25-200gramsflour(equal to or greater than the amount of starter, and equal to the amount of water)
25-200gramsfiltered water(equal to or greater than the amount of starter, and equal to the amount of flour)
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Measure 1 tablespoon (approx 25g) of sourdough starter into a bowl on a kitchen scale. Note the amount by weight in grams.
To maintain a small starter: Add the same amount of filtered water by weight to the starter. Stir to combine. Then add flour and mix until no dry spots or lumps remain. Let feed at room temperature for an hour or two, then transfer to the fridge.
To scale your starter up for baking
For a mellow starter (levain): Measure 1 tablespoon (25g) mature sourdough starter into a bowl on a kitchen scale. Add 75g-200g water (depending on how much starter your recipe calls for) and stir to combine. Then, add the same amount of flour by weight as you did water. Stir until no dry spots or lumps remain. Cover and let feed at room temp until it passes the float test.
For a sour starter: Transfer all of your small, mature starter (75g) to a bowl. Add 75g water, stir to combine. Then add 75g flour and stir until no dry spots or lumps remain. Cover and let feed at room temp until it passes the float test.
How quickly your starter feeds and when it's ready to bake with will depend on the ambient temperature and the temperature of your starter, water, and flour. Cooler ingredients = slower feeding. Room temperature ingredients = faster feeding. You can always check if it's ready by dropping a small amount in a glass of water. If it floats, it's ready to bake with.
To calculate how much flour and water to add to your starter when you want to scale it up, take however much starter your recipe calls for, divide it in half and add 20. That number is the amount of water and amount of flour you should feed 1 tablespoon of starter with. You will use most of the starter in your bake, and have about 25-30g of starter left to feed. It will become your new starter.