In custards, eggs are combined with milk and sugar. The chief protein of cow's milk is casein, which is not coagulable by heat. Milk contains some albumin and a small proportion of globulin, which are coagulable by heat. Zoller states that the protein coagulable by heat is about 0.75 per cent of cow's milk. In the mixed milk and egg, the egg furnishes the larger percentage of the heat-coagulable protein. As the egg, milk, and sugar mixture is heated, coagulation occurs, and the thickened mixture is known as custard. If the temperature of the custard is carried a little higher than the coagulation point, a point is reached at which the custard begins to show syneresis, or the liquid separates from the curd. This is called the curdling point.

Baked and soft custards. Custards may be cooked in two ways. They may be cooked without stirring. This type is usually baked in the oven and is called baked custard. The other type of custard is stirred continually while it is cooked and is called soft custard. If the proportion of ingredients in the custard are the same the one cooked without stirring is firmer in texture and appears to be in one piece or clot. The soft custard has a softer texture and is not in one piece but is a viscous fluid.

The stirred custard also tends to curdle more readily than the baked custard. In part, this is undoubtedly because of the effect of mechanical agitation and is similar to the separation of fibrin of blood when beaten.

Milk for custards is often heated before mixing with the egg and sugar. Forewarming of the milk may tend to prevent curdling of the milk, but the greatest advantage is probably in shortening the cooking period.

Coagulation temperature of custards. The temperature at which a custard begins to coagulate or thicken is higher than the temperature at which the egg alone coagulates. The exact point at which coagulation starts is more difficult to ascertain than in the egg white, since the coagulation must be determined from the thickening of the custard. But not much occurs below 80°C. at ordinary rates of heating nor below 78°C. at slower rates of heating. As the soft custard coagulates, a thick layer will cling to a spoon that is dipped into the custard. In cookery this is known as "coating a spoon" and is one method of estimating when the custard is sufficiently cooked. The temperature at which coagulation starts varies with the varying proportion of ingredients of the custard and the rate of cooking. In class results, the coagulation of a custard made of 1 cup of milk, 2 tablespoons of sugar, and 1 egg has never been perceptible below 78°C. and not at this temperature except when the rate of cooking is very slow. Occasionally a custard is heated slowly enough so that the best serving consistency occurs at 80°, provided the eggs are fresh so that the reaction has not become quite alkaline.

It is possible to heat a cup of custard mixture in a double boiler from room temperature to the curdling point in less than 3 minutes. With this rapid rate of heating the custard is too thin to serve at 85° or even at 87° to 89°, and generally curdles about 89°, often after the temperature has remained stationary at 89° for a time. It may reach 91° or 92° and then drop back to about 89°, the curdling occurring before a serving consistency is attained.

When the custard is cooked more slowly, and particularly if the heating is slow after 75°C. is reached, thickening is quite perceptible at 80°, it has a consistency for serving from 80° to 84°; that cooked more slowly being thicker at the lower temperature. A slower rate of cooking is preferable to the rapid one. With the slower rate of cooking the thickening is evident for considerable time before the curdling point is reached. With the rapid cooking, the custard requires very close watching and rapid work to remove it from the heat before the curdling temperature is reached. Curdling may occur at 84° if the temperature is raised very slowly, but in ordinary cooking it is more likely to occur between 85° and 87°C.

Coagulation and particularly curdling in custards is accompanied by absorption of heat. In the custards heated slowly the temperature is usually stationary for a considerable period before curdling occurs. In the rapidly heated custards the temperature often reaches 90° to 92° and then drops as curdling occurs.

The temperatures given in the following discussion are for custards requiring 12 to 20 minutes from the time cooking is started to reach the curdling point.

Concentration of egg and coagulation. If the proportion of egg in the custard is increased, thus increasing the concentration of the protein, coagulation starts at a slightly lower temperature, a firmer custard being obtained at a definite temperature as the proportion of egg is increased.

Sugar. If the proportion of sugar to 1 egg and a cup of milk is increased, the coagulation temperature is elevated and coagulation begins above 80°C. The elevation of coagulation temperature is proportional to the amount of sugar added. If the custard is saturated with sugar, coagulation does not occur at boiling temperature.

Yolks. If two yolks are substituted for 1 whole egg the coagulation temperature is higher than for the whole egg.

Whites. If two whites are substituted for the one whole egg the coagulation temperature is lower than for a custard made of the whole egg. With both the yolks and whites the coagulation temperature varies with the proportion of sugar used.

The optimum temperature for cooking custards. Williams has reported that the curdling point for a custard made of 1 egg, 1 cup of milk, and 1 tablespoon of sugar is 83.5°C. When the proportion of ingredients is varied the curdling point varies. If the proportion of egg is increased the curdling point is lowered; increasing the sugar elevates the curdling point.

The temperatures given previously and those reported by Williams are for soft custards. Baked custards may be cooked several degrees higher than soft custards without curdling. This lower curdling temperature for soft custards is probably due to the stirring, which increases the tendency for separation of the custards into curds. Hence, it is important in making soft custards to prevent heating after a certain temperature has been reached. This can be accomplished by putting the cooking pan in cold water or by pouring the custard into another utensil.

Between the temperature at which coagulation starts and the curdling point is a temperature at which the custard has the best texture and flavor for serving.

Williams has reported that the optimum temperature for custards made of 1 egg, 1 cup of milk, and 1 tablespoon of sugar is 82.5°C. When 2 yolks are substituted for the whole egg the optimum temperature is 83.5°, and when 2 whites are substituted for the whole egg the optimum temperature is 82°.

Custard pie. In cooking custard pie one of the major difficulties is to prevent soaking of the crust. Increasing the proportion of egg helps, as the custard then coagulates at a lower temperature. Thus 1 1/2 eggs to a cup of milk is better than 1 egg. Prewarming the milk before adding it to the other ingredients shortens the time before coagulation takes place.

One way to prevent soaking of the crust is to cook the pastry and the filling separately. Bake the pastry on the bottom side of a pie pan with sloping sides. Bake the custard in a pie pan of the same size as the one used for the crust, setting the pan in hot water and using as low an oven temperature as desired. After baking the custard, cool it until the pan feels warm but not hot to the hand. At this temperature it has set sufficiently to hold together, yet will not break so easily as when cold. Run a knife or spatula around the edge of the custard and, tilting it at about a 45-degree angle, shake and slide it out of the pan into the crust.