Once you've established a starter that's working well for you, how do you ensure that it stays alive when you can't tend to it?
Frankly, I've left starters in the refrigerator for months, and they've revived with no problem. They're a little sluggish at first, but they still spring back to life after a few feedings. But what about some sourdough insurance? What if you have a starter that seems really, really good, and you don't want to lose it in some terrible refrigerator accident? And what if you want to share your starter with a friend who lives in another state?
The simple answer is that you can dry it.
Drying is simple. Take some of your fully-active starter and spread it as thinly as possible on a piece of parchment paper or a Silpat. The thinner you spread it, the faster it will dry.
Recently, I dried one sample on a Silpat and one on parchment to see which would dry faster. Surprisingly, the one on the Silpat dried faster. The photo above is after a couple hours and the edges were already flaking up. Other surfaces are fine, too: you could even use a dinner plate. But since you're going to spread such a thin layer, you might as well use a large surface.
Leave your drying starter somewhere safe from random spills, pet attacks, and marauding insects. I leave mine inside the oven. Lights off, door closed.
When the starter is completely dry, it will flake off of the sheets. Crumble, crush, mash, or grind the flakes into a powder for easier hydrating later. Store the powder in a sealed container in the freezer.
The dried sourdough starter powder isn't a substitute for dried yeast—sourdough isn't that fast. To use it, you need to hydrate it and feed it and nurture it until you have enough starter to use for your baking. Since it's already active, this isn't like growing your starter from scratch—you should be ready to bake fairly quickly.
Let about a teaspoon of your dried starter come to room temperature, and add it to a tablespoon of room temperature water in a jar. Stir, and let it hydrate fully—just a few minutes. Then add about a tablespoon of flour. You want a thin batter. Cover it and let it sit until you have active bubbles. This should take from a few hours to 12 hours; maybe up to a full day.
When you have lively bubbles, it's time to increase the volume. Add about 2 ounces each of water and flour, stir, and wait until it's bubbling vigorously. This should happen fairly quickly, in a matter of hours, at most. Then add another 4 ounces each of flour and water. That should rise up in the jar within an hour or so. At that point, you're ready to use some of it for baking.
If your starter is more sluggish in coming to life, you can feed at a less accelerated pace and wait for it to regenerate. If you get no bubble activity at all, either your starter didn't survive the drying and freezing, or something went wrong with the re-hydration process. You can try again, making sure that your water isn't too warm and that the starter has reached room temperature. If it still doesn't bubble, it's probably dead. Needless to say, it's a good idea to test your dried frozen starter once in a while to make sure it's still alive.