Sourdough bread cracks in the hot oven as expanding carbon dioxide gasses in the dough look for the weakest point on the surface to break free. The dough cracks non-uniformly on the surface due to inadequate scoring, inadequate steaming, rough handling of the dough, dried out dough surface and excessive skin tension from shaping.
1. Inadequate scoring
The main reason why sourdough bread cracks is due to inadequate scoring or scoring that is too short and too shallow.
Scoring the bread intentionally introduce a significantly weaker area on the surface of the dough that gives room for the dough to expand in a controlled manner. Without scoring, the hot expanding carbon dioxide gasses in the dough pushes on every corner of the dough until the weakest point of the dough gives, resulting in cracking of the crust in a non-uniform and visually unappealing way.
Scoring is done using a bread lame which is essentially a sharp razor blade. To prevent cracking of the sourdough, be sure the cuts are about half an inch deep and that the cuts run across the entire length of the bread.
If the cuts are too shallow, there isn’t enough room for the dough to expand and it will find the next weakest point on the surface of the dough to break free.
If the cuts do not run the entire length of the bread, the areas where the dough is unscored will expand in an non-uniform manner.
However, we do not score weak breads such as those made with rye flour or those of very high hydration levels, as scoring ruptures the important gluten network that is already in a weak state to begin with, causing the dough to flatten.
2. Inadequate steaming
Steaming keeps the surface of the dough moist to delays the formation of the crust. This is important to allow sufficient time for the dough to fully rise in the oven (also known as oven spring) before the hard crust is set.
Without steaming, the hard crust sets prematurely before the loaf has finished rising, which causes a build up in internal dough pressure that results in either the surface cracking or in extreme cases a protrusion on the dough’s surface.
Steaming for the home baker can be done through two different method. The first method is to bake the dough sealed inside a dutch oven. The moisture in the dough evaporates into steam and is trapped in the sealed pot which provides sufficient moisture for the bake.
The second method for steaming is place a pan of hot boiling water inside the oven. The water in the pan evaporates into the oven throughout the bake and provides the necessary moisture to prevent cracking.
3. Excessive skin tension from shaping
The shaping of the dough essentially creates tension on the outer skin of the dough’s surface. Not enough tension and the dough does not hold its shape, but too much tension causes the dough to crack when baked.
As the dough expands in the oven, the outer surface of the dough expands as well and at the greatest length as compared to the rest of the dough. If the tension on the dough’s surface is too high when loaded into the oven, the gluten strand tears and the dough cracks as the tension applied to the dough’s surface from the expanding gasses has surpassed the dough’s limit.
Shape your dough just enough that it retains its shape and not too much that it becomes too tight and cracks in the oven.
4. Rough handling of the dough
Be gentle when working with the dough during shaping and when transferring the dough from one container to another.
Rough handling of the dough causes micro tears on the surface of the dough which is not visible to the naked eye.
These micro tears happens randomly and is distributed unevenly across the dough’s surface. When the dough expands in the oven, it looks for these micro tears as it is the weakest points on the dough’s surface, resulting in an non-uniform expansion and rips and tears the surface of the dough.
5. Dried out dough surface
The surface of the dough can get dried out from excessive air circulation and very low humidity levels. The dry surface is weakened and becomes brittle, and will expand and crack uncontrollably during baking.
Cover your dough with a plastic sheet when it is left to rest, especially during the bench resting and final fermentation phases of the baking process. The plastic sheet prevents any excessive air circulation around the dough’s surface which would otherwise pull moisture away from the dough and dry it up.
During months of very low humidity, the dough’s surface can also be dried out excessively. To circumvent this problem, lightly mist the surface of the dough with water during the bench resting and final fermentation phase. This will moisten the dough just enough to prevent a dry crust from forming.
Cracking in a sourdough is only bad when it happens uncontrollably to produce unsightly misshapened crust. All sourdough bread expands, and the key is to control that expansion by scoring the bread to produce uniform cracks on the surface that are beautiful.