Custard, an example of an egg thickened sauce often prepared in the kitchen.
Custard is prepared by mixing eggs, milk and sugar. After the milk has been heated, it is added to the
mixture of sugar and egg yolks, and then the combined mixture is heated, stirring constantly. Adding
the hot milk to the egg yolk mixture is preferred to adding the egg yolks directly to the hot pan of milk
because it will heat up the egg yolks much more gradually, so reduces the risk of rapid protein
coagulation and early curdling.
The milk is added to increase the liquid phase of the sauce and thus dilute out the proteins. Sugar is
added to sweeten the custard, and also to further reduce the overall protein concentration. In the
presence of sugar, the proteins present will be surrounded by a large number of sugar molecules, and
this makes it more difficult for the proteins to lump together. In the presence of both sugar and milk,
the egg yolks can be heated to a much higher temperature without the risk of lumping together. Also,
in the presence of sugar molecules, the likelihood of forming a sufficiently tight coagulated protein
network from which the trapped water molecules will be squeezed out is less, because the sugar
molecules prevent the protein strands from physically getting this close. It also means that the final
structure will be more tender and fragile.
The custard should be stirred regularly during cooking to cause an even distribution of heat, and to
reduce the chances that the egg yolk proteins will clump together before they have unwound.
A pinch of flour is sometimes added to a custard mixture to further reduce the likelihood of the custard
curdling. Pastry cream, which is similar to custard in composition except that it contains a much higher
quantity of flour, can be heated to boiling temperatures without risk of the sauce curdling. In the
presence of significant flour content, the starch granules contained in the custard will start to absorb
water and swell when the custard is heated. The starch granules will then start to leak out some starch