(These instructions have been adapted from a posting at thefreshloaf.com by Sourdolady.)

Procedure for Making Sourdough Starter

Day 1: mix...
2 T. whole grain flour (rye and/or wheat)
1 T. unsweetened pineapple juice or orange juice
Cover and let sit at room temperature for 24 hours.

Day 2: add...
2 T. whole grain flour
1 T. juice
Stir well, cover and let sit at room temperature 24 hours. At day 2 you may (or may not) start to see some small bubbles.

Day 3: add...
2 T. whole grain flour
1 T. juice
Stir well, cover and let sit at room temperature 24 hours.

Day 4:
Stir down, measure out 1/4 cup and discard the rest.
To the 1/4 cup add...
1/4 cup flour*
2 Tbs water

*You can feed the starter whatever type of flour you want at this point (unbleached white, whole wheat, rye). If you are new to sourdough, a white starter is probably the best choice. Unbleached all-purpose flour is fine.

Repeat Day 4:
Once daily until the mixture starts to expand and smell yeasty. It is not unusual for the mixture to get very bubbly around Day 3 or 4 and then go completely flat and appear dead. If the mixture does not start to grow again by Day 6, add 1/4 tsp. apple cider vinegar with the daily feeding. This will lower the pH level a bit more and it should kill off competitors to the yeast, allowing them to thrive.

How it Works
The yeast we are trying to cultivate will only become active when the environment is right. When you mix flour and water together, you end up with a mixture that is close to neutral in pH, and our yeasties need it a bit more on the acid side. This is why we are using the acidic fruit juice. There are other microbes in the flour that prefer a more neutral pH, and so they are the first to wake up and grow. Some will produce acids as by-products. That helps to lower the pH to the point that they can no longer grow, until the environment is just right for wild yeast to activate. The length of time it takes for this to happen varies.

When using just flour and water, many nascent starters will grow a gas-producing bacteria that slows down the process. It can raise the starter to three times its volume in a relatively short time. Don't worry--it is harmless. It is a bacterium sometimes used in other food fermentations like cheeses, and it is in the environment, including wheat fields and flours. It does not grow at a low pH, and the fruit juices keep the pH low enough to stop it from growing. Things will still progress, but this is the point at which people get frustrated and quit, because the gassy bacteria stop growing. It will appear that the "yeast" died on you, when in fact, you haven't begun to grow yeast yet. When the pH drops below 3.5--4 or so, the yeast will activate, begin to grow, and the starter will expand again. You just need to keep it fed and cared for until then.

Once your wild yeast is growing, the character and flavor will improve if you continue to give it daily feedings and keep it at room temperature for a couple of weeks longer.

After that time, it should be kept in the refrigerator between uses/feedings. Every week or so, take it out of the fridge, feed it by retaining only ¼ cup of starter and then feed it ¼ cup flour and 2 Tbs water.