Having clean, healthy yeast in sufficient quantity is a critical component of a good fermentation. Oftentimes the brewer will need to pitch more than one unit of the liquid or dry yeast to ensure the proper amount of yeast is being used. One way to accomplish this when using liquid yeast is to build up the yeast count by making a yeast starter. Do not make a yeast starter when using dry yeast; instead use multiple packages of dry yeast that have been properly re-hydrated.
A yeast starter is like a mini-batch of beer.
What you'll need:
- a saucepan large enough to hold the starter wort with room to spare
- measuring cups
- stirring spoon
- sanitized 1 gallon container
- sanitized funnel
- sanitized aluminum foil
- dry malt extract (DME)
- yeast nutrient (optional)
- 1 package of liquid yeast
The size of the starter will depend on the amount of beer being brewed, the age of the yeast, and how the starter is treated during fermentation. A tool like Jamil Zainasheff's Mr Malty Pitching Rate Calculator or Wyeast's Pitch Rate Calculator can help with deciding how large a starter to prepare, but in general 5 gallons of average-gravity ale will require a 2 L (approx. 2 qt) starter, when starting with fresh yeast. High-gravity ales and all lagers will require larger starters.
In a pan, combine 200 g (7 oz) DME per 2 L (2 qt) water. For maximum accuracy, solids should always be measured by weight, but if a scale isn't available, 1 cup (240 mL) can be assumed to contain about 200 g DME. Stir and bring the mixture to a boil. Boil for 10 minutes and then remove from the heat. Optionally, add 0.5 tsp yeast nutrient and gently stir into the liquid. Place a lid on the pot and immerse it in cold water, being careful not to use so much water that the pot can tip over. When the wort has cooled to the touch, pour it into the sanitized container. Using a funnel can help to prevent spillage. When the temperature of the wort is around 65-80°F (18-27°C), add the package of liquid yeast to the container.
In order to maximize yeast growth, the starter should have as much access to oxygen as possible. Loosely covering the container with aluminum foil will keep out dust and insects while still allowing for gas exchange. Put the yeast starter somewhere where it will be around 65-80°F and out of direct sunlight. As often as possible, gently swirl the starter to further increase oxygen contact. The starter should begin actively fermenting within 24 hours, and will likely be finished in less than 72 hours.
For yeast starters of smaller volumes, you can safely pitch the whole starter into your beer. If time allows, it is recommended that you chill the starter in the refrigerator for a day or two to get the yeast to drop out of suspension, then gently pour off the liquid and use only the yeast cake at the bottom of the starter for your beer.