Yeast Culturing Practices for Small-Scale Brewers
By Karl King
Republished from BrewingTechniques' May/June 1994.
High-quality yeast cultures and culturing kits are available, but how do you use them? Here's an overview of one successful method.
Five years ago, most small-scale brewers were using packets of dry yeast, and only a few brewers with professional microbiological training were able to culture yeasts for their own use. The introduction of high-quality commercial yeast strains in liquid and on gelled growth media (malt agar) changed the situation dramatically.
The number of strains available to small-scale brewers has grown remarkably in just a few years. As of this writing, over 30 unique brewing strains are available. Professional yeast banks have many more strains, but at present many discourage sales to the general public. Further, excellent tools for yeast culturing are now available in kit form, and they include easy-to-understand instruction manuals written for those without training in microbiological laboratory techniques.
High-quality yeast cultures make it possible to brew beers with lower levels of contamination from wild yeast and bacteria and with the characteristics that uniquely define specific beer styles. Consider the simple example of Dortmunder export lager. The grain bill and hopping schedule for this style are well known and unremarkable. A North German lager yeast, however, is needed to achieve the flavor characteristic of this style. Fermenting with a Munich lager strain, for example, will result in a taste profile more like that of Helles than of Dortmunder.
Maintaining your own yeast bank and culturing your own yeast also saves you money compared with having to buy packages each time, and it also facilitates the use of multiple yeasts in the brewing process. Active yeast can be reused if the time between batches is short, but yeast characteristics can degrade with continued use (diacetyl output will increase in lager yeasts, for example). Starting with a fresh culture avoids such problems.
The pioneering Wyeast (Hood River, Oregon) product line introduced many brewers to high-quality yeast cultures. The only significant criticism of the product is that one package supplies only a marginal amount of yeast for a 5-gal batch of beer. Pitching the contents of just one package can give an agonizingly long lag time. That problem has been addressed by kits with instructions for growing a 0.5-L starter from the contents of one package (1). Kits are not the only way to make a starter, but they make collecting the equipment a lot easier and include descriptions of the basic techniques.
Beyond making a one-step starter, you can also store yeast on slants and isolate single yeast colonies on plates (also called petri dishes, though strictly speaking they are petri dishes when empty and plates when they contain solidified wort). The mechanics of setting up slants and plates are well explained in commercial kits, so this article will not devote much space to those basics. But how are these slants and plates used?
Brewers that I've talked with know that yeast strains are available, and they know that the kits supply the tools, but they hesitate to use them because they lack an overall picture of yeast culturing and how it fits into the brewing process. This article presents an overview of how a small-scale brewer can apply the tools and the yeasts with confidence. I describe two example procedures to give practical form to the basic method. The method presented is not the only viable method, nor is it the most professional. Based on considerable experience with consistently excellent results, however, I am confident that any serious brewer can apply this method with success.
One key item generally not supplied with kits is a portable flame source; the flame is momentarily passed over the mouths of containers to ensure that surfaces are sterilized and is used to sterilize the inoculation loop. Cigarette lighters have too small a flame, and gas stoves are not portable. A propane torch from the hardware store works well.
Sanitation is a key requirement for success in yeast culturing, and it is easier than you might think. No heroic measures are necessary. Plan and practice what you are going to do so that sterile containers are open for as short a time as possible. Work on a nonporous surface, remove rings, watches, and so forth, and spray hands with rubbing alcohol. (DO NOT USE ALCOHOL NEAR AN OPEN FLAME; it is extremely flammable!) A small spray bottle for rubbing alcohol is very handy. Garden shops or hardware stores sell such bottles. In addition to the precautions described in the kit literature, a previous article in BrewingTechniques has much useful advice (2).
STEP-BY-STEP INSTRUCTIONSExample 1: A bottle-conditioned ale using yeast from a packet. To explain the general procedure, let's use a bottle-conditioned Belgian abbey beer as an example (recipe taken from reference 3). The conditioning yeast will be the same as the primary fermentation yeast. Using the same yeast for both is by no means necessary; extra complexity can be added to a beer if the strain used for bottle conditioning is different from the strain used for primary fermentation, especially if the bottling yeast has a nonneutral flavor.
Boil starter solution (see box) for 20 min, cover, and then cool before adding yeast. The small containers of wort are easily cooled by placing them in a cold water bath for a few minutes.
Step 1. The day before you plan to brew, pop a packet of liquid yeast (I use Wyeast #1214 for this recipe); it generally takes about 24 h to expand. On brewing day, prepare a 400-mL starter solution in a 1-L flask. It is important to leave a couple of inches of headspace at the top of the flask to accommodate gas transfer (carbon dioxide out, oxygen in). A typical Wyeast packet will supply 40-50 mL of liquid. Increasing the volume with 400 mL of starter, a 10:1 increase, is about as much of an increase as should be attempted. An increase of not more than 6:1 is the professional recommendation (4). Pitch the yeast mixture into the cooled 400-mL starter, fold over the top of the packet and set aside, upright, for use in step 2, then cap the flask with aluminum foil. Oxygen and agitation are important to maximize(see "Yeast Theory and it's Application to Brewing") yeast growth (5), and the foil cap allows oxygen to enter. Every few hours, hold the flask by the base and move it in a tight circular pattern to swirl the liquid gently. Store at constant room temperature (68 °F/20 °C). The solution will become hazy with growing yeast. Some yeast will settle to the bottom of the flask, and eventually bubbles of carbon dioxide will rise from the yeast. The yeast should be ready to pitch in 24 h (high kraeusen). Pitch into the primary fermentor and proceed as usual. The yeast should be up on the surface in 10-16 h.
Step 2. Some yeast is usually left behind in the packet. Cut off the top two-thirds of the packet and immediately use the dregs in the bottom to inoculate a plate. Seal the edges of the plate with tape or Parafilm (American National Can, Greenwich, Connecticut). Label the plate with the type of yeast and date. Put the plate in a zipper-type sandwich bag and invert so that the inoculated surface is face down. In 4-5 days, the yeast will have grown about as much as it can. Dots of yeast are usually colonies grown from single yeast cells. Healthy yeast will typically appear creamy white. Anything else signals gross contamination and the plate should be discarded. After the yeast has grown, store the plate in a refrigerator. Plates are not intended for long-term storage, but I have many that are good after six months of refrigeration.
Step 3. Two days before bottling,* open the plate and use the inoculation loop to transfer a loop or two of yeast to each of two starter tubes. Flame the loop before starting and between each transfer. Reseal the plate and store. Leave starter tubes at room temperature for 24 h. The caps should be one-quarter-turn loose to permit gas transfer.
Step 4. Make a 200-mL starter solution in a 500-mL flask. Pitch the yeast from the two starter tubes into the 200 mL of starter solution and cover with aluminum foil. In 24 h the yeast will be ready to pitch for bottle conditioning.
Step 5. If the yeast on the plate is not contaminated (if it looks healthy as described above), a colony can be transferred to a blank slant for long-term storage in a refrigerator. Tape the junction at the cap or store in a plastic bag. Yeast on a slant will remain viable for 6-15 months, depending on the strain. Label the slant with yeast type and inoculation date so that you know when it is about to expire.
Example 2: A Weissbier using yeast from a slant. Now let's use the example of growing yeast from a commercial slant to make a Hefeweizen. In my recipe I use Brewers Resource CL-920, German Wheat (Brewers Resource, Woodland Hills, California), for primary fermentation because it produces the phenolics appropriate to this style. For the conditioning yeast, I use a neutral ale strain, Brewer's Resource CL-10, American Microbrewery Ale #1. If you want a yeast that stays plastered to the bottom of the bottle, Brewers Resource strain CL-300, Belgian Ale #1, works well.
Step 6. Transfer a loop of yeast from the slant to a 10-15 mL starter tube. If this is the first time the source slant has been opened, immediately transfer a loop of yeast from that slant to a fresh slant; this step maximizes the possibility of propagating the yeast without contamination. Cap and label the starter tube as usual, wait 4-5 days, then store in the refrigerator.
Step 7. 24 h after step 6, make a 100-mL starter in a 250- or 500-mL flask. Pitch the yeast from the starter tube.
Step 8. Make a 400-mL starter in a 1-L flask and pitch the 100 mL of yeast, which should be at high kraeusen after 24 h.
Step 9. As an alternative to using a 100-mL starter, you could inoculate three or four starter tubes and use those to pitch into the 400-mL starter. This method works well when you are pulling yeast from a plate, which usually has plenty on its surface. Be aware that using three or four starter tubes also increases the probability of contaminating your pitching yeast by three or four times. Hold off on using this method until you are sure of your ability to manipulate yeast without introducing contamination.
Step 10. Each starter tube usually has a few drops of yeast left over after pitching. Those few drops can be used to inoculate a plate. Plating this yeast not only gives you another plate of the yeast, but is also a quality control check on your starters.
Step 11. If you find contamination in your starters, you can inoculate a commercially prepared plate directly from a slant. If the yeast grows properly and without visible contamination, it is almost surely a sign that your starters or techniques are faulty.
You may then culture the conditioning yeast from a slant, using steps 3 and 4, or 6, 10, 3 and 4.
HOMEMADE STARTER TUBES, PLATES, AND SLANTSIf you culture yeast frequently, you will probably want to prepare your own starter tubes. Starter tubes are fairly easy to prepare, but slants and plates are more challenging (6). Capped glass tubes for starters and slants are available from scientific supply houses. You may be able to reuse empties from prepared commercial starter tubes.
After boiling starter wort for 10 min, let it cool and decant the clear wort. Wort can be injected into tubes with a clean plastic syringe. Steam the tubes in a pot of boiling water for 20 min. Cool, and make sure the caps are on tight before storage. Some sediment in the sterilized, cooled tubes is normal. Place these blanks in a sandwich bag and refrigerate. The starter solutions should remain clear over time. Any haziness or turbidity throughout the solution is a sign of contamination.
The wort can also be thickened with agar for use with slants and plates (2-3% [w/w] is common, but the ratio can vary widely with brand). Wide-bore syringes are useful for injecting the more viscous wort-agar mix into tubes. Slants must be steamed and then cooled at an angle to generate the necessary surface configuration. Store the same as the starter tubes.
If you have never worked with these items before, it would be good to start with the commercially prepared versions so that you have fewer variables to contend with.
PRACTICE MAKES PERFECTIt may take a few tries to get comfortable with the culturing procedures, but once you do, you will be able to make full use of the yeast strains that are now available.
REFERENCES(1) J. Mellem and M. Raines, Growing Commercial Liquid Yeasts (Brewers Resource, Woodland Hills, California, 1992).
(2) M. Raines, "Methods of Sanitation and Sterilization," BrewingTechniques 1 (2), 30-33 (1993).
(3) P. Rajotte, Belgian Ale (Brewers Publications, Boulder, Colorado, 1992).
(5) G. Reed and H. Peppler, Yeast Technology (AVI Publishing Company, Westport, Connecticut, 1973), pp 271-272.
(6) P. Farnsworth, "Yeast Stock Maintenance and Starter Culture Production," Zymurgy 12 (4), 32-35 (1989).
*The fermentation yeast is usually in poor shape at this point because it has exhausted its energy and nutrient reserves; it will ferment slowly and impart off flavors. Freshly cultured yeast ferments easily, even in strong beers. Most beers will be nicely carbonated in one week. Freshly cultured yeast will also remain viable for months, helping to keep the beer fresh. return to step 3
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