While many breads are made only with added yeast, traditional sourdough bread uses sourdough starter to help the dough rise -- and gives fresh, homemade loaves that elusive tang and deep, rich flavor. Sourdough starter uses wild yeast, yeast found naturally in the environment, which is what gives the bread such deep, rich flavor. Proofing sourdough starter means you need to give it enough time to ferment and strengthen, before using it for baking.
In baking, proofing often refers to the final rise before a loaf of bread is baked. However, it more generally refers to fermentation, which is how it functions in relation to starter, as starter does not require a final rise. Proofing sourdough starter simply means letting the starter ferment and caring for the starter so that it continues to be usable.
If your starter doesn't start bubbling by the third or fourth day, add around 1/8-teaspoon, of commercial yeast to help the starter along. Make sure you are storing the starting in a warm location, but not one that is too hot. The ideal temperature is between 70 to 75 degrees F.
If liquid has collected on the top of the starter, but it otherwise looks and smells fine, pour off the excess liquid. Reduce the amount of liquid used when feeding the starter.
Once its established, sourdough starter is easy to take care of. Unless you are baking everyday, store the starter -- tightly sealed -- in the fridge. The cooler temperature means that it will require less feeding. Feed the starter by discarding half the starter, then stirring in 1 cup of flour and a 1/2-cup of water every week.
If you want to feed your starter even less often, add 2 cups of flour to the starter when giving it its final feeding before storage. This will produce a thicker starter that will keep for longer in the fridge.
Dry starter for even longer-term storage. Spread the starter out in a thin layer on a silicone mat and let air dry. Once fully dried, break into pieces and store in an airtight container. To use dried starter, dissolve 1/4-cup of starter flakes in a 1/2-cup of water and add 1 cup of flour. Continue discarding and feeding the starter until it is once again active -- bubbling and with a fruity smell.