How Temperature Affects Sourdough Fermentation

The Basic Principle

Fermentation is a biological process driven by yeast and bacteria. Like all living things, they're more active when warm and sluggish when cold.

As a rule of thumb: every 5°C (9°F) change roughly doubles or halves fermentation speed.

Temperature Zones

What Actually Happens

Yeast Activity

Wild yeast is most active between 24-27°C (75-80°F). Below this, it slows down. Above 35°C (95°F), yeast begins to die.

Bacteria Activity

Lactic acid bacteria (LAB) have a wider comfortable range. They produce two types of acid:

• Lactic acid (mild, creamy) – dominant at warmer temperatures
• Acetic acid (sharp, vinegary) – dominant at cooler temperatures

This is why cold-fermented bread tastes tangier—more acetic acid develops.

Timing vs Temperature

Approximate bulk fermentation times at different temperatures:

These are estimates. Your starter's strength also affects timing.

Controlling Temperature

Water Temperature

The easiest lever. Use warmer water (30°C/85°F) to raise dough temperature, cooler water (18°C/65°F) to lower it.

A simple formula: desired dough temp × 2 = flour temp + water temp. So if you want 24°C dough and your flour is 20°C, use 28°C water.

Finding Warm Spots

• Oven with light on (often 27-30°C)
• On top of refrigerator
• Near a radiator or heating vent
• In a microwave (unplugged) with a cup of hot water

Finding Cool Spots

• Basement or cellar
• Inside closed oven (no light)
• Near exterior wall in winter
• Refrigerator for dramatic slowdown

Seasonal Adjustments

Summer (Hot Kitchen)

• Use cold water straight from fridge
• Reduce starter amount (15% instead of 20%)
• Shorten bulk fermentation
• Consider overnight room-temp bulk + morning shaping
• Cold retard is almost mandatory

Winter (Cold Kitchen)

• Use warm water (30°C/85°F)
• Find warm spots for proofing
• Extend bulk fermentation significantly
• Use more starter if needed (25%)
• Be patient—cold kitchens just take longer

Effect on Bread Quality

Too Cold (Under-Fermented)

• Dense, gummy crumb
• Poor oven spring
• Tight, uneven crumb
• Bread may tear in unexpected places

Too Warm (Over-Fermented)

• Flat loaf, spread sideways
• Weak structure
• Overly sour taste
• Large irregular holes

Dough Temperature vs Air Temperature

What matters is the dough temperature, not the room temperature. Dough has thermal mass—it changes temperature slowly.

If your kitchen is 21°C but your dough started at 28°C (from warm water), fermentation will be faster than recipes suggest until the dough cools down.

Measuring Dough Temperature

An instant-read thermometer is invaluable:

• Measure dough temperature after mixing
• This tells you how fast fermentation will proceed
• Adjust expectations accordingly

The Desired Dough Temperature (DDT)

Many professional recipes specify a "desired dough temperature"—the target temperature right after mixing. Common targets:

• Standard: 24°C (75°F)
• Quick fermentation: 27°C (80°F)
• Slow fermentation: 21°C (70°F)

Temperature and Flavor

Slower, cooler fermentation develops more complex flavors because:

• More time for enzyme activity
• Greater acetic acid production
• More complete fermentation of sugars

Hot, fast fermentation produces bread with less complex flavor—fine for same-day bakes, but not artisan quality.

Practical Application

Scenario: Recipe Says 4 Hours, Your Kitchen is Cold

1. Measure your dough temperature
2. If significantly below 24°C, expect longer fermentation
3. Find a warmer spot, or just wait longer
4. Judge by dough appearance, not clock

Scenario: Recipe Says 4 Hours, Your Kitchen is Hot

1. Use colder water in the mix
2. Check dough earlier (3 hours)
3. Be ready to shape sooner
4. Consider refrigerating if fermentation is racing

Key Takeaways

• Temperature is the master variable—learn to control it
• Warmer = faster fermentation = milder flavor
• Cooler = slower fermentation = more complex flavor
• Use water temperature to adjust dough temperature
• Judge fermentation by dough appearance, not time
• The refrigerator is your pause button