# Sourdough Starter: A Microbiologist's Notes

I'm a microbiologist who bakes. These are the notes I wish someone had given me — the science behind what actually matters and what's ritual.

## What's actually happening

Your starter is an ecosystem of ~30 species competing for resources. Two groups dominate:

**Wild yeast** (*Saccharomyces* and *Kazachstania* species) — produce CO₂ (leavening) and ethanol. They prefer slightly acidic environments (pH 4–5) and thrive at 25–28°C.

**Lactic acid bacteria** (LAB, primarily *Lactobacillus* and *Leuconostoc*) — produce lactic acid and acetic acid. These create the sour flavor and, critically, lower the pH to suppress pathogenic bacteria. The starter is self-preserving.

The ratio of yeast to LAB stabilizes around 1:100. This sounds yeast-poor, but each yeast cell produces far more CO₂ than each bacterium produces acid. The system is balanced.

## What actually matters

**Hydration ratio** (flour:water by weight): This is the single most impactful variable.
- 100% hydration (equal parts): More lactic acid, milder sour, looser texture
- 60% hydration (stiff starter): More acetic acid, sharper tang, slower fermentation

**Temperature**: LAB and yeast have different optimal ranges.
- 25–28°C: Yeast-dominant. More rise, less sour.
- 30–35°C: LAB-dominant. More sour, less rise.
- This is why the same starter behaves differently in summer and winter.

**Flour type**: Whole grain flour has more wild microbes on the bran and more nutrients. Use whole wheat or rye for the first week to establish the culture faster, then switch to whatever you prefer.

**Feeding schedule**: When the starter runs out of simple sugars, LAB switch to metabolizing amino acids, producing unpleasant flavors (nail polish remover = ethyl acetate). Feed before this happens — when the starter has risen and just begins to fall.

## What doesn't matter

- **The water**: Chloramine in tap water is a non-issue at typical municipal concentrations. LAB are far more chloramine-tolerant than the literature suggests. I've tested this with plate counts. Use tap water.
- **The container**: Glass, plastic, ceramic — doesn't matter. The microbes don't care.
- **"Catching" wild yeast from the air**: Your flour already contains the microbes. The air contributes negligibly. You don't need to leave it uncovered.
- **Stirring direction**: Obviously.

## The discard problem

Most recipes say "discard half, feed half." People feel guilty about waste. Here's the biology: you're diluting to keep the population in exponential growth phase. Stationary phase cultures (no dilution) accumulate waste products and metabolic byproducts that change flavor unpredictably.

Use the discard: pancakes, crackers, pizza dough, compost. But don't skip the step.

## Troubleshooting with actual science

| Symptom | Likely cause | Fix |
|---------|-------------|-----|
| Rises fast, falls fast | Yeast overpopulation, low food | Use less starter per feeding (1:5:5) |
| Smells like acetone | Starvation — amino acid metabolism | Feed immediately, more frequently |
| Hooch (dark liquid on top) | Ethanol layer — extended starvation | Pour off, feed, not harmful |
| Pink/orange streaks | *Serratia marcescens* contamination | Discard. Start over. |
| No rise after day 4 | Normal. *Leuconostoc* die-off. | Keep feeding. *Lactobacillus* will establish by day 7–10 |

## The patience principle

Days 1–2: Explosive growth. This is *Leuconostoc*, a fast-growing LAB. It produces gas. You think it's working. It is, but these are the opening act.

Days 3–5: Nothing. *Leuconostoc* acidified the environment past its own tolerance. It dies off. The starter looks dead. This is the most common point of abandonment.

Days 6–10: *Lactobacillus* species, which are acid-tolerant, slowly establish. Yeast populations grow. The starter begins to rise and fall predictably.

Day 14+: The ecosystem stabilizes. Congratulations, you've cultivated a self-regulating microbial community that, with minimal maintenance, will outlive you.