The Dead Starter: Pathogen Takeover in Neglected Jars
A sourdough starter is resilient. It can survive being dried, frozen, or forgotten in the back of the fridge for months.
But it is not invincible.
There is a point of no return. A moment when a neglected starter stops being a culture of beneficial microbes and becomes a Petri dish for pathogens.
This transition is chemical. It is driven by a single variable: pH.
The Acid Firewall
A healthy starter is safe because it is acidic.
The Lactic Acid Bacteria (LAB) produce lactic and acetic acids, maintaining a pH between 3.5 and 4.0. This is the Acid Firewall. It is a biological border wall that kills almost every common food pathogen, including E. coli, Salmonella, and most moulds.
As long as the bacteria are fed and active, the wall stands [1].
The Biology of Neglect
When you stop feeding your starter, two things happen:
Starvation: The bacteria run out of maltose. They stop producing acid.
Proteolysis: Desperate for fuel, the bacteria begin to consume the protein (gluten) in the flour. This releases ammonia and other basic compounds as byproducts.
The Result: The pH begins to rise.
As the environment shifts from acidic (safe) back towards neutral (unsafe), the Acid Firewall crumbles. The starter loses its immune system.
The Invasion
Once the pH rises above 4.5, the door opens for invaders that were previously locked out.
1. The Pink Streak (Serratia marcescens) This bacterium thrives in damp, nutrient-rich environments. It produces a bright pink or orange pigment called prodigiosin. Unlike your friendly LAB, Serratia can be pathogenic.
The Sign: Neon pink or orange streaks on the surface or down the side of the jar.
The Verdict: Lethal. Do not scrape it off. The toxins can penetrate the dough. Discard the entire starter and sterilise the jar.
2. The Fuzzy Islands (Mould) Mould spores are everywhere. In a healthy starter, they land on the surface and die, unable to germinate in the acid. In a neglected (high pH) starter, they bloom [2].
The Sign: Furry patches of green, black, or white.
The Verdict: Lethal. Mould roots (mycelium) go deeper than you can see. Discard.
The "Black Liquid" (Hooch)
Do not confuse invasion with hunger.
If you see a layer of dark grey or black liquid on top, this is Hooch. It is alcohol produced by stressed yeast. It indicates severe starvation, but the Acid Firewall may still be intact underneath.
The Fix: Pour off the hooch. Scrape off the top layer. Take a small amount of the cleaner starter from the bottom and feed it 1:5:5. If it rises and smells sour, it is safe. If it smells like rotting meat or has pink streaks, it is dead [3].
Summary
Neglect is not passive; it is an active decay.
By starving your starter, you are dismantling its defences. The best way to keep your culture safe is not to sterilise your kitchen, but to maintain the acidity of your jar. Feed it, and it will defend itself.
References
Gobbetti, M., et al. (2005). Biochemistry and physiology of sourdough lactic acid bacteria. Trends in Food Science & Technology.
Schnürer, J., & Magnusson, J. (2005). Antifungal lactic acid bacteria. Applied and Environmental Microbiology.
Messens, W., & De Vuyst, L. (2002). Inhibitory substances produced by Lactobacilli isolated from sourdoughs—a review. International Journal of Food Microbiology.
