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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 starch such as flour, cornstarch, arrowroot and potato starch can be found in some custard recipes, while not in others.
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, 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.
Curdling or Weeping: The term curdling is usually used in connection with a stirred mixture such as custard sauce, while weeping or synersis are more often used with reference to pie meringues or baked custards. When egg mixtures such as custards or sauces are cooked too rapidly, the protein becomes overcoagulated and separates from the liquid leaving a mixture resembling fine curds and whey. If curdling has not progressed too far, it may sometimes be reversed by removing the mixture from the heat and stirring or beating vigorously. To prevent synersis or curdling, use a low temperature, stir, if appropriate for the recipe, and cool quickly by setting the pan in a bowl of ice or cold water and stirring for a few minutes.
SARAH SAYS: No matter how custard is cooked or baked, it is important to remove the custard the minute it's done from the stove or oven AND the double boiler or waterbath; the ingredients continue to cook if you don't.
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.
Eggs coagulate at (as measured with an Instant Read Thermometer): whites at 140 degrees F with yolks at 150 degrees F.
Gentle cooking, such as baking custard without starch in a waterbath, keeps the egg proteins loose and soft until the mixture gets hot enough, at around 160 degrees F. The constant stirring with a stove top recipe maintains an even temperature so the protein bonds don't form too early, which are allowed to set undisturbed in baked versions.
When starch is present in a custard recipe, it HELPS to prevent the unwound egg proteins from joining together too soon, thus when heated to too high a temperature and then cooled, the custard won't scramble or crack.
But despite the gentle heat, if custard is heated above 180 degrees F, the more tightly the proteins join together. They becoming thicker, curdling and squeezing out all the water which you see evidence of coming from little tunnels in the custard, called synersis. The egg proteins will overcoagulate from too much heat which eventually shrinks when cooled.
You can also see overcooking or exposure to too much heat in an cracked cheesecake. Overheated eggs shrink when cooled causing with a large crack running through its center or tiny cracks all over its top.
When a starch-based custard mixture gets hot enough and boils at around 212 degrees F, the egg proteins simply join together. However, if you undercook such a custard they won't join together and gel. Yolks have a starch digesting enzyme called alpha-amylase. In order for a successful gelling of a starch in the recipe, the enzyme has to be killed by cooking the custard almost to boiling (a little less than 212 degrees F). Otherwise the left-over enzymes digest all of the nice firm starch gel and your custard is nothing but liquid.
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 (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.
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. However, they can be used, if desired. An example is the Pastry Cream Recipe.
- Without Starch: Stirred custards or those cooked on the stovetop,in a double boiler or waterbath or in a heavy-bottomed pot; 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 Crème Anglaise recipe.
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.
SWEETENERS
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.
FLAVORINGS
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.
BAKING911 IN THE NEWS
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Take your time, use lower heat for curdle-free boiled custard ASK MAMA! February 4, 2004
QUESTION: Dear Mama: During the holidays, I made boiled custard twice. The first time it was smooth and had a good consistency. The second time, it had a rather curdled appearance. What causes this? Is it because it is cooked too fast? Is there something you can do to reverse that once it happens? - Marilyn Tilghman
ANSWER: Dear Marilyn: Mama's so sorry this happened to you. I'm a big fan of homemade boiled custard at the holidays, and I know how disappointed you must have been.
And yes, it's basically because it was cooked too fast or at too high a temperature.
Whether you can reverse it depends on whom you ask. Jean Anderson, who wrote "1,001 Secrets of Great Cooks," says, "If stirred custard curdles, not to worry. Put through a fine sieve. Or buzz until smooth in a food processor or blender." The American Egg Board, at its Web site says, "If curdling has not progressed too far, it may sometimes be reversed by removing the mixture from the heat and stirring or beating vigorously."
But Linda Lau Anusa sananan, writing in Sunset magazine, says, "Once a custard curdles, there is no way back. It's wiser to undercook slightly when in doubt."
Eggs are the key to the chemical reaction that creates custards, and the success of the custard depends on what happens to the eggs.
"Eggs are the main thickener in most custards and the yolks make them smooth and rich," says Sarah Phillips, author of "Baking 9-1-1." "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, giving you the custard's texture."
The American Egg Board says, "When egg mixtures such as custards or sauces are cooked too rapidly, the protein becomes overcoagulated and separates from the liquid leaving a mixture resembling fine curds and whey." This is known as curdling, or syneresis. (Actually, the Egg Board's Web site says "sunrises" but the chemical process of a gel losing moisture is called "syneresis." I guess somebody taking dictation got as close as they could.)
Weeping, or syneresis, usually refers to pie meringues or baked custards and curdling is used with stirred custards.
"To prevent sunrises (there they go again!) or curdling," the Egg Board says, "use a low temperature, stir, if appropriate for the recipe, and cool quickly by setting the pan in a bowl of ice or cold water and stirring for a few minutes."
So, how do you know when to stop cooking and thereby prevent sunrises, er, uh, syneresis or curdling?
At her Web site, baking911.com, Phillips says, "For stirred custard mixtures, the eggs are cooked to the proper doneness when a thin film adheres to a metal spoon dipped into the custard. This point of coating a metal spoon is 20 to 30 degrees below boiling. Stirred custards should not boil. The finished product should be soft and thickened but not set. Stirred custards will thick en slightly after refrigeration."
So that's it, Marilyn. Next time, slow down, take your time, keep your burner turned down and enjoy the process of making smooth, curdle-free custard.
