On Food and Cooking

molecules, and the proteins must be hotter and
moving around more rapidly in order to find
and bond to each other at a noticeable rate.
Sugar also raises the thickening temperature,
and for the same reason: its molecules dilute
the proteins. A tablespoon of sugar surrounds
each protein molecule in a one-egg dish with a
screen of several thousand sucrose molecules.
Combine the diluting effects of water, sugar,
and milk fat, and a custard mix containing a
cup of milk, a tablespoon of sugar, and an egg
begins to thicken not at 160ºF/70ºC, but at 175
or 180ºF/78–80ºC. And because the protein
network is stretched out into such a large
volume — in a custard, the proteins from a
single egg have to embrace not three
tablespoons of liquid but 18 or 20! — the
coagulum is far more delicate, and easily
disrupted by overheating. At the extreme, in a
concoction like eggnog or the Dutch brandy
drink advocaat, the egg proteins are so diluted
that they can’t possibly accommodate all the