Culture In Your Kitchen: Step-by-Step Guide to Growing Yeast
Homebrewers go through many stages in the hobby. A typical path is to buy a kit and make a beer with malt extract, sugar and dry yeast. My first few homebrews had more corn sugar than malt!
But since we are trying to craft the best beer possible, many go to the next stage: We buy more equipment and try different kinds of malt extract and yeast. We use specialty grains to add color and flavor, or even plunge into all-grain brewing. One direction some homebrewers take is yeast culturing.
What is yeast culturing? First we have to know the role of yeast in beer fermentation. Yeast ferments sweet wort into alcohol, carbon dioxide, flavor and aroma compounds. During beer fermentation, yeast undergoes a growth cycle in which they multiply by approximately five times the number of starting cells. Therefore, a beer fermentation can be thought of as one large yeast culture.
Brewers control fermentation, balancing good yeast health and minimal off-flavor production. Each strain of yeast will produce different flavor and aroma compounds. The amount and ratio of compounds determine an individual yeast strain’s signature. Natural yeast found in the wild (on plants, in the air) do not usually produce acceptable-tasting beer. It is generally phenolic, yeasty and dry. So the particular culture (called a strain) of yeast used is very important. Thousands of individual strains of yeast are stored in many yeast banks and breweries worldwide. White Labs has over 300 strains of yeast stored in a cryogenic freezer. Breweries that depend on yeast for their beers’ signature character heavily protect them. When growing yeast cells from small quantities, we call this yeast culturing.
Why culture yeast?
Homebrewing is an adventure, a quest for knowledge. The more we can learn about the hobby, the better we will understand the process. Also, yeast culturing is fun.
Do you need to do it? Not really — there are many sources of pure yeast cultures today, including my own company. But you might not be close to a source of yeast; you may brew larger quantities than economically feasible to purchase; you may want to use strains of yeast that are not commercially available; or you may just want to try it. Possible disadvantages of home yeast culturing are planning, learning the skills, and the possibility of contamination.
This article describes yeast culturing from an at-home point-of-view. It borrows methods used in the professional laboratory, but is limited by equipment differences and environment. (You can buy all of the equipment I mention in this article — such as test tubes, petri dishes and alcohol lamps — from most homebrew shops.)
Where Can I Culture?
You can culture yeast almost anywhere, if you take the necessary cleaning steps and are aware of potential contamination sources. If culturing in the kitchen, be aware that it’s one of the largest sources of microbial life in the house.
Microorganisms, such as bacteria and wild yeast, are everywhere, but can be controlled. Drafts, blowing fans and dirty countertops are all threats to pure yeast culturing. Work surfaces should be cleaned thoroughly, and drafts eliminated. Drafts create a wave for bacteria to ride throughout the house on.
Microorganisms are constantly descending in the air. By lighting a flame, such as an alcohol lamp, in the work area, you effectively put up a barrier that pushes them up and away from your culture area. (Be careful when working with alcohol lamps, as it’s always dangerous to work with an open flame.)
Alcohol is also used to clean and sanitize surfaces. A 70-percent isopropanol solution is commonly available and works very well. Just pour some alcohol on the counter and wipe up with a paper towel. Again, be careful when working with flammable alcohol.
KEEPING THINGS CLEAN
Sanitation is very important in the brewing process. A beer’s flavor is delicate, and can be thrown off by bacteria or wild yeast. In yeast culturing, sanitation is critical. Sterile conditions and sterile media are required to isolate and propagate yeast from small colony sizes.
There is an important difference between sterile and sanitized. To be sterile is to be free of all microbial life. The killing of all living things — sterilization — is achieved with a high pressure-temperature autoclave in the laboratory, or at home with a pressure cooker. To be sanitized is to be mostly free of microorganisms. Sanitation can be achieved by methods such as boiling, chemical treatment or pasteurization. In yeast culturing from single cells, if the starting material is not sterile, other organisms can be propagated along with the yeast.
In summary, brewing beer, starting with a large population of healthy yeast, requires only sanitation. The yeast will out-compete other organisms, and quickly create an inhospitable environment. But in yeast culturing, sterile conditions are required because of the small initial population of cells.
SLANTS AND PLATES
Slants and plates are commonly used in all yeast and bacteria culturing. A semi-solid media (we’ll just call it agar, even though agar is just one ingredient) is put into a test tube (called a slant) or a petri dish (a plate). Agar is derived from the cell wall material of red seaweeds (the algae Gelidium and Gracilari). Agar is a gelatin-like material that is liquefied at temperatures over 107° F and forms a gel under 99° F. Yeast will grow on the semi-solid agar surface, and provide a means of semi-long-term storage.
Yeast grow in globs or in single colonies (each colony contains millions of cells) as round creamy-white domes on the agar surface. Wild yeast can look similar to brewers’ yeast, but there can be clues to wildness. For example, if you want to collect yeast from a bottle-conditioned beer, it’s best to “plate” it first, to check for purity. Even when the results appear to be pure yeast, there can be “blisters” around the yeast colony, a sign of wild yeast formation. If any foreign growths occur, your plate is contaminated and should be discarded.
A slant is your “mother” culture, which should remain pure due to its infrequent usage. A blank slant is made by pouring liquefied agar into a test tube and cooling at an angle, creating a large surface area.
Once inoculated with yeast, the mother culture should be used to create a “working” culture. A plate serves as your working culture, which in turn provides the yeast for the “starter” culture that you pitch into a batch of beer. The plate serves two functions — it’s a working store of your yeast culture, and it offers a look at the purity of your yeast. Most contaminants grow and become a visible colony. This allows the average person without a microscope to identify possible contaminants with the naked eye.
Contaminants will look different on plates than normal brewers’ yeast. Molds are common contaminants and are easily spotted as they look similar to those that grow on bread, cheese and fruit. Bacteria are harder to identify. They may at first look like yeast, but are usually more translucent and often colored or slimy. Bacteria can appear clear, white or reddish-pink.
If you contaminate a plate, you can make a new one from your pure slant. But if you contaminate a slant you lose your mother culture. You may make backups or new slants from older ones by following the sterile technique of transfer. Yeast on slants last longer because it has a screw cap, and does not dry out. Yeast continues to grow on plants and slants, just very slowly, since they are kept cold.
If yeast grows, they will mutate. Mutations can occur on slants in as little as two months. At White Labs, our method of long-term storage is cryogenic freezing, which reduces the likelihood of mutation.
To prolong the life of your plates and slants, store them in the fridge. A plate will last several months, stored wrapped in parafilm with the agar side up. Slants are the best possible semi-long-term storage. They should be re-cultured every 4 to 6 months before they start to mutate and die. Note color changes, indicating cell death. Healthy yeast should be a creamy white color.
Sources of yeast
There are many primary sources of yeast for your plates and slants. Yeast is everywhere, on plants and in the air. Throughout history, it has been selectively cultivated by brewing and good brewing strains have been naturally isolated. Once pure culture techniques were developed in Europe during the late 19th century, yeast strains from all over the globe were banked in newly created “yeast banks.”
Yeast can be bought from these yeast banks today, but it is expensive for the average homebrewer. Typical costs are $100 per slant. The most affordable sources of homebrewing yeast are a vial of liquid yeast, a bottle-conditioned beer, a dry yeast pack or the sediment at the bottom of your carboy. Each source has its own advantages and disadvantages.
Yeast from your carboy, for example, is good because it is a plentiful source of yeast, and if you follow good, sanitary brewing procedures, it should be easy to purify. The disadvantage is that the yeast may be in poor condition following fermentation. Yeast from a bottle-conditioned beer is difficult to obtain, and in most cases you will also find wild yeast and bacteria. Separating that mix and selecting the appropriate brewing strain can be a challenge! But that’s what makes yeast culturing fun, and it’s especially rewarding if you can capture a rare yeast strain.
Once you select a source of yeast, making a mother culture is easy. The mother culture will be a slant. It is best to “plate” your source of yeast first (see step-by-step directions for “streaking a plate” below), inspect the colonies and replate if necessary. You want to obtain isolated colonies, free from contaminating bacteria, wild yeast and mold. You can streak from any yeast source onto plates: Simply dip an inoculation loop (see below) into a sample of yeast — the bottom of a bottle, yeast slurry in a jar — and streak the yeast onto a plate. Once you have obtained an isolated, healthy-looking colony, take a colony or two and spread on a slant. Grow the slant for two days as explained later in the article, and then grow up the colony to make a starter culture for brewing. Make sure your test-brew yeast comes from the new slant, so if the yeast works well, you know you already have it “banked.” If it does not ferment correctly, or has off-flavors, the slant can be discarded, and you start all over again.
INOCULATION LOOPS
Inoculation loops can be disposable or metal. Disposable loops usually come sterile, and are easy to work with. The problem is they are wasteful, since they can’t be reused. Metal inoculation loops need to be sanitized by flaming prior to any transfer. You should hear it sizzle, and always get the loop red-hot prior to cooling. Metal loops require cooling prior to transfers, which can be done by immersing the loop in sterile water (sterilized in a pressure cooker or boiled) or by touching the hot loop to an agar surface prior to picking up a colony.
STREAKING A PLATE
Streaking an agar plate is a quick and easy way to isolate yeast, to check for purity, and to re-culture yeast from aging plates or slants. A sterile inoculation loop is dipped into a sample of yeast and streaked over the agar surface in a pattern of decreasing cells. The last cells to rub off the loop are wide-enough apart so that they grow into isolated colonies.
To begin, clean an area and light the alcohol lamp. Place the plate near the flame, agar side up. If streaking from a slant, open slant and pass opening through flame. Lightly dip the inoculation loop into the yeast and remove a tiny bit. Re-flame and close slant. Pick up agar side of plate and turn over near flame. Run the loop back and forth several times in a small 1/4-inch section. Streak the loop through this section and re-streak a new section. Repeat this step about three times.
Grow the plate for two to three days at room temperature (70° F), agar side up. Dense yeast will grow in the initial area, getting more diluted in the later streaks. If isolated colonies are not obtained, a new plate can be re-streaked. If you don’t get individual cells, you’re transferring too many cells each streak. Take a smaller portion for re-streaking, or flame the loop and cool between each re-streak. After growth period wrap edges with parafilm and refrigerate.
Making a starter culture for brewing
Yeast has a creamy white color. Plates usually have many colonies growing on them, while slants usually have a slick of yeast. This is typical of a healthy sample.
Each dot on a plate is a colony or group of colonies of yeast, any of which can be selected to make a starter culture. A starter culture is the yeast that you’ll pitch into a batch of beer.
To make a starter culture, first make a sterile wort starter by boiling dried malt extract and water, adding it to a test tube and letting it cool. Then pick either a single colony (small white disc of yeast) from a plate or a loopfull from a slant. It’s better to select single colonies, grown from single cells, as they are the purest form of yeast identifiable.
Once you have the yeast on a sterile loop, transfer it the 10-mL wort starter, being careful to open containers for the minimal time required. Note: Just before you inoculate the wort starter, you must flame the opening of the test tube. Then insert the loop into the test tube and shake the tip of the loop in the liquid to transfer the yeast.
Flame cap and tube, then snug cap. Unscrew cap slightly to allow oxygen transfer.
To inoculate a 10-mL wort starter from a plate, set the plate upside down on the counter and loosen the cap on a test tube that contains 10 mL of sterile wort. Working close to the flame, quickly lift up the plate and with a sterile loop select a colony by scraping the surface of the yeast with the small end of the sterile loop. Replace the cover and open the test tube, flame the lip, dunk and shake the end of the loop holding the tiny amount of yeast into the liquid. Flame opening, screw on cap snug, shake, then unscrew slightly, permitting the transfer of gas for optimum growth.
Shake starter to aerate and to mix yeast into suspension. Leave upright in a warm place (70° to 80° F). Your 10-mL wort starter is now inoculated and will grow over a period of 24 to 48 hours. It won’t look like a normal fermentation, because very little action will be seen at the top. A white sediment will appear on the bottom, assuring you that growth has occurred. You should use this starter after approximately two days. It will keep if refrigerated, after growth has occurred, up to seven days. Warm it to room temperature before proceeding to the next step.
When you are ready to grow your starter, boil 400 mL of water with four tablespoons of dry malt extract and some dry yeast nutrient. Add mixture to your sanitized Erlenmeyer flask. If you use a one-liter Erlenmeyer flask, you can boil directly in the flask. If you use a 500-mL flask, it is preferable to boil separately, in order to avoid boilover. There are two acceptable methods of sanitizing your flask. You may make up a chlorine solution of 1 tsp. bleach, add to Erlenmeyer and fill to top with water. Allow flask to soak for 15 minutes, then rinse with hot water and cap top with aluminum foil. Another method of sanitation is to place the flask, with a foil cap tightly molded around lip of flask, in the oven at 350° F for two hours. The bake method is preferred and may be done days in advance. Once you have filled the Erlenmeyer with your hot boiled media, quickly place foil over the top, creating a cap. Allow to cool to room temperature, or cool to touch. You are now ready to add the 10-mL wort starter to the Erlenmeyer flask.
Tighten cap on 10-mL vial and shake to re-suspend yeast into solution. Unscrew cap slowly as there may now be pressure in the 10-mL vial, then flame opening while removing foil cap from flask. Flame opening of flask and dump in 10-mL vial, quickly replace foil and form tightly around opening. Shake Erlenmeyer flask to aerate and to mix yeast into solution. Allow this to sit at warm temperature (70° F) for one to two full days before use. Activity should be present within 12 to 24 hours. Again, activity will only be slight and will not produce the vigor of a five-gallon fermentation.
After 24 to 48 hours, swirl flask to mix yeast into solution, remove foil, flame lip and pour into five-gallon batch. Shake fermenter to aerate, then leave at recommended temperature. Activity should be present in 6 to 24 hours. Ferment ales at 65° to 75° F, and ferment lagers at 48° to 55° F.
PLANNING AND TIMING
Creating your own yeast starter requires planning. Each step is not very time consuming, but yeast requires a growth period in order to build up to a size sufficient for use in five gallons of wort.
Suppose you want to brew on a Sunday. A sensible time frame would be as follows:
WEDNESDAY
Inoculate 10-mL sterile wort starter
from plate or slant.
THURSDAY
Growth begins in 10-mL starter. Keep in a warm place (70° F).
FRIDAY
More growth, followed by the
transfer to Erlenmeyer flask
starter to build additional cells.
Keep at 70° F.
SATURDAY
Growth occurs and visible signs
of fermentation. Keep at 70° F.
SUNDAY
Pitch starter culture into
five-gallon batch of beer!
Although this schedule seems regimental, performing each task, such as inoculating a 10-mL starter, can be done in a few minutes. Growth occurs with little attention and preparing the pitchable starter takes about 30 minutes. The suggested temperature for growth is approximately 70° F. Don’t worry if you are a few degrees off.
Allowing a total of four days from inoculating to brewing should provide an adequate cell count to pitch into a five-gallon batch. If you go to a higher volume than 400 mL in an Erlenmeyer flask, then you will have to add one to two days for each additional transfer. If you’re brewing a high-gravity beer or a lager, it is best to make a one- to two-liter starter for five gallons.
QUALITY CONTROL
How do you know your culture is pure, and not contaminated? There are many methods available to check the purity of your yeast. In most cases, homebrewers do not do these procedures and instead rely on experience, smell and luck. Smell the contents of the used test tube and other containers of yeast culture. It should not smell sour, but should smell like good, clean yeast.
Methods to test your yeast cultures include anaerobic plating, aerobic plating, wild yeast plates, and microscope. Details of these testing procedures are better left for other articles. It takes two to four days after the yeast is ready in order to do the tests and interpret the results. At White Labs, it takes three laboratory technicians to test all of the yeast we produce.
AND IN CONCLUSION…
There are many good references to investigate these procedures in more detail. A very good resource is Pierre Rajotte’s book, “First Steps In Yeast Culturing.” Joining a local homebrew club is also a good way to meet people who can teach the skills described in this article.
Many of these yeast culturing techniques can be used in other parts of homebrewing, such as making a starter and saving yeast from batch to batch. The quality (health, viability, purity) may not be the same as laboratory grown yeast kept under constant conditions, but it will be very good. And it’s fun!
Some material in this article is excerpted from “The Fungus Among Us, 2nd Edition,” by Chris White and Yuseff Cherney, published in 1997.
