Lots of beginning bakers (especially those fooling around with sourdough or natural levains) often start with recipes that give sourdough amounts by volume. For example:
To feed your starter, take 1/4c of starter and feed it 1c flour and 1c water.
The issue that I have with this is that using this starter (a very liquid starter) is this: while easy to remember the proportions for feeding, it's much harder to tell when the sourdough starter is fully ripe and ready to use.
Just so we're clear: feeding 1c flour and 1c water does not make a 100% hydration starter. Starter hydration percentage always represents the ratio of flour to water by weight, not volume.
- 1c of white flour weighs typically around 125g, but can weigh 120-140g, depending on how you measure.
- 1c of water weighs around 236g
This means that a starter fed equal volumes will result in a ~188% hydration starter! (236g water/125g flour)
So what's the problem with this? Here's a very common sourdough recipe that I was investigating:
1 cup active starter (fed using 1c flour and 1c water)
3.5 cups flour
1 cup water
1 tsp salt
Let's translate this common recipe to a weight-based formula:
1/2c flour + 1/2c water = 62.5g + 118g = 180.5g
3.5c flour @ 125g/cup = 437.5g
1c water = 236g
1 tsp salt = 7g
So how does this help? What does this tell us?
First, let's total the flour in the recipe (62.5 + 437.5 = 500g) and water (118+236 = 354g)
354 / 500 = .708, or 70.8% hydration, which is slightly on the high side for a sourdough bread, which tends to get wetter and floppier the longer it ages.
Second, we are using 180.5 / 437.5 = 41% of the final flour in the recipe in starter, which is somewhat high, especially with a liquid starter...this means it will probably ferment really fast, perhaps more quickly than desired for a sourdough, which like all doughs, will develop deeper flavor during longer fermentations.
BTW, if you want to calculate dough hydration easily, Joshua Cronemeyer's online calculator is one of the easiest.