Hi Abigail,

Crusty potato bread. mmmmm sounds good. Not sure what the recipe means by "leaven" or if it's active dry or instant yeast you'd like to use. In my experience, there are 3 types of yeast: fresh (also known as baker's or compressed yeast); active dry which must be dissolved in lukewarm water before incorporating in dough; and instant dry which can be included directly, without first dissolving, with other ingredients in the dough.

Here are some conversion formulas that might be useful:

From fresh yeast to active dry: For each pound of yeast, use 0.4 lb of active dry. Or if you're dealing in ounces, use 0.4 oz of active dry for each ounce of fresh yeast.

From fresh yeast to instant dry: For each pound of fresh yeast, use 0.33 pound instant dry yeast or 0.33 oz instant dry for each ounce of fresh.

Dry yeast (active or instant): 0.3 oz = 2.7 teaspoons.

Good luck! Carol

Abigail, when I was searching for the opposite information (how to alter from active dry yeast to natural starter), I came across the following in the sourdough section of Artisan Baking Across America by Maggie Glezer. She wrote:

Any sourdough-based recipe can be converted into a yeast-based recipe. The bread will not have the complex flavor [...] of a true sourdough, but it will still be a very fine loaf.

To convert a recipe from sourdough to commercial yeast, you will just use a small amount of yeast in the levain and omit the sourdough starter. [...] Dissolve 1/4 teaspoon yeast in 1/4 cup warm water and use 2 tablespoons of the yeasted water per cup (150 grams, 5.3 ounces) flour. [...] Be sure to reduce the water measure in the levain by the same amount as the added yeasted water.

Let the levain, which is now technically a pre-ferment, ferment for 2 to 3 hours, or until it has risen to about half again its original volume, then refrigerate it overnight until ready to use. Let it come to room temperature before adding it to the final dough. Continue with the recipe as directed - there is no need to add more yeast.

And Richard Packham wrote the following on his webpage Sourdough and Sourdough Starter:

The general rule for substituting sourdough starter for yeast is to use one cup of starter for each one-ounce yeast cake, and then reduce the amounts of flour and liquid each by about one cup.

So my guess is that you could go in the opposite direction to use one one-ounce yeast cake (28.5gm) instead of one cup of starter and add a cup of flour and liquid to the mix. (I believe that Mr.Packham is from Idaho, where 1cup = 240ml)

Hope that helps!

-Elizabeth

P.S. I did manage to make a sourdough rye using my natural starter converting a recipe in 'Joy of Cooking' that called for 1+1/4 tsp active dry yeast. Please excuse the shameless self-promotion; here is the post: http://etherwork.net/blog/?p=430

edit:  OOOPS!!!! I am new to this site and just noticed the date of Abigail's query. Please pardon my very late reply.