The Science of Open Crumb: Why Hydration Is Key

If you’ve ever sliced into a sourdough loaf and felt disappointed by a tight, heavy interior, you’re not alone.

Most bakers assume open crumb is about:

• Better flour
• Stronger starter
• Longer fermentation

Those things matter — but they’re not the deciding factor.

Crumb structure is primarily a hydration problem.

Once you understand how water interacts with gluten, fermentation, and gas retention, “open crumb” stops being mysterious. It becomes predictable.

What “Open Crumb” Actually Means

Before talking about hydration, we need to clarify what bakers mean by open crumb.

Open crumb

A bread interior with irregular, well-distributed air pockets formed by gas retention during fermentation and baking.

Closed crumb

A dense interior with small, uniform holes caused by limited gas expansion or poor dough extensibility.

Open crumb is not about size alone. It’s about structure — the dough’s ability to stretch without tearing while holding gas.

That ability is controlled by one variable more than any other: water.

Why Hydration Controls Crumb Structure

Hydration refers to the ratio of water to flour, usually expressed as a percentage.

In baker’s math:

• 100% flour is the baseline
• Water is calculated relative to that flour weight

So a dough at 70% hydration contains 70 grams of water for every 100 grams of flour. This ratio affects everything that happens next.

Water determines:

1. How gluten forms
2. How extensible the dough becomes
3. How gas expands during fermentation
4. How steam inflates the loaf in the oven

Without enough water, the gluten network stays tight and restrictive. With sufficient water, it becomes elastic and capable of expansion.

Gluten Development: Strength vs Extensibility

Gluten isn’t a single thing. It’s a network formed when flour proteins hydrate and align.

Two forces must balance:

• Strength — the ability to hold shape
• Extensibility — the ability to stretch

Low hydration doughs lean heavily toward strength. They resist stretching, which limits gas expansion. The result is a tight crumb.

As hydration increases:

• Proteins hydrate more fully
• Gluten strands slide instead of locking
• Dough stretches rather than tearing

This is why higher hydration doughs naturally produce more open crumb — if they’re handled correctly.

Why 65% Hydration Feels “Easy” (and Why It Limits Crumb)

Many beginner sourdough recipes hover around 60–65% hydration. There’s a reason for that.

At lower hydration:

• Dough is easier to handle
• Shaping feels controlled
• Mistakes are more forgiving

But the tradeoff is structural. At 65% hydration, gluten tightens quickly, gas pockets remain small, and dough resists dramatic expansion.

This hydration range is excellent for sandwich bread or toast loaves, but it’s not ideal for large, irregular holes.

Why 75% Hydration Unlocks Open Crumb

Once hydration reaches the mid-70s, dough behavior changes noticeably.

At 75% hydration:

• Dough feels looser and stickier
• Gluten develops more slowly
• Fermentation has more influence

This environment allows larger gas bubbles to form and more dramatic oven spring, creating those sought-after irregular crumb patterns.

But this hydration level also exposes errors quickly. Too much water without proper structure leads to spreading dough, weak shaping, and flat loaves.

The problem isn’t hydration itself. It’s uncontrolled hydration.

Autolyse: Why Water Timing Matters

Hydration isn’t just about quantity. It’s also about when water interacts with flour.

Autolyse — resting flour and water before adding salt and starter — allows:

• Full hydration of proteins
• Enzymatic activity to begin
• Easier gluten formation

At higher hydration levels, autolyse becomes more valuable, not optional. Without it, dough feels chaotic. With it, dough becomes cohesive and structure develops without excess mixing.

Fermentation Rate and Hydration

Water accelerates fermentation. Higher hydration doughs allow enzymes to move freely and increase yeast activity.

This means fermentation timing becomes more critical as hydration increases.

If bulk fermentation runs too long, gluten weakens and structure collapses. This is why high hydration requires shorter, more attentive fermentation, not longer neglect.

Why “Flat and Dense” Loaves Often Start with Math Errors

Many sourdough failures blamed on technique actually begin earlier — in calculation.

Common issues include:

• Accidentally exceeding target hydration
• Misjudging starter hydration
• Forgetting to account for water in starter

A dough intended to be 72% hydration can easily become 78% without noticing. At that point, the dough lacks strength and oven spring disappears.

Precision matters most at higher hydration levels.

Baker’s Percentages: The Language of Predictability

Baker’s percentages remove ambiguity. Instead of “a splash of water,” you work with exact weights. This transforms hydration from a feeling into a controlled variable.

Once hydration is predictable, results become repeatable. Open crumb stops being luck.

When Not to Chase Open Crumb

It’s worth stating clearly: open crumb is not always the goal.

Avoid high hydration if:

• You need clean sandwich slices
• You’re working with very low-protein flour
• You prefer predictable shaping over dramatic structure

A 65% hydration loaf isn’t “inferior.” It’s just optimized for a different purpose. The problem isn’t choosing lower hydration. It’s choosing blindly.

FAQ: Hydration and Open Crumb

Open Crumb Is a System, Not a Secret

The difference between tight crumb and open crumb isn’t talent. It’s control.

Once hydration becomes intentional — not intuitive — sourdough baking becomes calmer, cheaper, and far more consistent.

You don’t need more tricks. You need numbers that work together.

Continue Learning

• Learn why excess starter creates waste in Sourdough Discard 101
• Troubleshoot dense loaves in Why Is My Sourdough Flat and Dense? (Coming Soon)
• Design your next loaf with the Sourdough Hydration Calculator