Question

When baking bread, the toasty brown layer (i.e. the crust) will only form on the very outside of the bread. Why?

Answer 1

The formation of the toasty brown layer on the very outside of the bread can be explained by following phenomena

1) Caramelization: It is the process, in which the colourless sugars in the presence of heat become golden brown colour (browning of sugars) and also new flavours ( caramel).

2) Maillard reaction: In this, chemical reaction between reducing sugars and amino acids gives browned food its distinctive flavour. This process is also called non-enzymatic browning.The nucleophilic amino group of the amino acid reacts with the most reactive carbonyl group of the sugar, that in turn forms a complex mixture of unknown molecules which are responsible for a range of aromas and flavours.

During Baking, at high temperature Maillard reaction and caramelization occurs hand in hand to give the brown colour toasty layer on bread with caramel aroma.

3) To answer question why on very outside on the bread, we need understand the biochemical process of heat transfer and movement of water within the dough.

In the oven, the heat energy is transferred to the dough by convection and radiant transfer. When it touches the dough, further it transfers to inside of the dough by conduction. Hence , the bottom and side-wall of the dough ( surface) warms quickly, and then interior of the dough warms progressively and slowly depends on the distance from the surface. Now when the temperature inside the oven is raised, the oven atmosphere is saturated with quasi-moisture. There is slight condensation of water take place on the surface of the dough. The surface temperature of the dough is risen, due to continuous supply of heat energy. The crust formation is initiated and the moisture on the outer layers of the bread turns into steam. Most of the steam is evaporated; however some amount is transferred to inside of the dough.

In summary, High radiant and convective heat transfer on the surface of the dough makes it more warmer than progressively heated interior of the dough by conduction. Moreover, the high presences of residual moisture inside the dough make it difficult to reach high temperature till the end of the baking. On the hand, the surface of the dough has low moisture dries out completely due to heat, non enzymatic browning occurs ( Maillard reaction and caramelization) and forms nice browny toasty layer on the surface.