1. What Are the Crust and the Crumb of Sliced Bread Called?
The outer skin of sliced bread is called the crust, and the white part in the middle is called the crumb. [1] In Japanese, this part is also nicknamed the bread’s “ear” (耳). [1]
If you look closely at the crumb, you will see many tiny air bubbles packed tightly together. [1] These bubbles are what create the soft, fluffy feeling of the inside. [1] From here, we will follow how the same dough separates into two different textures.
2. Why Does the Inside of Bread Stay Cooler While Baking?
The dough contains a lot of water. When it is heated in the oven, that water turns into steam. Water takes in heat as it evaporates, which is thought to be one reason the temperature does not rise quickly.[3]
According to one source, the center of the crumb reaches about 98°C (about 208°F) during baking, which is close to the boiling point of water. [3] This is much lower than the temperature of the oven. The evaporation of moisture is thought to be connected to these temperature changes. [3]
The sources do not explain how the temperature is kept steady, so it is safest to take this only as "about 98°C." Wikipedia's article on bread crust states that the moisture inside the crumb during baking is about 40 to 45%. [2] This is just one rough guide.
3. Why Does the Outside of Bread Get Hotter as Water Leaves?
The outside is directly exposed to the hot air of the oven. On the outside, moisture is lost by evaporation and the temperature rises. [3] One source states that the crust eventually reaches a temperature close to that of the oven. [3]
The moisture in the part that becomes the crust is said to be 20% or less, and on the very outer edge, it is 10% or less. [2] Compared to the 40 to 45% in the crumb, the crust has about half or less the moisture, and the outer edge has less than about one-quarter. [2]
When the surface becomes hot, reactions between sugar and amino acids, as well as reactions where sugar changes due to heat, create the brown color and fragrant smell. [3] This is the "browning" effect. For the browning reaction between sugars and amino compounds (the Maillard reaction), sources say it speeds up greatly above 160°C (about 320°F), under certain conditions such as acidity (pH 5.0 to 5.5). [2][3]
4. Why Does the Crust Become Softer After Baking?
Right after baking, the crust is firm, and the inside is very soft. Over time, moisture from the inside slowly moves to the outside, and the crust may absorb that water and become moist and somewhat softer. [1] This happens when conditions make it hard for water to escape. [1]
Another source also notes that moisture moves from the inside to the outside, increasing the crust’s moisture and changing its feel from crisp to more like skin. [2] Right after baking, the outside is drier than the inside. As time passes, this difference gradually levels out.
5. How Does Storage Change the Softness of the Crust?
The story about moisture moving from inside to outside applies only when conditions prevent water from escaping easily. The change in the crust’s softness also depends on storage conditions, so it is not uniform. [1]
Also, how hard and thick the crust becomes can change based on the dough’s ingredients and whether the baking pan has a lid or not.
6. How Can You Investigate the Crust and Crumb Yourself?
You can compare the crust and the crumb of sliced bread by pressing them with your fingers or tearing them apart. Describing the differences in color, smell, and hardness can help you imagine what happened during baking. [1]
It is also recommended to store one piece of the same bread in a bag and leave another piece exposed to the air, then compare the feel of the crust the next day. If you want to learn more, you can read explanations about bread structure and methods in the "Mini Encyclopedia of Bread" by Yamazaki Baking. [1]