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I know that there are formulas for switching from and to sourdough and or preferments, but even though my library grows I haven't found such a formula. I have a general idea of the fact that a sourdough bread usually requires around 40% of sourdough and a regular yeasted bread requires something from .1% to a stunning value of 2% of yeast. Other that this general idea I have not found anything in the literature I have at home since most of the books usually treat these subjects separately.

Is there a formula that transforms a recipe from fresh yeast to sourdough and vice versa? What about preferments?

EDIT: The question is under the assumption that a started is intended as a 100% hytration one, if you know of a conversion for a 50% hydration starter or pasta madre feel free to pitch in.

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    I've seen a factor of nearly 3 difference in the amount of sourdough starter for the same size loaf, so I doubt you'll find a direct conversion. You would of course have to take into account the water in the starter. It will be interesting to see whether more experienced bakers can point to a rule of some sort, even if it's not a simple one – Chris H May 9 at 18:39
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I've seen numbers ranging between 3x to 20x for going between Madre or starter to yeast.

That said, as @Chris H noted, there are multiple other variables and factors involved here that would make such a formula completely moot. Here are some of the factors:

  • The actual strain of yeast in the starter
  • Relative age of the starter and last feed time
  • Temperature of the starter
  • Length of desired rise/fermentation time
  • Percentage of yeast

In either case of yeast or starter, what you are trying to do is give an environment to the yeast to digest the sugars and produce CO2 to lift the dough for a given time span.

A starting point:

Have a look at the sour dough's recipe's timeline and try to match that with a yeast based recipe of similar time-line.

If it's a few (8-10) hour rise type of bread, then you'll need a good amount of starter (e.g. 80-125g for 600g flour). See other factors above.

If it's a multi-day yeast based recipe, it'll call for 1/10th or 1/20th of the yeast by comparison (.1% to 2%), but if you go with 1/20th of the starter (e.g. 4g), you may not get much activity in the dough. So you're probably better off erring on the high side in those cases.

make sure you adjust for the hydration present in the starter (count the water in it as a part of the water you need for your hydration percentage)

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I am not sure if you will find a formula that simply translates between the two. Store bought yeast (fresh or dry) is nearly 100% yeast but sourdough starter is mostly flour and water. The timelines for sourdough are almost always much longer than with store-bought yeast.

The sourdough recipe I have been using calls for 500g flour and 75g of starter. The starter itself is half flour and half water. I ferment my dough for 12-24 hours at room temp which I would never do with fresh yeast or it would be crawling out of the bowl.

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  • How much fresh yeast do you use in percentage for those loaves? – Lolman May 12 at 13:25
  • I have recipes that call for 2-3 cups of flour which is probably around 250-400g and 2 tsp of yeast. The quantity of yeast is relatively forgiving because it grows so fast, and adding too much or too little, even half as much or twice as much, probably doesn't have much impact on the recipe. The sourdough yeast is much harder to manage and is generally a lot less active. – Uncle Long Hair May 12 at 14:59
  • I'm sorry, I thought we were talking about fresh yeast and not dry yeast. The conversion from dry to fresh is 1 tsp dry=10g fresh. Thus 2 tsp will be 20g thus giving us 20/400=1/20=5% which is more than double the already very high 2% I was talking about. Of course the dough would explode, but it's not the yeast's fault. P.S. I suggest you to get a scale to help you in the kitchen. – Lolman May 12 at 15:49

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