Acetic vs. Lactic Acid in Sourdough: The Flavor Lever

Sourdough's flavor comes mostly from two acids: lactic acid and acetic acid. Both are produced by bacteria during fermentation, but in different ratios depending on conditions. Understanding the lever lets you steer toward mild yogurt-like flavors or sharp vinegar-like ones.

Lactic acid

• Mild, smooth, yogurt-like, slightly sweet
• Produced abundantly at warm temperatures (75–85°F)
• Produced abundantly in wet doughs (high hydration)
• Produced abundantly in fast fermentations
• Easy to taste in fresh starter

Acetic acid

• Sharp, vinegary, "sour" in the strong sense
• Produced abundantly at cool temperatures (55–70°F)
• Produced abundantly in dry doughs (stiff starters)
• Produced abundantly in slow fermentations
• The hallmark of "very sour" sourdough

The key insight

Same starter, different conditions = very different flavor.

A 100% hydration starter at 80°F → mostly lactic, mild

A 50% hydration starter at 60°F → mostly acetic, sharp

The bacteria are the same. The conditions are different.

How to make milder sourdough

If your bread is too sour:

• Use a wet starter (100% hydration or higher)
• Bulk ferment warm (78–82°F)
• Use shorter fermentations
• Skip cold retards (or keep them short, 6–8 hours)

How to make sharper sourdough

If your bread is too mild:

• Build a stiff levain (1:2:1 starter:flour:water)
• Bulk ferment cooler (68–72°F)
• Long cold retards (24+ hours)
• Add 10–25% rye flour (boosts acetic acid production)

What controls the acid balance

Three main factors:

1. Temperature

Warm = lactic. Cool = acetic.

This is the biggest lever. A 10°F temperature change shifts the acid balance noticeably.

2. Hydration

Wet = lactic. Dry = acetic.

Wet doughs let lactic-acid bacteria thrive. Dry doughs favor acetic-acid bacteria.

3. Time

Short = milder. Long = sharper.

The first 8 hours are mostly yeast and lactic acid. Acetic acid accumulates over longer periods.

The combinations

For each goal, here's the recipe:

Maximum mild

• 100% hydration starter
• 80°F dough temperature
• 4-hour bulk
• No cold retard
• White flour
• Result: gentle, almost sweet sourdough

Balanced

• 100% hydration starter
• 75°F dough temperature
• 5-hour bulk
• 12-hour cold retard
• 10% whole wheat
• Result: classic sourdough flavor

Maximum tang

• 50% hydration starter
• 70°F dough temperature
• 7-hour bulk
• 24-hour cold retard
• 25% rye flour
• Result: pronounced sour, almost vinegary

Why young starters are mild

A new starter (under 2 months) tends toward mild flavor. The bacteria are establishing, and acetic-acid producers take longer to become dominant.

If you want a sour bread quickly with a young starter, lean on the recipe levers (cold, dry, long) rather than waiting for the starter to mature.

A flavor experiment

Make the same dough recipe twice on the same day:

• Loaf A: 80°F dough, 4-hour bulk, no cold retard
• Loaf B: 70°F dough, 6-hour bulk, 24-hour cold retard

Same flour, same starter. Bake side by side. Taste comparison should be obvious.

This one experiment teaches more about sourdough flavor than any blog post.

The "tartine" style

The famous Tartine sourdough is mostly lactic — relatively warm, fast, wet. Mild flavor, very open crumb. That's by design.

The "rustic country sour" style is acetic-leaning — cooler, slower, includes rye, longer cold retard. Sharp flavor, slightly tighter crumb.

Both are valid sourdough. Different goals.

What about pH?

A finished bread's pH is around 4.0–4.5. The lower the pH, the more sour the perceived taste. Both acids contribute to lowering pH, but acetic acid is perceived more strongly.

You don't need a pH meter. Taste tells you everything.

The myth of "real sourdough"

Some bakers insist real sourdough must be very tangy. Others insist real sourdough should be mild with subtle complexity.

Both are real sourdough. The difference is how the dough was managed. Neither is more "authentic" than the other.

A controllable variable

Once you understand the acid lever, sourdough flavor stops being random. You can dial in mild bread for guests who don't like tang. You can dial in sharp bread for sourdough purists. Same starter, different process.

This is what experienced bakers do without thinking. It's a learnable skill, not a mystery.