A yeast starter is a small volume of low gravity wort that is used to begin the first two stages of fermentation: respiration and reproduction. Over the course of 1-2 days, a yeast starter can turn a weak or standard pitch of yeast into a strong, healthy sugar-eating machine. Wyeast recommends on their label to make a starter, if the gravity is over 1.060, if the yeast is 6 months past manufacture date, or if a faster start is desired.

Equipment:

2000ml Erlenmeyer flask (or a 2-quart saucepan and sanitized jug)

Rubber stopper to fit flask or jug

Sanitary air filter

1 inch of 1/4-inch ID tubing

Ingredients: (for a 1-Liter/1-quart starter)

100 g / just under 1 cup dry malt extract (Pils, Light, or Organic Light)

1/8 tsp yeast nutrient (e.g. Wyeast nutrient, Biotin, or GoFerm)

Your yeast of choice

Procedure:
Use a 10:1 ratio of water to dry malt extract (DME). For example, when making a 1-Liter/quart starter, add 100g (slightly less than 1 cup) DME to your flask or saucepan, and add water to 1000ml/1qt. Add 1/8 tsp yeast nutrient. Place flask on burner (bend a wire or metal hanger into a coil for use on electric stoves), and boil gently for 15 minutes. Chill to room temperature and add yeast. Place rubber stopper on flask, affix the small piece of tubing to one end of the sanitary air filter, and put it in the stopper.

For aeration, there are a few options ranging in levels of time and expense. The simplest way to give yeast oxygen is to periodically shake the flask. This works, but is not optimal for the yeast. Using a stir plate with a stir bar in the flask to continuously move the wort works best. If you can, add a dose of pure oxygen to the wort just prior to pitching the yeast. If you can’t, don’t worry.

You shouldn't notice signs of an active fermentation. This is because the yeast are busy absorbing oxygen and nutrients from the wort in order to reproduce, rather than consuming the sugar and releasing CO2 (creating bubbles). Let the yeast settle for about an hour, carefully pour out the wort on top, and pitch your yeast starter into your brew; it should start faster and ferment more vigorously than pitching directly from a packet.

If you wish to make an even larger starter (and/or increase the alcohol tolerance of the yeast), you can chill the starter to drop the yeast, carefully decant the wort, and then add more sterile wort to the yeast. As always, sanitation is key!