1. Why Only the Crust Turns Brown: Water Escapes From the Surface First
The crust is the place where water escaped first. When bread is baked, moisture on the surface evaporates first, and this "evaporation front" moves slowly inward. The outer layer, where water has left, exceeds 100°C and approaches the oven's temperature. This layer becomes the crust and has less moisture [1].
The inside crumb, however, does not change much in moisture content, and its temperature stays around 100°C [1]. Note that "about 100°C" is a rough guide; it does not mean the temperature is exactly 100.0°C every time.
As long as water is present, heat is used to evaporate it, so the temperature does not rise easily. This difference creates two regions with different temperatures inside the same loaf. So, what happens when the temperature goes above 100°C?
2. What Makes the Brown Color: Sugar and Amino Acids Reacting in Baking
The browning that occurs during baking is thought to be caused by two reactions: the Maillard reaction and the caramelization of sugar [2]. Japanese explanations also state that the crust color of bread is produced by these two reactions [5].
Caramelization is a reaction where only sugar changes due to heat, and it does not involve nitrogen. On the other hand, the Maillard reaction is a reaction between reducing sugars and amino compounds, such as amino acids [1]. On the surface of the bread, it is thought that these two reactions overlap to create the color.
The brown substances form in the final stage. The Maillard reaction has initial, intermediate, and final stages. In the final stage, large brown molecules called melanoidins are formed [1].
3. Where the Fresh-Baked Smell Comes From: The Middle of the Browning Reaction
During the intermediate stage of the reaction, a change called Strecker degradation occurs, which produces substances that become the source of the aroma [1]. In other words, the smell appears in the middle of the reaction, while the brown substances appear at the end. The aroma comes from the middle stage, and the color comes from the final stage. It is also thought that other changes, such as caramelization, are involved in the aroma of bread.
It is thought that the savory smell rises when the bread turns brown because of this timing. The types of aromas change depending on the conditions. The aroma varies with the type of amino acid that reacts with glucose and with the temperature range. In bread baking, it is explained that burnt smells, caramel smells, nutty smells, and typical bread-like smells are produced [6]. Even with the same reaction, different conditions create different aromas.
4. Who Discovered the Browning Reaction: Louis-Camille Maillard of France
The reaction where sugar and amino acids turn brown when heated was reported in 1912 by Louis-Camille Maillard, a researcher from France [3]. Later, it came to be called the Maillard reaction after his name.
The year 1912 is simply when the existence of such a reaction was reported. The detailed explanation of the stages through which the reaction proceeds was organized by subsequent research.
This is not just a story about food. It is said that this reaction also occurs within the human body [3].
5. How Hot Does It Need to Be? Temperatures for the Crust and the Inside
General explanations state that the Maillard reaction proceeds quickly at approximately 140–165°C [4]. This is an encyclopedic guideline, not a strict boundary for bread. It does not mean the reaction does not happen at all below 140°C. The reaction also proceeds at room temperature, but it takes a long time.
The crust and the crumb differ in both moisture and temperature. The crust's temperature approaches the oven's temperature, so it enters the temperature range where the reaction proceeds quickly. The crumb stays around 100°C, so it does not reach the temperature range for a fast reaction [1].
The crust browns as its moisture decreases and its temperature rises. The crumb stays mostly white because its moisture changes little and its temperature stays around 100°C. The speed of the reaction probably also depends on the amount of water, so temperature alone does not decide it.
6. How to Investigate It Yourself: Toasting a Slice of White Bread With an Adult
With an adult, try baking one slice of white bread in a toaster. The inside of a toaster is very hot, and there is a risk of burns, so an adult should handle the removal of the bread. Compare the color of the crust and the crumb before and after baking. Also, compare the smell before and after baking to see if it has changed.
If you want to read more, see the Blue Backs article published on Gendai Media (in Japanese) [5].