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All custards are made basically the same ingredients: mainly eggs and/or yolks, as well as cream or milk, sugar and usually salt and flavorings. A small amount of thickening starch such as flour, cornstarch, arrowroot and potato starch can be found in some custard recipes, while not in others.
See also Custard: Problems and Solutions
EGGS, A PRIME INGREDIENT IN CUSTARDS
A large portion of the discussion of stirred and baked custard hinges on eggs, such as whole eggs, whole eggs plus yolks and just separated yolks and whites.
Eggs are the main thickener in most custard and the yolks make them smooth and rich. Both egg yolks and whites contain proteins, which change from liquid to solid, called coagulation, when cooked or baked. This means that the liquid egg becomes firmer. As heating continues the egg eventually becomes semi-gelled or fully gelled when cooled, especially under refrigeration, giving you the custard's texture. The less eggs in a custard recipe, the cooking time increases and so does the coagulation time.
Because eggs are the primary structural ingredient of custard, it's important to use fresh, large Grade A ones -- sometimes a negative difference in the custard can be seen when using frozen, old, dried or egg substitutes. The heating of egg proteins must be under low heat, and done slowly, thus egg-based custards are typically cooked or baked in waterbaths (water baths) or Bain-Maries (Bain Maries). Double boilers are another type of pan used to cook stove top custards in.
SARAH SAYS: Because egg proteins are sensitive to heat, it's easy to overbake or overcook recipes containing them. If you do, problems occur.
How eggs thicken: Both egg yolks and whites contain protein. They are shaped like coils or springs or complex wads that are all separate from one another. You can see through an egg white because the proteins are not attached!
When eggs are heated, their proteins unwind (called denaturing) and break apart from their tightly bound bundles, bump up against one another, and adhere to form loose, flat and long strands. These strands are linked together in a three-dimensional mesh. You can see the result of this process with egg whites because they turn from clear to opaque, forming a solid gel. Liquid gets trapped in these strands, and this causes the mixture to thicken.
Some recipes depend only on the thickening power of egg yolks. A raw egg yolk is filled with tightly curled protein molecules, many of which contain sulfur atoms that are interspersed along their lengths. When egg yolks are heated, the proteins denature and the sulfur atoms are able to link up. The result is a tangled network of protein molecules that hold water or a thickened custard, as a result.
STARCH - FLOUR OR CORNSTARCH
Eggs are sensitive to high heat and thus, all custard recipes require slow cooking or baking and gentle heat. If a recipe contains starch thickeners (flour or cornstarch) it HELPS to prevent the eggs from overcooking and gives more lead way between success and failure of the egg-rich mixture, but problems can still occur. The starch molecules slow protein coagulation, making the egg proteins more resistant to overcooking and curdling. While basic custards should never be boiled,
Whether a recipe contains starch, it determines how the custard is cooked or baked:
- With starch: Custard can be cooked or baked under direct heat, without a double boiler or waterbath. Starch-thickened recipes need to reach just below boiling (while stirring) to ensure that they’re fully cooked. An example is the Vanilla Bean Pastry Cream Recipe Tutorial.
SARAH SAYS: But, moisture during baking always helps prevent cracking when baking a custard dessert with starch; I bake my Creamy and Luscious No-Crack Cheesecake Recpe, that includes a small amount of flour, in the oven with a pan filled with hot water on a shelf just below.
- Without Starch: Stirred custards or those cooked on the stovetop,in a double boiler, and should never e boiled; and, baked custards are typically placed in a waterbath in the oven. The water insulates the recipe from high heat and moderates the cooking or baking temperature of the custard. This guarantees that the eggs in the custard approach their set point slowly and thicken gradually. An example is the Vanilla Flan with Caramel Sauce Tutorial.
LIQUIDS: MILK OR CREAM
Besides eggs, custards contain either milk or cream or both. Cream contributes to the thickness of the custard, as those made with water or skim milk will not gel or thicken. Many recipes direct you to scald them before using; this is a holdover from the days of unpasteurized milk. Scalding does, however, shorten cooking time because the milk is already hot; it also ensures that the sugar dissolves completely in the custard base before baking, so I recommend this step.
Sugar is also important to custard as the addition of it in a recipe results in a softer custard. Sugar also increases the coagulation temperature and time. Don't dump the sugar directly onto the eggs and let it sit; this causes the yolks to "burn" into hard little lumps that detract from your creamy custard. Rather, add the sugar while your whisk is moving; this way, the sugar will be gradually incorporated into the eggs. However, too much sugar prevents the eggs from coagulating.
Custards contribute to a plain back drop and can be easily paired with various flavorings to create exciting and exotic variations on the basic theme. Some flavorings can include melted chocolate, vanilla extract (always use pure extract, not imitation) or a medley of aromatic spices, fruits, and citrus flavors. Before baking, grate a little nutmeg over the top. Remember to add other flavorings AFTER the custard has cooked and is still hot and not firmly set (except for vanilla bean seeds which are added to infuse flavor into the cream). You don't want to loose flavor when the extracts are cooked with the rest of the ingredients and you don't want to add when the custard has cooled. Acids, such as freshly squeezed lemon or orange juice decreases coagulation time and temperature so, always add after the custard has finished cooking. Any stirring when the custard has cooled to incorporate the flavorings will thin it.
SARAH SAYS: Try using vanilla paste in your custard recipe—it's easy to use and imparts as much flavor as a whole vanilla bean.