"Sourdough discard" refers to a portion of a sourdough starter that is removed and not used in the final bread or recipe. This removal is typically done during the regular feeding and maintenance of a sourdough starter. Here's how it works:

To keep a sourdough starter healthy and active, it needs to be regularly fed with a mixture of flour and water. This feeding provides fresh nutrients for the wild yeast and bacteria in the starter. During each feeding, bakers often discard a portion of the existing starter before adding fresh flour and water. This discard helps maintain a consistent ratio of fresh ingredients to the existing culture.

There are a few reasons for discarding part of the starter. First, it helps control the overall quantity of starter, especially if you don't bake with it every day. Second, it ensures that the ratio of fresh flour to old starter remains balanced, preventing the accumulation of byproducts that might affect the flavor and performance of the starter. While the term "discard" might suggest waste, many bakers find creative ways to use the discarded portion rather than throwing it away. Sourdough discard can be used in various recipes so none has to go to waste!