Yeast is a single-celled organism that is vital to most bakers and brewers around the world because of its ability to convert sugar into carbon dioxide and alcohol. You can make a yeast-filled bread starter, or sourdough starter, using only flour, water, and regular maintenance. Brewer's yeast cultivation is more complicated because it requires a sterile environment, but this process is also carried out by experienced or ambitious breweries. Both types of yeast culture can easily last for months in the refrigerator, so you can make the perfect bread or beer time and time again.
If you want to know how to prepare yeast before baking bread, you might want to read about how to activate yeast instead of reading this article.
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Method 1 of 2: Growing Yeast from Bread Starters
Step 1. Choose a large, clean bottle
Ideally, a glass bottle should be enough to hold at least two liters, as the starter will grow quickly, and you'll be forced to throw away a lot of starter if the bottle is too small. Plastic, clay, or stone bottles can also be used, but glass bottles are the easiest to clean, and make checking your bread starter easier. It is recommended to sterilize your bottles in boiling water if your bottles are heat resistant. However, washing the bottle with hot soapy water, then rinsing is also sufficient.
Step 2. Pour 120 ml of water without chlorine
If your home tap water contains chlorine, you can buy de-chlorinating tablets to remove the chlorine in the water, or let the water sit for 24 hours. The minerals found in "hard" water can help yeast cultures thrive, so using distilled water is not recommended.
If you do not have access to water that has ideal properties, use any water that is safe to drink
Step 3. Mix well 180 mL of flour
Use unbleached all-purpose flour if you plan to use a starter to make white bread, or whole wheat flour to make whole wheat bread. Wheat flour naturally contains wild yeast, microorganisms that produce carbon dioxide and other substances that cause bread to rise and provide added flavor.
- Stir vigorously, thus adding air to the mixture.
- Many other types of flour can be used to make starters with different flavors, including brown rice flour and spelled flour.
Step 4. Add unwashed organic wine (optional)
If using white flour instead of whole wheat flour, the starter may not contain the specific type of yeast that produces the tangy sourdough taste. Alternatively, you can try to correct this by adding a little fruit, most commonly a handful of grapes, to the mix. Use organic wines that are not pesticide or waxed, so you can add unwashed grapes to the mix.
While wine does contain yeast, how well it thrives in bread starters is debatable. Some bakers recommend this step, while others question how much of an effect it will have
Step 5. Cover, but not tightly
Avoid using a lid that is too tight, as a successful starter will produce gas that can crack the lid, and may require additional oxygen to expand. Instead, cover it with cheesecloth, tissue paper, or a clean cloth tied with a rubber band, or use a loose lid that doesn't close tightly.
Step 6. Store in a warm place for two days
To increase yeast activity, store the bread starter in a warm place, at least 21ºC. After two days, the mixture should appear bubbly or foamy, and emit a strong odor. However, some starters take longer to develop, so don't worry if you haven't noticed any changes yet.
If your home is cold, keep the yeast near the stove or heater, but not too close to it so the yeast doesn't overcook or get hot or steamy. Yeast thrives best in warm environments, but dies if it gets too hot
Step 7. Add 120 ml of water and 180 ml of flour
Stir in water and flour of the same type as before in small amounts, until well blended. Cover and leave for 24 hours while the yeast eats its new food.
Step 8. Replace some of the starter daily with new flour and water
Every day, partially remove the starter, leaving at least 120 ml in the bottle. At this stage, the starter is still not safe and effective to use in recipes, so discard the raised part. Add more water and flour to replace the portion - the exact amount you use doesn't matter, as long as you're using 3 parts flour instead of 2 parts water. Do not try to add more than triple the current amount of the mixture.
Step 9. Monitor the progress
Initially, the starter may produce a yellowish liquid on top, and give off an alcohol-like odor. Hopefully, this will go away within a week, as the yeast colonies develop and produce an odor that is more like the smell of bread. Once the yeast has formed, the mixture should rise consistently until it doubles in size between each feeding. Continue feeding until this is achieved, and for at least a full week to minimize the chances of rival microorganisms overpowering the yeast. Some starters may take more than a month, or even longer, to be ready.
If the mixture produces a dark brown liquid instead, this is a sign that the yeast has run out of food. Discard the brown liquid and feed it more often, or with more flour and water at each feeding
Step 10. Transfer to refrigerator and reduce feeding frequency
Once the mixture has doubled in size daily for at least three days, and is not producing any liquid or foul-smelling odor (not smelling like bread), cover it tightly and transfer it to the refrigerator. The yeast will become dormant, or at least slow down, and you'll only need to feed the yeast with flour and water once a week, removing some of the yeast if needed to keep it from overflowing. As long as you remember to feed the yeast, the starter can be stored in the refrigerator indefinitely, resulting in a yeast-filled bread starter for months or even years.
Brown rice flour starter needs to be fed every few days even when it's been refrigerated
Step 11. Use starters in bread recipes
Before using some of the starter in a bread dough recipe (as a substitute for bread yeast), reactivate the yeast by moving it to room temperature, covering it loosely with paper towels or cheesecloth, and feeding the yeast at least three times 8-12 hours apart.. Knead the bread dough evenly until the gluten is activated, which will allow the dough to be stretched thin enough for light to shine through it without tearing the dough. Since wild yeast tends to act more slowly than commercial yeast types, let the dough rise for 4 - 12 hours, or even 24 hours for a more tart loaf.
- Make sure the bread dough doesn't get too hot, as this can kill the yeast. Touch the bread dough occasionally if you are kneading it with a mixer, as the mixer can overheat the dough.
- You can also use sourdough starter in other recipes that involve wheat flour, but keep in mind that the starter will give the sourdough a tangy taste. Many people make sourdough pancakes to use the extra starter, which is picked up when feeding the yeast, which is usually thrown away.
Method 2 of 2: Growing a Brewer's Yeast Culture
Step 1. Start by using a high-quality yeast culture that is used for beer
While you can start a yeast culture using store-bought liquid brewer's yeast, growing it is usually more difficult and longer if you start with just the commonly available varieties. Typically, breweries grow yeast cultures starting with a yeast deposit from a very successful home brew, beerhouse favorite, or some other rare or expensive breed that they wish to grow for reuse.
- Growing your own long-term yeast culture takes a lot of time and effort. No need to have to brew beer at home, just to keep a certain favorite type of yeast.
- It should be noted that the yeast deposit in the beer bottle may not be the same as the yeast used in the primary (initial) fermentation, so your final result may not be what you want.
Step 2. Work on a clean area
Airborne contaminants can destroy yeast cultures, as can bacteria. Avoid wet areas or places where food is prepared, such as in the kitchen and in the basement. Close the windows in the room where you grow yeast, especially during warm weather.
Always wash your hands with antibacterial soap before handling yeast cultures
Step 3. Clean and sanitize the work surface
Wash the workbench as clean as possible. Kill most remaining microorganisms with a sanitizing product such as rubbing alcohol. Let it dry.
Step 4. Purchase equipment
The easiest way to get the necessary equipment is to purchase a brewing kit, which may or may not come with a starter yeast and guide. If you're collecting equipment piece by piece, or checking to see if it's all-encompassing, check out the Things You'll Need section for the full list. Try looking at pharmacies, or looking for laboratory equipment providers on the yellow pages or the internet.
- Ordering laboratory equipment in the United States can be delayed or involve inquiries from government agencies.
- Agar powder is available in many Asian grocery stores. If you can't find one, use powdered, unflavored gelatin, but remember that gelatin-based cultures need to be stored in a cooler place so they don't melt.
Step 5. Sterilize suitable containers
Steam heat-resistant glass containers and lids in the pressure cooker for at least 10 minutes to kill any sources of contamination. Petri dishes, or “plates,” are often used, but you can use any small glass container. A “starter tube” is sometimes included in brewing kits for this purpose.
- If you don't have a pressure cooker, soak the containers in water and simmer for 30 minutes. However, it is not as effective as a pressure cooker in killing contaminants, which is likely to result in a large number of yeast cultures failing to grow or spoiling due to mold.
- If you have sterile plastic bags to store the containers, you can prepare the containers in advance.
Step 6. Allow the containers to cool, then pass through the heat
Since sterilization is essential for brewer's yeast cultures to prevent other microorganisms from overpowering the yeast, this step is recommended in addition to the steps above. Using a propane torch, or other portable high-temperature ignition source (not a regular lighter), pass the tip of the flame along the lip of the container.
Step 7. Use soft or distilled water
If the tap water in your area is “hard” water, which means it contains high amounts of calcareous carbonate minerals, it could lead to bacterial growth in your yeast culture. Use distilled water to be safe, or measure the pH of your water and use it only if it's 5.3 and below.
Step 8. Boil 240 mL of water and 60 mL of dry malt extract
Heat the water in a pressure cooker if possible to prevent the water from overflowing, or use a clean Pyrex jar or saucepan. Add the dry malt extract and stir to dissolve it. Simmer for 15 minutes, and watching, lower the temperature if it threatens to overflow.
This is called the “starter filtrate.”
Step 9. Lower the temperature, add 2.5 mL of agar powder, and stir until dissolved
The starter filtrate already contains the nutrients that brewer's yeast needs to thrive, but the agar powder will eventually thicken the mixture into a gelatinous base on which the yeast resides. It should be noted that thickening will not occur at this stage.
Use unflavored gelatin powder only if you cannot obtain agar powder, as cooked gelatin may melt in a warm room
Step 10. Bring to a boil again
Simmer again for 15 minutes. Again, be careful not to overflow.
Step 11. Remove from stove
Allow it to cool to 50ºC or less, or slightly cooler if using gelatin instead of agar. The mixture will thicken, but will not completely harden.
Step 12. Fill each container with a small layer of the mixture
Take your sterilized containers and fill each container with a small amount of the boiling mixture, called starter filtrate. The petri dish should be about a quarter full; larger containers do not require thicker layers.
Step 13. Cover the container and wait
Cover the container or cover it with plastic. Allow to cool for about half an hour, and watch as the filtrate hardens due to the agar powder. Once the container can be tilted without the mixture in it flowing, then the container is ready.
Step 14. Sterilize the inoculation loop / ose
An ose, available at lab supply stores, is a small loop of wire at the end of a stick, which is used to transfer microorganisms such as yeast. Sterilize the loop end of the loop by heating it over a fire until the entire wire glows orange or red. Cool the ose slightly to room temperature or slightly warm it by dipping it in a shallow cup of isopropyl alcohol, or wiping it with a cotton ball moistened with alcohol.
- If you don't refrigerate the oven, the heat in the loop can kill the yeast.
- Cooling the ose in water or air increases the chance of contaminating microorganisms, which must be killed using alcohol.
Step 15. Gently rub the ose wire over the liquid yeast precipitate
Do not attempt to pick up any visible amount of yeast. All you need to do is slightly run the loop of wire through the sediment that has accumulated on top of the liquid.
Step 16. Add yeast to the surface of the filtrate, following this step carefully
Don't leave the container uncovered for too long, so you have to work as quickly as possible. Move the loop lightly over the surface of the starter filtrate in one of your containers. This transfers the yeast to, hopefully, a germ-free, nutrient-rich filtrate. To minimize the chance of contamination, close it again immediately. Place the petri dish upside down (lid down), or close the starter tube until it is about 3/4 density.
By microbiologists, the process of adding microorganisms to a petri dish is called the "streaking method."
Step 17. Repeat sterilization before adding yeast to each container
Use the same process to add yeast to each container, but remember to heat the oven to sterilize it between transfers, then cool in the alcohol. Yeast cultures grown at home have a fairly high chance of contamination, so using multiple cultures grown separately increases the chance that some of your cultures will eventually work.
Step 18. Check the yeast culture for the next few days
Store containers at 21–26ºC, the ideal temperature range for active yeast growth. Discard cultures that have a single strand or cluster of mold, or fail to show any yeast growth at all after a few days. A successful yeast culture will produce a milky white layer on the surface, and you can see individual yeast colonies forming a dotted trail on the surface.
Step 19. Transfer the successful cultures to the refrigerator
Now that the successful cultures have been activated, wrap the container completely with electrical tape or other light-blocking material, as light can destroy or damage yeast colonies. Store the containers in the refrigerator, ideally at 1–2ºC or slightly warmer, to slow yeast growth and prevent the yeast from running out of nutrients. When you want to use one to make beer, first take it out of the fridge to get it to room temperature before adding it to the filtrate.
