OK, so someone (maybe me!) gave you some sourdough starter and it is sitting in your fridge. You might want to use it someday… like maybe soon… or maybe not… to make bread. In the meantime, you don’t want it to die. (If you do want it to die, that is OK. Just throw it out.) You also don’t want to wade through that long post about natural leavening. Well, you have come to the right place.
How long will it keep in the fridge? I frequently go two weeks between feedings and I’ve never had a problem. My guess is that 3 or even 4 weeks would be fine.
So here is the quick-and-not-very-dirty. You need to feed it, let it go through a growth cycle, then feed it again and stick it back in the fridge where it will go through another growth cycle, veerrrry slowly, which is what it is doing right now in your fridge, hopefully.
To feed it, put a bowl on your scale, zero the scale and dump your starter in. You are going to roughly double it, by weight, and then let it sit for about 12 hours (like maybe 8-16 hours… I just made that up). I usually have about 450 grams (450g) of starter sitting in my fridge, but you may have less. Let’s say your starter is 243g. To roughly double it, add 100g of white flour (I use Trader Joe’s All-Purpose Flour), and 100g of water. Mix it up with a rubber spatula, cover it with plastic wrap, and leave it in a warm place like a countertop or the top of your fridge. It will expand some, so if it’s close to the top of the bowl, put it in a bigger bowl.
Leave it all day long, or overnight. Hopefully when you come back to it, it will be nice and bubbly, like this:
Now you need to feed it and put it back in the fridge. Let’s say that, like me, you want to have about 450g in your fridge for next time. Take about 250 of the starter, put it the container you’ll use (I use a mason jar), add 100g of flour and 100g of water, mix it with a rubber spatula, cover with plastic wrap and stick it in your fridge. Done. You can stop reading here.
Water temperature: for this last round, just use cold water. For the first round, where you’re leaving it out, you can use lukewarm water, though you are probably fine without it. In my big honking microwave, I warm the water 10 seconds for every 100g of water. (Your measuring cup may have “ml” markings on it; for water, grams and milliliters are the same. If not, a cup is about 250ml.) Of course, you know not to use warm tap water for cooking, right?
What if your starter is barely bubbly at all? You might want to take it through a few cycles out of the fridge. So instead of sticking it back in fridge in the last step, leave it out for another 12 hours. Hopefully it will be enthusiastic by then. If not, you can try another cycle or two. If you’re leaving it out, you might want to warm the water up when you feed it.
What if you want to leave it out of the fridge but you only want to feed it once a day, not twice? Then instead of doubling it, you want to quadruple it. So if you start with 200g of starter, you want to add 600g of flour-water mix, so add 300g of flour and 300g of lukewarm water.
Why plastic wrap? The starter expands and makes gases. If your seal it tightly, it can blow up your container.
What do you do with the unused starter? Really, it’s just flour and water (albeit magic flour and water), so don’t feel bad about throwing it away. But there are quite a few good things you can make with it, too!