The Starter Invaders: Mold, Kahm, and Pink Streaks
A sourdough starter is a fortress. It is built on a simple, brutal principle: acid warfare.
The Lactic Acid Bacteria (LAB) in your jar produce lactic and acetic acids to drive the pH down to a hostile 3.5–4.0. This "Acid Firewall" is designed to kill almost everything that tries to invade. It is why a jar of wet flour can sit on your counter for decades without becoming a biohazard.
But sometimes, the wall is breached.
When you open your jar to find fuzzy islands, wrinkled skins, or neon streaks, it is a sign that the pact has been broken. The acid was not strong enough, or the invader was too resilient.
This is your field guide to the enemy.
1. The False Alarm: Hooch (Grey/Black Liquid)
Diagnosis: Not an infection.
The Look: A layer of grey, black, or brownish liquid floating on top.
The Biology: This is alcohol (ethanol) produced by yeast under stress. It means your starter is starving. The yeast has consumed all the sugar and is now dormant.
The Fix: Pour it off (or stir it in for a boozier flavour) and feed your starter immediately. It is not dead; it is just hungry [1].
2. The Tourist: Kahm Yeast (White, Wrinkled Skin)
Diagnosis: Annoying, but usually harmless.
The Look: A thin, matte, white film that looks like crepe paper or delicate lace. It is not fuzzy.
The Biology: "Kahm" is not a single species but a colloquial term for aerobic surface yeasts (often Debaryomyces, Mycoderme, or Pichia). They thrive when the starter is weak, under-acidified, or exposed to too much oxygen. They consume the acid your LAB worked so hard to create, potentially weakening the firewall [2].
The Fix:
Scrape off the skin carefully.
Transfer a small amount of clean starter from the very bottom (underneath the infection) to a fresh, sterilised jar.
Feed aggressively (1:3:3 ratio) to re-acidify the environment quickly.
3. The Biohazard: Pink/Orange Streaks
Diagnosis: DANGER. DO NOT EAT.
The Look: Streaks of pink, orange, or neon red running through the dough or on the surface.
The Biology: This is often Serratia marcescens, a bacterium that produces a bright red pigment called prodigiosin. Unlike the beneficial bacteria in your starter, Serratia can be pathogenic, causing infections in humans [3]. It thrives in damp, nutrient-rich environments (like your bathroom... or a weak starter).
The Fix: Discard, there is no fix. The toxins can persist even after baking. Dispose of the entire starter immediately. Sterilise your jar with boiling water and bleach before starting over. Do not try to scoop it out; the microscopic colony has likely spread further than you can see.
4. The End Game: Mould (Fuzzy/Green/Black)
Diagnosis: FATAL.
The Look: Fuzzy, hairy patches in green, white, black, or blue.
The Biology: Common bread moulds (Penicillium, Aspergillus). While some are used to make cheese or penicillin, in a starter, they are a sign of total ecosystem collapse. Mould roots (mycelium) can penetrate deep below the visible surface, carrying mycotoxins with them [4].
The Fix: Discard. Do not scrape. Do not save "just a little bit." The presence of mould means the Acid Firewall has failed completely. The culture is compromised.
How to Strengthen the Walls
The best defence against invasion is a strong offence.
Maintain Acidity: A weak, hungry starter has a higher pH, making it vulnerable. Regular feeding keeps the acid production high.
Clean the Sides: Mould loves the dried, crusty flour on the rim of the jar where the acid doesn't reach. Scrape down your jar after every feeding.
The "Lid" Protocol: Kahm yeast needs oxygen. If you are prone to surface skins, ensure your lid is tight enough to limit airflow but loose enough to let CO2 escape (or use an airlock).
Your starter is a living city. Most of the time, it can defend itself. But when the neon flags of Serratia or the fuzz of mould appear, it is nature's way of telling you the city has fallen. Listen to it.
References
Gobbetti, M., & Gänzle, M. (Eds.). (2012). Handbook on Sourdough Biotechnology. Springer Science & Business Media.
Fleet, G. H. (1992). Spoilage yeasts. Critical Reviews in Biotechnology.
Hejazi, A., & Falkiner, F. R. (1997). Serratia marcescens. Journal of Medical Microbiology.
Legan, J. D. (1993). Mould spoilage of bread: the problem and some solutions. International Biodeterioration & Biodegradation.
Last updated: 31 December, 2025