Why Does Bread Crust Turn Dark?

Crust color begins as the surface dries. Once the water evaporates, the temperature rises, and above 140°C / 285°F, proteins and sugars bond and produce hundreds of new color and aroma molecules. Most of bread’s aroma comes from this reaction. The inside never reaches this temperature, so it stays pale; color forms only on the outside, in a layer no more than a few millimeters thick. The same reaction occurs when meat browns and coffee is roasted; the aroma of bread crust belongs to the same family.

Sugar and milk in the dough speed up this reaction. Milk contains both sugar and protein, so the crust on dough made with milk becomes noticeably darker and softer. Honey, grape molasses, and sugar have the same effect. That is why doughs such as savory pastries and rolls are baked at a lower temperature than plain bread; if left in the same oven for the same amount of time, their surfaces darken before the insides are cooked. Milk-based doughs are therefore generally baked on the middle rack with both top and bottom heat, reducing direct radiant heat from above.

No sugar is added to plain bread dough, but it still develops color. During fermentation, some of the starch in the flour breaks down, producing sugars that the yeast does not consume; these sugars color the crust. In doughs that ferment for a very long time, more of these sugars are consumed, and the crust may stay pale. That is why a small amount of honey or malt is sometimes added to doughs that undergo a long, cold fermentation. The amount is very small: about one 5 ml measure for 1 kilogram of flour noticeably improves the color.

Color also depends on oven conditions. High heat speeds up crust formation. Steam delays it: the surface stays moist during the first minutes and dries later, giving the bread time to rise; once the steam stops, color develops quickly. On a rack close to the upper heating element, the top browns early, so place the pan on the middle rack and turn it halfway through baking if needed. Browning also happens faster in a convection oven because circulating air dries the surface sooner, and the temperature is lowered by about 10°C / 18°F.

What is brushed onto the surface also affects its color. Brushing bread with water gives it a matte, crisp crust. Egg yolk leaves a dark, glossy surface, while egg white leaves a pale, matte one. Milk darkens the color, and butter softens the crust. Bread dusted with flour stays pale and floury. Color is a matter of preference, but it does not indicate doneness; that is still determined by the internal temperature. Brush the surface just before putting the bread in the oven; egg brushed on too early dries and cracks in the oven, leaving marks.