How do I make a yeast starter? Why do I need one?

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A starter is basically a mini batch of beer that has been tailored for optimized yeast reproduction. Making a yeast starter is a fairly simple task that can greatly improve your chances for having a good batch of beer.

Yeast undergoes a series of metabolic steps when it is pitched into wort. It takes a while for the yeast to figure out what to do. When yeast is poured from the tube or package directly into wort, they are in a dormant state. They don't immediately begin fermentation. The first thing they do is to assess the level of nutrients, including sugar, amino acids and oxygen levels. Once they've adapted to the environment, they begin a reproductive phase. They will continue to reproduce as long as oxygen and other nutrients are available. Fermentation begins in earnest only once all the oxygen has been depleted.

No matter how good a job you do at sanitation, there is bound to be some bacteria or wild yeast floating around. Some of it is going to end up in the wort. While the yeast is trying to figure out what to do, these bacteria are having a field day. The more time they have, the worse off you'll be. They'll begin their own reproductive and metabolic rampage, turning your sweet wort into foul slime.

A starter will minimize the impact of bacteria in two ways. First, the yeast will already be accustomed to the environmental conditions. In essence, they will have been pre-trained so they already know what to do. Secondly, the starter will drastically increase the number of yeast cells. This greatly increases the odds that a yeast cell will grab a bit of nutrient before any bacteria does. Once the wort has been depleted of nutrients, the bacteria have a much harder time. Some will even be killed off in the onslaught of alcohol and acidity.

Starters are more important if you're asking the yeast to do a tougher-than-normal job. Yeast have a hard time adapting to high gravity worts. Hence, a starter is particularly important for styles such as Barleywine and Imperial Stout. Also, slower-fermenting lager yeasts greatly benefit from a starter.
Starters also impact the course of fermentation, even in the absence of any bacteria. Higher numbers of yeast cells will attenuate the wort more, and will more quickly consume fermentation byproducts such as diacetyl. Starters also minimize the risk that the yeast will mutate, which can lead to a host of off-flavors. Finally, making a starter will leave you with finished beer sooner.

Making a Starter:
You want to get a starter going the day before you brew, so it is best to plan ahead. If you're using the Wyeast slap-packs, hit one two days before you brew, and plan to make the starter the next day. If you use the big Wyeast tubes or the White Labs tubes, you don't need to get the yeast ready ahead of time.
This procedure is the minimum recommended for a five- gallon batch. You can make a bigger starter if you like. Scale it up accordingly if you're making a bigger batch.

Put a pint of water in a saucepan and start it heating. Measure out an ounce of dry malt extract, or a fluid ounce of extract syrup. Mix it into the hot water. I prefer to use dry malt extract, so that is dissolves quickly. If you use liquid extract, turn off the heat until it dissolves. Once dissolved, bring it up to boiling. Turn the heat down to simmer and cover. Let it simmer 5 to 10 minutes. Shut off the heat. Leave the pan covered while it cools. You can either let is sit for an hour or so, or chill it by placing in the sink with cold water. You should get an original gravity of about 1.020. It's better not to check, though, since the extra handling could result in contamination.

While the wort is cooling, you need to sterilize a container, an airlock, and a stopper to fit. A 22 oz bottle works well, but just about any glass container that is big enough will do. Some people use an Erlenmeyer flask. That way you can boil the wort in the same container. Whatever you pick, make sure you have enough headroom for fermentation.

Rinse out the container and pour in the cool wort. Give the yeast package a quick external dunk in sterilant and open it up. Pour the yeast into the bottle and shake well to aerate. Put on the airlock and let it sit at about 70oF overnight. You should see gentle foaming and a thick yeast layer on the bottom. When you're ready to pitch the starter into the main batch, make sure you swirl the container vigorously so that is all stirred in.