Milk, Cream, and Sugar Dilute, Delay, and Tenderize

When we dilute eggs with other liquids, we raise the temperature at which thickening begins. Dilution surrounds the protein molecules with many more water 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 160°F/70°C, but at 175 or 180°F/78–80°C. 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 liquid, and instead merely give it some body.