Grow Yeast

Yeast is a single-celled organism vital to most bakers and brewers all over the world, due to its ability to turn sugars into carbon dioxide and alcohol. You can create your own yeast-filled bread starter, or sourdough starter, with nothing more than flour, water, and regular maintenance. Cultivating brewer's yeast is more complex due to the need for a sterile environment, but this process is also included for experienced or ambitious home brewers. Either type of yeast culture can easily last months in the refrigerator, letting you recreate that perfect bread or beer many times over.

If you want to know how to prepare yeast before baking, you may be looking for how to activate yeast instead.

Steps

Growing Yeast from a Bread Starter

1. Select a large, clean jar. Ideally, use a glass jar that can hold at least two quarts (two liters), as the starter will grow rapidly and force you to throw away more of it if the jar is too small. Plastic, earthenware, or stoneware containers are also usable, but glass may be easiest to clean, and makes it easy to view your bread starter.Sterilize-Bottles-and-Jars-for-Canning in boiling water is recommended if your container is heat-safe. Washing the jar in hot, soapy water, then rinsing may be sufficient, however.
2. Pour in 1/2 cup (120 mL) dechlorinated water. If your tap water is treated with chlorine, you can purchase de-chlorination tablets to remove it, or let it sit out for 24 hours. The minerals found in "hard" water may help the yeast culture develop, so using distilled water is not recommended.
   • If you do not have access to water with ideal characteristics, use any water that is safe to drink.
3. Mix in 3/4 cup (180 mL) flour thoroughly. Use unbleached all-purpose flour if you will be using your starter to make white bread, or whole wheat flour to make brown bread. Flour naturally contains wild yeast, a micro-organism which produces the carbon dioxide and other substances which cause bread to rise and add additional flavors.
   • Stir vigorously, adding air into the mixture.
   • Many other types of flour can be used to make different flavors of starter, including brown rice flour and spelt flour.
4. Add organic, unwashed grapes (optional). If you are using white flour instead of whole grain flour, your starter may not have certain types of yeast that produce a tangy, sourdough flavor. Optionally, you can attempt to correct this by adding a little fruit, most commonly a handful of grapes, to the mixture. Use organic grapes that have not been treated with pesticides or wax, so you can add them unwashed to the mixture.
   • While grapes definitely contain yeast strains, how well they thrive in a bread starter is disputed. Some bakers recommend this step, while other question how much effect it has.
5. Cover but do not seal it. Avoid using an airtight lid, as a successful starter will produce gas that could break the lid, and may need additional oxygen to thrive. Instead, cover it with a cheesecloth, paper towel, or clean dishcloth tied on with a rubber band, or use a loose fitting lid not fully tightened.
6. Keep in a warm place for two days. In order to encourage yeast activity, keep the new bread starter in a warm environment, at least 70ºF (21ºC). After two days, the mixture may look bubbly or foamy, and take on a noticeable smell. Some starters will take longer to get off the ground, however, so don't worry if you don't notice any changes yet.
   • If your house is cold, store the yeast near the stove or heater, but not so close that it cooks or becomes hot or steaming. Yeast thrives in warm environments, but dies if it gets too hot.
7. Add 1/2 cup (120 mL) water and 3/4 cup (180 mL) flour. Stir in the same type of water and flour in smaller quantities, until it is mixed thoroughly. Cover and leave an additional 24 hours while the yeast eats its new food.
8. Replace part of the starter each day with new flour and water. Each day, remove part of the starter, leaving at least 1/2 cup (120 mL) in the jar. The starter is not yet safe and effective to use in recipes, so throw out the removed portion. Add in more water and flour to replace it – the exact amount you use isn't important, as long as you use 3 parts flour to 2 parts water. Don't try to add more than triple the current size of the mixture.
9. Keep an eye on its progress. At first, the starter may produce a yellowish liquid at the top, or smell like alcohol. Hopefully, this should disappear within a week, as the yeast colony grows and produces a more bread-like smell. Once the yeast is established, the mixture should consistently expand to double its size between each feeding. Continue to feed until this is accomplished, and at least for a full week to minimize the chance of competing micro-organisms taking over. Some starters may not be ready for a month or longer.
   • If the mixture produces a dark brown liquid instead, this is a sign that it is running out of food. Pour off the liquid and feed more often, or with larger amounts of flour and water per feeding.
10. Move to the refrigerator and feed less often. Once the mixture doubles in size every day for at least three days, and does not produce any unpleasant (non-bread-like) aromas or liquids, cover it tightly and move it to the refrigerator. The yeast will go dormant, or at least slow down, and you will only need to feed it once a week with flour and water, discarding part of it if necessary to avoid overflow. As long as you remember to feed it, the starter can be kept in the refrigerator indefinitely, producing yeast-filled bread starter for months or even years.
    • Brown rice flour starters need to be fed every few days even in the refrigerator.
11. Use it in bread recipes. Before using a portion of the starter in a bread dough recipe (in place of baker's yeast), make it active again by moving it to room temperature, covering loosely with a paper towel or cheesecloth, and feeding it at least three times at 8–12 hour intervals. Knead the bread dough thoroughly until the gluten is activated, which will create a dough that can be stretched thin enough for light to shine through without the dough breaking. Because wild yeast tends to act slower than commercial yeast strains, allow the bread dough to rise for anywhere from 4 – 12 hours, or even 24 for a more sour bread.
    • Make sure not to overheat the bread dough, which can kill the yeast. Touch the bread dough occasionally if kneading in a mixer, as these can overheat the dough.
    • You may also use sourdough starter in other recipes that involve flour, but be aware that it will add a tangy sourdough taste. Many people make sourdough pancakes to use up extra starter that would otherwise be thrown away during feeding.

Growing Brewer's Yeast Cultures

1. Start with a high quality yeast culture intended for brewers. While you can begin a yeast culture using store-bought liquid brewer's yeast, the process of growing it is usually too difficult and time consuming if you are only starting with a commonly available strain. Typically, home brewers grow yeast cultures starting with yeast sediment from a particularly successful homebrew, a favorite brewpub, or another rare or expensive strain they wish to grow for repeated use.
   • Growing your own yeast cultures long-term can take a lot of time and effort. It is not required to Brew-Your-Own-Beer, only to maintain certain favored yeast strains.
   • Note that the yeast sediment in a beer bottle may not be the same as the yeast used in primary (initial) fermentation, so your results may not be what you expect.
2. Work in a clean area. Airborne contaminants can ruin the yeast cultures, as can bacteria. Avoid damp areas or places where food is prepared, such as kitchens and basements. Close windows to your yeast growing room, especially in warm weather.
   • Always wash your hands with antibacterial soap before handling the yeast cultures.
3. Clean and sanitize a surface. Wash a workbench or table as thoroughly as possible. Kill most remaining micro-organisms with a sanitizing product such as rubbing alcohol. Allow to dry.
4. Purchase equipment. The easiest way to acquire the necessary equipment may be purchasing a brewer's kit, which may or may not come with starter yeast and instructions. If you are acquiring the equipment piece by piece, or checking whether the kit contains everything, see the Things You'll Need section for a full list. Try pharmacies, or look for laboratory equipment suppliers in the yellow pages or online.
   • Ordering laboratory supplies in the United States may be delayed or involve questioning by government agencies.
   • Agar powder is available at many Asian grocery stores. If you cannot find any, use unflavored gelatin powder, but be aware that gelatin-base cultures needs to be kept in cooler locations to avoid melting.
5. Sterilize suitable containers. Steam heat-safe, glass containers and their lids in a pressure cooker for at least 10 minutes to kill sources of contamination. Petri dishes, or "plates," are often used but you may use any small, glass container. "Starter tubes" are sometimes included in brewer's kits for this purpose.
   • If you don't have a pressure cooker, immerse the containers in water and boil for 30 minutes. However, this is not nearly as effective at killing contaminants, which will likely result in a larger number of yeast cultures failing to grow or being ruined by mold.
   • If you have sterilized plastic bags to store the containers in, you may prepare the containers in advance.
6. Let the containers cool, then run them through a flame. Because sterilization is so important for brewer's yeast cultures to prevent other micro-organisms from taking over, this step is recommended in addition to the above. Using a propane torch or other high-temperature, portable flame source (not an ordinary cigarette lighter), run the end of the flame across the lips of the container.
7. Use soft or distilled water. If the tap water in your area is "hard," meaning it contains a high quantity of chalky, carbonate minerals, it may cause bacterial growth in your yeast culture. Use distilled water to be safe, or Measure-the-pH-of-Water and use it only if the result is 5.3 or below.
8. Boil 1 cup (240 mL) water and 1/4 cup (60 mL) dried malt extract. Heat the water in a pressure cooker if possible to avoid boiling over, or use a clean Pyrex flask or saucepan. Add in the dried malt extract and stir to dissolve. Bring to a boil for 15 minutes, taking care to turn down the heat if it is in danger of boiling over.
   • This is called a "starter wort."
9. Turn down heat and stir in 1/2 tsp (2.5 mL) agar powder until dissolved. The starter wort already contains the nutrients brewer's yeast cultures need to thrive, but the agar powder will eventually thicken the mixture into a gelatinous base for the yeast to rest on. Note that the thickening will not occur during this step.
   • Use unflavored gelatin powder only if you cannot acquire agar powder, since cooked gelatin may melt in a warm room.
10. Bring to boil again. Boil for an additional 15 minutes. Once again, keep a careful eye on it to stop it from boiling over.
11. Remove from heat. Allow the mixture to cool to 122ºF (50ºC) or below, or a bit cooler if using gelatin instead of agar. The mixture should thicken, but not solidify completely.
12. Fill each container with a small layer of the mixture. Take your sterilized containers and fill each one with a little of the boiled mixture, called starter wort. Petri dishes should be filled approximately 1/4 of the way full; larger containers do not require a thicker layer.
13. Cover the containers and wait. Put lids on the containers or cover with plastic wrap. Let them cool for about half an hour, and watch as the wort solidifies due to the agar powder. Once the containers can be tilted without the mixture running, they are ready.
14. Sterilize the inoculation loop. The inoculation loop, available from laboratory supply stores, is a tiny wire loop at the end of a wand, used for transferring micro-organisms such as yeast. Sterilize the loop end by heating it in a flame until the entire loop grows orange or red. Cool the loop to room temperature or slightly warmer by placing it in a shallow dish of isopropyl alcohol, or wiping it with a cotton ball soaked in alcohol.
    • If you do not cool the loop, the heat could kill the yeast.
    • Cooling it in water or air increases the chance of micro-organism contamination, which should be killed by the alcohol.
15. Draw the loop lightly over the liquid yeast sediment. Do not try to pick up a visible amount of yeast. All you need to do is barely draw the loop through the sediment gathered on the top of the liquid.
16. Add the yeast to the surface of the wort, following this step carefully. Leaving the lid off for as short a time as possible, move the inoculation loop lightly over the surface of the starter wort in one of your containers. This transfers yeast onto the hopefully germ-free and nutrient-rich wort. To minimize the chance of contamination, immediately attach the lid again. Turn petri dishes upside down, or cap starter tubes to about 3/4 tightness.
    • The process of adding a micro-organism to the plate is called "streaking" by microbiologists.
17. Repeat the sterilization before adding yeast to each container. Use the same process to add yeast to each container, but remember to heat the inoculation loop to sterilize it between each transfer, then cool it in alcohol. Yeast cultures grown at home have a relatively high chance of contamination, so using multiple, separately-grown cultures increases the odds that some of your cultures end up usable.
18. Check on the yeast cultures for the next few days. Store the containers at 70–80ºF (21–26ºC), the ideal temperature range for active yeast growth. Discard any cultures that grow fuzz or balls of mold, or fail to grow any visible yeast after several days. Successful yeast cultures will produce a milky layer over the surface, and you may see individual yeast colonies forming trails of dots over the surface.
19. Move successful cultures to the fridge. Now that the successful cultures have been activated, wrap the containers completely in electrical tape or another light-blocking material, since light can destroy or damage yeast colonies. Store these in the fridge, ideally at 34–36ºF (1–2ºF) or slightly warmer, to slow their growth and prevent them running out of nutrients. When you wish to use one in a brew, remove it from the fridge in advance to bring it up to room temperature before adding (pitching) into the wort.

Tips

• You can also grow a yeast starter in a jar of fruit and water, or with potato, sugar, and water.

Warnings

• Know that fully sealing growing yeast inside a container may lead to the container exploding due to CO₂ gas buildup. In the case of glass containers, this may propel glass shards in all directions.
• Leave the bread starter jar half-filled with starter or less after each feeding, since it will expand greatly in size.

Things You'll Need

Bread starter:

• Large jar
• Loose lid, cloth, or paper towel
• Flour
• Water
• Refrigerator

Brewer's yeast culture:

• Brewer's yeast
• Propane torch or other high-temperature, portable flame source
• Inoculation loop ("smear loop")
• Agar powder
• Pressure cooker
• Soft or distilled water
• Pyrex flask
• Dried malt extract (or other yeast nutrient)
• Small, sealed glass containers (usually petri dishes, "slants," or starter tubes)
• Isopropyl alcohol
• Electrical tape
• Refrigerator

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