In custard, egg acts as thickening agent. A true custard consists only of eggs, milk, sugar and flavouring compounds. Custards are of two types, the stirred or soft custard which is given a creamy consistency by being stirred while it is cooking and the baked custard which is allowed to coagulate without stirring, thereby producing a gel. Recipe for custard is as follows.
1 – 1 ½
Baked Custard This is made at 350oF. It is done when the tip of a knife inserted halfway between the centre and outside, comes out clean. When custard is overcooked, some clear liquid separates from the gel structure that is syneresis occur. In an overcooked custard, the egg proteins that form the mesh-like gel structure apparently shrink and squeeze out some of the liquid that was held in the mesh. Fruits, dry fruits and caramelized sugar can be added to give variety.
Soft Custard Same ingredients are used as in baked custard. The vanilla, because of its volatility is added after the other ingredients are cooked. Custards that are cooked more slowly coagulate more completely at a lower temperature than custards that are cooked rapidly. There is less danger of curdling and both consistency and flavour are better in stirred custards cooked relatively slowly. The total cooking time, in a double boiler should be 12 to 15 minutes, heating more rapidly at first and then more slowly (70oC-75oC water temperature) while stirring thoroughly and rapidly should be done during the entire process. Stirring separates the coagulated particles, resulting in a creamy consistency. The custard will be thicker when it is cold. When the knife comes out coated, the end point has come.
Success in making stirred and baked custards depends on cooking just enough to coagulate the protein of egg. Excessive heat results in over coagulation and syneresis of protein characterized by curdling in stirred custards and in baked custards. Scalding (heating) the milk used in custards shortens the cooking time and may also improve the product. Baked custards are better when made from curdle at 90oC than to 50oC. Custards cooked over rapidly boiling water thicken at 87oC and curdle at 90oC thus leaving a narrow margin of safety. However if the water in the bottom of the double boiler was cold when cooking started and was brought to boil slowly the custard thickens at 52oC and curdles at 87oC. The consistency of curdled custard can be improved greatly by pouring it into a cool dish and immediately beating with a rotatory beater.
The temperature at which gelation occurs depends on many factors.
Proportion of ingredients: The temperature required is higher than that required to coagulate the egg because the protein of egg is diluted with milk. The higher the proportion of egg to milk the lower the temperature at which the custard sets. But two egg yolks in the place of one raises the coagulation temperature. The protein of the egg is not readily denatured by heat when sugar is present. The sweeter the custard the weaker is the gelation at the given temperature.
Rate of cooking: When the cooking is rapid the margin of safety is reduced and curdling can occur easily.
Acid: Acid lowers the setting temperature. The acid supplied by the fruit in custards lowers the gelation temperature and it gets done sooner than one without fruit.