17.2 The starter

A sourdough starter is a living culture. Once established, I’ve found it to be pretty robust, but not immortal. If you don’t feed it, it will eat everything it can and then, eventually, die. The usual recommendation I see is to double the weight of the starter at feeding time. So if you have 40 g of starter, you’d want to add around 40 g at feeding time. It doesn’t have to be exact, I think what’s more important is keeping track of hydration. But, these two things give you two fixed values to use:

\(X\): The amount of weight that will be added to the culture.
\(H\): The hydration that you’re keeping the starter at.

Final Formulas

The summary version to compute the amount \(F\) of flour and amount \(W\) of water so that you’re adding \(X\) weight at \(H\) hydration is:

Compute flour: \(F = 1 / (1 + H)\)
Compute water: \(W = HF\)

Derivations

To derive those formulas, start from the following two. These should be pretty straightforward. Since we’re adding \(X\) weight, this has to be the combined weight of \(F\) and \(W\). And I already defined \(H\) as the ratio of water to flour.

\(H = W / F\)
\(X = W + F\)

In the first equation, multiply both sides by \(F\) to get \(HF = W\). This will be an important formula later. Then substitute this in for \(W\) in the second equation to get \(X = HF + F\). This simplifies to \(X = F(1+H)\). So if you pick a value for \(X\), you can compute the amount of flour needed by dividing \(X\) by \((1+H)\). So that gets us the “Compute flour” equation above: \(F = 1/(1+H)\).

To get the amount of water, use the first formula we derived when we started this, \(HF = W\). Since we select a total weight and derived the amount of flour, we just need to multiply the amount of flour by the hydration to get the amount of water needed.

Some examples are in the table below. The first time I tried switching my starter to a lower hydration, I used 66%, and it was a pain in the ass to mix. So, I’ve switched to 75% for the time being.

X	H = 0.75		H = 0.85		H = 1.00
(Weight)	F	W	F	W	F	W
40	22.9	17.1	21.6	18.4	20.0	20.0
80	45.7	34.3	43.2	36.8	40.0	40.0
100	57.1	42.9	54.1	45.9	50.0	50.0
150	85.7	64.3	81.1	68.9	75.0	75.0
200	114.3	85.7	108.1	91.9	100.0	100.0
300	171.4	128.6	162.2	137.8	150.0	150.0

17.2.1 Other comments on the starter

What if you need to feed your starter, but you’re not ready/don’t have time to bake, and don’t need any more?
- “Discard” some starter. That is, throw it out.
- There are some easy recipes for discard starter. I haven’t tried them, but sourdough crackers look incredible. I’ve made pancakes and waffle batter with excess starter.
- I usually don’t have this problem, as once the starter is established, it can live for a while in the fridge between feedings. So I just … don’t feed it for up to a week at a time (and it probably can go longer). It’s mainly when you’re just getting the starter going that you’ll have this problem.
How to store your starter:
- Keep it in some sort of jar (preferably glass), and with some sort of a loose covering. Don’t seal it, but don’t leave it completely exposed. I used a 2-cup measuring glass with a rubber lid (but which has holes to pour through).
- I do not recommend a mason jar. You may want to periodically clean the container, and I find cleaning bits of dried starter off the inside of a major jar annoying. I switch the starter to a larger measuring glass (8 cup, which is probably overkill) when I’m bulking it up for a loaf.
To start a sourdough starter:
1. Choose your hydration, and mix the appropriate amounts of flour and water.
2. Let it sit on your counter. Feed it every 12 hours. Eventually you should start to see it making bubbles.
3. Stay on this process for a few days at least, until the starter reliably bubbles/expands after each feeding (it may take a few hours to show).
4. At this point, you can put the starter in the fridge, which will slow the yeast down so that you can go longer between feedings.
I occasionally feed the starter with whole wheat and/or rye flour.