Discarding a portion of your sourdough starter before feeding it fresh flour and water is a standard practice for several critical reasons:
1. Control Growth
Without discarding, your starter would grow exponentially. Because sourdough is fed based on ratios (typically 1:1:1 or similar), keeping all the old starter would require you to add increasing amounts of flour and water with every feeding. Within a week of daily feedings, you could easily end up with gallons of starter.
2. Maintain Microbial Balance
Prevent Underfeeding: A large population of yeast and bacteria requires more food. If you have too much starter but only add a small amount of flour, the microorganisms will quickly run out of nutrients, leading to a weak or sluggish starter.
Manage Acidity: As the starter ferments, it produces acids (lactic and acetic) and alcohol byproducts. If you never remove old starter, these acids build up, which can eventually become toxic to the yeast and negatively affect the flavor and rise of your bread.
3. Ensure Predictability
Regularly discarding helps maintain a consistent environment for the wild yeast and bacteria. This ensures your starter peaks at a predictable time, making it much easier to schedule your baking sessions.
Practical Tips
Don't actually throw it away: "Discard" is often just a term for the portion removed. You can save it in a jar in the refrigerator to use in recipes like pancakes, crackers, or muffins.
"No-Discard" Methods: Some bakers use a "scraping" or "micro-starter" method where they keep only a small amount (e.g., 20g) in the fridge and build up only what they need for a specific bake, effectively eliminating daily waste.
A sourdough starter deflates after feeding as part of its natural fermentation cycle. This occurs for two primary reasons: the exhaustion of its food supply and the weakening of its internal structure.
1. Depletion of Food
After you feed your starter fresh flour and water, the wild yeast and bacteria begin consuming the available sugars and starches.
Gas Production: As they "eat," they produce carbon dioxide (CO2) as a byproduct. This gas gets trapped in the starter's gluten network, causing it to rise and expand.
Exhaustion: Once the microorganisms have consumed all the available nutrients, gas production slows down significantly. Without new gas being generated to maintain the pressure, the starter begins to sink.
2. Breakdown of Gluten Structure
A starter's ability to stay risen depends on its gluten network—the "structure" that holds the gas bubbles in place.
Acid Accumulation: As the bacteria in the starter ferment the flour, they produce lactic and acetic acids.
Gluten Degradation: Over time, these acids and certain enzymes begin to break down the gluten proteins. As this structure weakens, it can no longer hold the weight of the starter or trap the gas effectively, leading to a collapse.
Timing and Environment
When it happens: Most starters will peak and begin to deflate 4 to 12 hours after feeding, depending on the temperature and the feeding ratio used.
Temperature: Warmer environments speed up the microorganisms, causing the starter to rise and deflate much faster than in a cool kitchen.
Hydration: Thinner, runnier starters often deflate faster because their weaker gluten structure cannot hold gas as long as a thicker, stiffer starter.
Note: Seeing your starter deflate is actually a positive sign; it proves that your yeast is active and healthy. For the best results in bread baking, you should ideally use the starter when it is at its peak volume, just before it begins to fall.