If you were playing hockey, you would not simply hop on the ice at game time and hope for the best. You would warm up beforehand so that as soon as your skates hit the ice, you're ready to go.
Yeast needs the same kind of warm up to properly ferment your beer! If you just toss your yeast out of the package into your fermenter, your yeast are going to be a little shocked by the sudden climate change. A yeast starter gives your yeast a chance to warm up before you pitch it in your beer.
What is a yeast starter? In some ways, it is like a small beer batch that wakes up your yeast so they reproduce and get ready to eat up your wort and produce alcohol. Since yeast can reproduce a lot quicker in beer with a lower gravity, you will use a low gravity wort for your starter.The gravity should ideally be around 40% of the starting gravity of your actual beer recipe. And if you're brewing a 5 gallon recipe, you should make a starer that is about 2 quarts by volume.
Step 1: Step By Step: Making A Yeast Starter for Beer
Step 1. Bring about 2 quarts of water in a pot to a boil.
Step 2. To your boiling water, add 1 1/2 cups of some sort of unhopped dry malt extract
Step 3. Optionally, add any kind of yeast nutrient to the water (about 1/4 tsp.)
Step 4. Let the solution boil for between 15 to 20 minutes, but keep an eye out to prevent boilovers!
Step 5. Turn off the heat and let the solution cool to below 90 degrees F.
Step 6. Pour the starter into a sterilized and cleansed container - something that can be made airtight and something that gives you at least 1 inch of headspace above your starter solution. Pop bottles work well for this.
Step 7. Shake your container to aerate the wort.
Step 8. Add your yeast to the wort - make sure to follow any guidelines provided by the manufacturer of your yeast.
Step 9. Attach a lid to your container - such as a stopper. Then outfit the lid with an airlock making sure to fill the airlock half full with water. An airlock is important as it will release any carbon dioxide gas that will build up in the starter.
Step 10. Find a clean, dry, dark, and warm place to store your yeast starter! This is the best sort of environment to keep your yeast happy.