature setting; most home freezers can reach temperatures of about -15 °F (-20 °C).
The frozen yeast can be stored in a home freezer for up to several years. This assumes there are no power outages that would allow it to thaw. In a laboratory freezer at -80 °C (-112 °F), frozen yeast has successfully been stored for decades.
Waking the dead
Of course, at some point in the future, you will want to revive the refrigerated or frozen yeast and use it again. For yeast that has been cultured and stored on agar plates, the procedure is to find a single colony of pure yeast and use it as the starting point, growing it up until you have a “pitchable” population for your brewing session. With frozen yeast, you may have a similar purpose, or you may wish to culture a plate from the frozen sample in order to ensure that it is pure.
If frozen, remove the test tube or vial from the freezer and first place it in the refrigerator. Allow three to five days for it to thaw. At that point, the thawed yeast is treated the same as a refrigerated agar plate. Remove from the refrigerator and keep at room temperature overnight.
Now the process is one of increasing the population by making yeast starters while maintaining good sanitation throughout. If you have experience with making a starter, you should be familiar with the instructions for preparing sterile wort. A frozen tube or vial can be stepped up first to about 70 mL (2 oz.) of wort, then to about 500 mL (one pint) and finally to as much as a gallon (3.8 L), if desired. Allow each starter to incubate for about 48 hours in a somewhat warm (70–80 °F/21–27 °C), undisturbed location between steps.
Reviving yeast from agar plates requires an additional step. In that case, start with a single colony on the surface of the plate. Select a round and relatively uniform “bump” or “dot” that is physically isolated from the others. Prepare a sterilized test tube with about 10 mL (0.33 fl. oz.) of sterile wort. Use the sterilized inoculation loop to gather the single yeast colony from the plate and immerse it in the starter wort, swirling it until the yeast is mixed well. Place the cap loosely on the tube, but do not seal it. Set it in a warm location for about 48 hours. You may see bubbles during that time, but the only definitive indication of activity will be yeast sediment at the bottom of the tube. This is used as the source for successive starters in the same manner as frozen yeast.
Yeast culturing is somewhat involved and may make you seem like a “lab rat.” But, it is also the key to a ready supply of yeast at lower cost and greater flexibility. If you are a conscientious, detail-oriented brewer, you can become an experienced and committed yeast rancher and successfully maintain your own relatively pure source of this most valuable of brewing ingredients.