What Are Bread Crust Blisters?

Blisters, also known as “bird’s eyes”, are clustered micro-pouches of air on the surface of bread crust, sometimes emulating the appearance of chickenpox or warty toads.

Comparison of warty toad and blistered San Francisco style sourdough bread

Can you tell the difference? (Bread photo courtesy of David)


The formation of blisters can be encouraged by using one or a combination of the following techniques:

Of the four mentioned, the most common implemented technique to induce blisters is extending the proofing period. This is done by incubating the dough at cooler temperatures, often in a refrigerator or dough retarder.

Warning: Conjecture ahead.

When dough is chilled to as low as 3°C / 38°F for several hours, gluten degrades and the dough retains a reservoir of excess gas produced by active yeast. Because gases are more soluble in water at cooler temperatures, the carbon dioxide generated by the yeast slowly dissolves and diffuses throughout the dough. (Have you ever noticed that chilled soda is fizzier than lukewarm soda? The same effect applies.) When the dough is loaded into the hot, preheated oven, the retained gas dissipates and seeps to the dough surface, forming pockets of carbon dioxide and thus expanding the exterior.

Voila! You’ve achieved blistering.

Red cherry floating in carbonated soda

Carbonated soda with a cherry on top


In France, bakers traditionally refresh their sourdough starters more frequently and do not chill their dough to minimize or eliminate sourness. Consequently, blisters are seldom observed and generally regarded as defects by the French. Not surprisingly, the majority of French bakers avoid blisters—on their breads and likely their bodies.

Blisters are strong indications that the dough was fermented for hours, perhaps days, and often characterized by a piquant tang. In San Francisco, USA, where tangy breads are not considered unusual, this attribute may be sought after.

What’s your opinion on crust blisters? Yay or nay?


1: Blistered Glaze-Like Crust. Sourdough Companion.
2: Carbon Dioxide and Solubility. Newton.
3: Decree No. 93-1074. LegiFrance.
4: Gluten Hydrolysis and Depolymerization During Sourdough Fermentation. Journal of Agricultural and Food Chemistry. [PDF]
5: Retarding Dough for Flavor Enhancement and Process Control. Sourdough Home.

Author: Zita (bakingbadly)
Published: September 4th, 2013
Last Modified: October 8th, 2013

4 Responses to “What Are Bread Crust Blisters?”

  1. sandy says:

    IMHO blisters on loaves not dusted with flour or seeds are very beautiful…

  2. I’m pro-blister all the way. The more sour, the better.

  3. Enjoyed reading through this, very good stuff, appreciate it.

  4. Doc3 says:

    Your conjecture about CO2 coming out of solution and getting trapped under the surface layer of gelatinized starch is supported by the physics. So long as active fermentation is ongoing while the dough is cooling during retardation, much of the CO2 created will be dissolved in the liquid of the dough with the remainder either contributing to volume expansion or diffusing to the surface and escaping (depending on the specifics). When it goes into the oven, steam rapidly gelatinizes the starch when it condenses on the surface of the dough. This forms the gas-tight layer that traps the CO2 as it effervesces out of solution.

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