A basic sauce
Making it is quite simple: first a "roux" mixture of flour and butter, which is heated and colored (hence its name).
Once the roux has reached the right color, cold milk is added, the mixture is very liquid, and you continue to cook and stir over low heat until it thickens (you'll find the full recipe here).
But why does it thicken? As you'd expect, there's no magic involved, just a little physics and chemistry.
Flour and starch
The secret lies in the roux's flour, which contains grains of starch, a complex sugar made up of glucose molecules rolled up into tiny little balls.
It's around 70°C that the magic happens: the grains of starch in the flour break up into smaller grains, starch molecules that begin to absorb the milk around them, up to 20 times their volume, thus increasing the sauce's viscosity.
This process, known as starch gelatinization, takes place between 70°C and 85°C, giving béchamel its characteristic thick, creamy consistency.
Note in passing that this process of starch gelatinization is also at work in, among other things, crème pâtissière or flan, where flour is often replaced by maïzena, a corn starch.
In short: Béchamel thickens because it contains flour that is heated, causing its starch grains to burst and absorb the milk around them.
So béchamel is not just a culinary classic, it's also a little chemistry lesson in action.
