A sourdough starter is a living culture that requires regular attention. To keep it healthy, you mix fresh flour and water into the starter on a regular schedule in a process called feeding. The most common feed ratio is one part starter, one part flour, and one part water by weight, often written as 1:1:1, though other ratios are used to control fermentation speed and flavor. A starter is at peak activity when it has roughly doubled in volume and is full of bubbles; this is the point at which it has the most leavening power and is most useful in a dough. A simple way to gauge readiness is the float test: a small spoonful of starter dropped into a bowl of water will float when it has trapped enough gas to leaven bread. Typical sourdough recipes call for ten to twenty percent of total flour weight in active starter, though this varies with hydration, temperature, and desired fermentation time.
Every feeding produces more starter than you can usually use, which is why bakers talk about discard. Discard is simply the portion removed before feeding, and it is not wasted; it can be used in pancakes, crackers, waffles, and many other recipes. With time, bakers learn to recognize the warning signs of a struggling starter. A starter that is lazy or dormant may need several feedings to revive. Sometimes a dark liquid forms on top of a hungry starter; this is hooch, an alcoholic by-product of yeast metabolism that smells sharp and should be poured off before the next feeding. A stinky starter often signals the same condition and is generally corrected by a thorough feeding. The most common reasons starters fail are underfeeding, inconsistent temperature, and contamination from dirty utensils, all of which are easy to prevent with a little routine care.
One of the great advantages of a sourdough starter is its longevity. A starter can rest in the refrigerator for weeks or even months between feedings if kept cool, and many home bakers establish a weekly baking rhythm that requires feeding only once a week. There is also flexibility in how a starter is built and used. Bakers often maintain a split starter, keeping one culture of white flour and one of rye, since rye is naturally rich in microbes and ferments very actively. A stiff starter, fed at fifty to sixty percent hydration, ferments more slowly and produces more lactic flavor. To accelerate a sluggish starter, a small amount of rye flour added to the next feed can give it a noticeable boost. Building a new starter from scratch typically takes five to fourteen days of daily flour-and-water feedings, with the baker watching for the first signs of bubbles and rise.