The Science of Sourdough Fermentation: A Plain-English Explanation

Short answer: sourdough fermentation is a partnership between wild yeast (which produces gas) and lactic acid bacteria (which produce flavor compounds). Both feed on sugars in flour and create CO2, alcohol, lactic acid, and acetic acid.

The two main microbes

Sourdough culture has:

Wild yeast (Saccharomyces, Candida, others)

• Eats sugars
• Produces CO2 (rise)
• Produces alcohol (ethanol — burns off in baking)
• Smells yeasty

Lactic acid bacteria (LAB)

• Eats sugars
• Produces lactic acid (yogurt-like flavor)
• Produces acetic acid (vinegar-like flavor)
• Smells tangy

Both are present in sourdough at all times, but their balance shifts based on conditions.

What sourdough microbes feed on

Flour contains:

• Starch (long sugar chains)
• Enzymes (amylase) that break starch into sugars
• Maltose (the main sugar yeast/bacteria can use)
• Glucose, fructose (smaller amounts)

Wild yeast prefers maltose. Bacteria prefer glucose. They work in tandem because of these different preferences.

What they produce

The byproducts:

How temperature shifts the balance

For "tangy" sourdough: ferment cold and slow. For "mild" sourdough: ferment warm and fast.

How a starter develops

When you start a sourdough from flour and water:

Day 1–3: random microbes from flour and air

• Bacteria that can survive low pH
• Wild yeast spores germinate
• Often initial bloom of unwanted bacteria

Day 3–7: pH drops as LAB produce acid

• Unwanted bacteria die off (can't handle low pH)
• LAB and wild yeast establish dominance
• Activity stabilizes

Day 7–21: stable culture

• Consistent rise
• Reliable timing
• "Mature" starter

Day 30+: complex flavor

• Multiple LAB strains
• Multiple yeast strains
• Distinct character per location

Why every starter is unique

Your starter is a unique microbial community based on:

• The flour you use
• Your kitchen environment
• Your feeding schedule
• The temperature

Two bakers using the same recipe in different kitchens will have different starters with subtly different flavor profiles.

What "feeding" the starter does

Feeding (adding flour and water):

• Provides fresh sugars for microbes
• Dilutes the acidic environment
• Supports microbe reproduction
• Refreshes the culture

A well-fed starter has:

• Active yeast (rises and produces CO2)
• Healthy bacteria (produces flavor)

A starved starter has:

• Hungry, slow yeast
• Stressed bacteria
• Excess acid (no fresh food)

The role of salt

Salt:

• Slows fermentation (mild inhibitor of yeast and bacteria)
• Tightens gluten (provides structure)
• Adds flavor

Without salt: dough ferments too fast, structure is weak. Too much salt (>2.5% of flour): yeast severely inhibited, slow rise.

What happens during bulk

During the 4–6 hour bulk:

• Yeast multiplies
• Gas (CO2) accumulates
• Gluten develops
• Bacteria produce acid

The dough rises 50–60% as gas pockets form within the gluten network.

What happens during shape

Shaping:

• Degasses the dough partially
• Builds surface tension (gluten alignment)
• Doesn't reset fermentation
• Sets up the final proof

After shaping, the dough continues to ferment.

What happens during proof

Final proof (1.5–2 hours):

• Yeast continues producing gas
• Gluten relaxes (allows expansion)
• Surface develops a slight skin
• Dough domes and rises

The dough is ready when:

• It's risen 50%
• Finger-dent springs back slowly
• Smells fermented

What happens in the oven

The oven phase:

Minutes 0–5:

• Yeast becomes very active (gas production accelerates as it warms)
• Steam expands existing gas pockets
• Massive oven spring (initial rise)

Minutes 5–15:

• Yeast dies (above 140°F)
• Crust starts to form
• Crumb sets

Minutes 15–25:

• Maillard reactions create crust color
• Internal temp rises
• Bread becomes golden

Minutes 25–45:

• Internal temp reaches 205°F
• Bread is done

Why sourdough has unique flavor

The combination of:

• Multiple yeast strains
• Multiple bacterial strains
• Acid byproducts (lactic + acetic)
• Long fermentation (vs. yeast bread, which is hours)

Creates flavor compounds (esters, organic acids, aromatic chemicals) that yeast bread doesn't have.

A simplified mental model

Think of sourdough as:

• A garden of microbes
• That you feed and water
• That produce gas (rise) and flavor (acid)
• And bake into bread

The microbes do the work. You provide the conditions.

A scientific deep-dive (optional)

If you want to go deeper:

• Read "The Bread Builders" by Daniel Wing
• Read "Tartine Bread" by Chad Robertson (less science, more art)
• Search "sourdough microbiome" on Google Scholar

The science is fascinating but not required for great bread.

A final note

You don't need to understand the microbiology to bake great sourdough.

But understanding helps when:

• Things go wrong (you know what to adjust)
• You want to manipulate flavor (cold vs warm)
• You want to maintain a healthy starter

The science is the foundation; the art is the practice.