Sourdough Starter Guide 🦠

The Science of a Sourdough Starter

A sourdough starter is a symbiotic culture of bacteria and yeast (SCOBY). The wild yeast (primarily Saccharomyces cerevisiae and Candida milleri) is responsible for producing carbon dioxide, which makes your bread rise. The lactic acid bacteria (LAB) produce lactic and acetic acids, which give the bread its signature sour flavor, lower the pH to prevent mold, and break down gluten to make the bread more digestible.

Why Discard?

Many beginners hate throwing away flour. However, discarding before feeding is mathematically necessary. If you don't discard, you must feed an exponentially growing amount of flour to maintain the proper food-to-microbe ratio. If you keep 100g of starter, you must feed it 100g of flour. The next day, you have 300g of starter, which requires 300g of flour, and so on. Discarding keeps the culture at a manageable size and prevents the acidity levels from becoming too toxic for the yeast to survive.

Understanding Hydration in Starters

This guide assumes a 100% hydration starter, which is the standard for most modern recipes. This simply means you feed it equal parts by weight of water and flour. Because water is denser than flour, a 100% hydration starter will look like thick pancake batter. If you use volume measurements (like cups), equal parts water and flour will result in a hydration closer to 200%, which is too watery and will struggle to trap gas bubbles.