Questions regarding how to convert one kind of starter into another are frequently asked on The Fresh Loaf. The easy answer is to just take "a little bit" of seed starter and add enough flour and water to make a mixture of the desired thickness. This is fine and it generally works very well. However, sometimes a recipe calls for a precise hydration level levain and changing this, even a few percentage points, will make the dough consistency quite different from that intended by the formula's author. For those times, one needs to be more precise in making up the levain.
To convert a starter of one hydration to a starter of another hydration - For example, if you have a 50% hydration starter and want to build a 100% hydration starter from it.
Here's a general method for a precise conversion:
First, you need to know four things:
1. What is the hydration of your seed starter?
2. What is the hydration of your final starter?
3. How much of the total flour in your final starter comes from your seed starter?
4. How much (weight) final starter will you be making?
Second, you need to calculate the total amount of flour and the total amount of water in your final starter.
Third, you need to calculate the amount of flour and the amount of water in the seed starter.
Fourth, you can now calculate the ingredients of your final starter. They will be:
1. Seed starter
2. Flour (from seed starter plus additional)
3. Water (from seed starter plus additional)
So, let's see how this method works with some specific assumptions.
The four things you need to know:
Assume you have a 50% hydration seed starter that you want to use. Assume you want to make 100 g of a 100% hydration starter. And assume you want the seed starter to provide 25% of the total flour in the final starter.
Note: Using "Baker's Math," Flour is always 100%, and all other ingredients are proportionate to the flour. So, in a 50% hydration mix, the water is 50% (of the flour, by weight). If hydration is 125%, the water is 125% (or 1.25 times) the flour.
To calculate the total amount of flour and water in your final starter:
Flour (100 parts) + Water (100 parts) = 100 g
So, the 100 g of starter is made up of 200 "parts." The weight of each part is calculated by dividing the total weight by the number of parts. So, 100 g /200 parts = 0.50 g. This number is sometimes called "the conversion factor."
Then, since there are 100 parts of flour, its weight is 100 parts x 0.5 g = 50 g.
The total water in the final dough is 100 parts x 0.5 g = 50 g.
To calculate how much flour will come from the seed starter and how much will be added to make the final starter:
We now know that the total flour in the final starter will be 50 g. But we decided that 25% of this flour is going to come from the seed starter. This means that the seed starter must contain 50 g x 0.25 = 12.5 g of flour, and the flour added to this to make the final starter will be 50 g - 12.5 g = 37.5 g.
To calculate the total weight of the seed starter and the weight of water in the seed starter:
We now need to calculate how much seed starter it takes to provide 12.5 g of flour, and how much water is in this amount of seed starter.
If the seed starter is 50% hydration, it contains 100 parts of flour and 50 parts of water. We know then that the amount of water is 50 parts water/100 parts flour = 0.5 parts of the flour. Since we already know that the flour has to weigh 12.5 g, then the water must weigh 12.5 x 0.5 = 6.25 g and the total weight of the seed starter is the sum of the water and flour or 12.5 g of flour + 6.25 g of water = 18.75 g.
To calculate the weight of water that must be added to the seed starter to make the final starter:
Now we can calculate how much water must be added to the seed starter to make the final starter. It is the total water in the final starter minus the water in the seed starter or 50 g - 6.25 g = 43.75 g.
Now we know "everything!" To make 100 g of 100% hydration starter, beginning with a 50% hydration seed starter, we would mix:
1. 18.75 g Seed Starter.
2. 37.5 g Flour
3. 43. 75 g water
This method can be used to build any amount of starter of any hydration using a seed starter of any (known) hydration.