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.
The formation of blisters can be encouraged by using one or a combination of the following techniques:
- Using highly acidic preferments (dough fermented in advance);
- Delaying fermentation during proofing (final dough-rise step before baking);
- Misting the dough with water prior to baking;
- Injecting hot steam into the oven during the initial phase of baking.
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.
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