A custard is defined as a dairy product that is thickened with eggs (crème brûlée, crème anglaise, crème caramel, flan, quiche) or with eggs and starch (pastry cream, chocolate pudding). It is a usually sweet, very moist, tender gel of egg protein. Custards are typically classified as boiled, stirred, and baked and are grouped in this book under the boiled custard or pastry cream, baked custard, or stirred custard or anglaise methods.
A boiled custard is thickened with eggs and cornstarch over direct heat. The idea is that heat will coagulate the proteins in the starch and gelatinize the cornstarch, which will in turn thicken the liquid, which is generally milk. The mixture, in theory, will have to boil for this to happen, but as you will read in the method below, boiling is not necessary. There is a way to make a thick, smooth pastry cream without submitting it to intense heat. Because we do not need to boil this custard, it is not necessary to call it a boiled custard; we will simply call it pastry cream.
This method can get extremely complicated if you do not follow the steps to the most exact detail. It is not easier than the traditional boiled custard method; it is different and not without its complications. However, it turns out a superior-quality custard with a very smooth mouthfeel.
There are some cons to this method. The first is that there is an enzyme in the egg yolk called amylase that breaks the cornstarch in the pastry cream down into sugar (retrogradation). The only way to neutralize this enzyme is by boiling the egg yolk, which can be done with the boiled custard method. However, the custard made by the following method will not be affected within 48 hours of making the pastry cream. Making too much pastry cream is not necessarily ideal, so this method is well suited for the pastry shop. The second con is that this custard is cooked until all the proteins are just cooked and no further, which means that the addition of a flavored liquid such as rum could potentially loosen the consistency of the custard too much.
Stirred custards are similar to boiled custards in that they are cooked over direct heat, but they differ in that they do not need to boil to reach the desired consistency (between 80°C/175°F to 85°C/185°F) and are thickened only by eggs. The most common example is crème anglaise.
Baked custards use only eggs and, as the name indicates, are baked in order to coagulate the egg protein. They are usually baked inside a ramekin in a hot water bath in an oven between 135°C/275°F and 160°C/325°F.
The Boiled Custard or Pastry Cream Method is as Follows:
Warm the eggs to 21°C/70°F in a bowl over a hot water bath. Set aside. This is an important step, since it will help the egg yolks coagulate faster than if they were refrigerator cold (see Step 5).
Pour the first amount of milk in a deep pot or rondeau. It should be deep enough to hold the milk when it is at a rolling boil. Add the full amount of sugar, along with any flavors (vanilla, coffee, and so forth).
In a small bowl, combine the cornstarch with the second, smaller amount of milk. Mix well. Whisk in the tempered egg yolks.
Pour through a fine-mesh sieve into a large bowl. The bowl should be large enough to hold the entire amount of milk that is in the pot, but it should not be so big that it cools down the hot milk too much once it is added to the bowl.
Bring the milk and sugar to a rolling boil. Let it boil for 10 seconds, and then, in one motion (very important step: It must be one dumping motion, not a slow pour), dump it into the bowl with the yolks, cornstarch, and milk while whisking vigorously (preferably another person is doing the whisking while you dump). It is imperative that you do not hesitate when dumping the milk in one motion and that you do not use a slow pour; this is necessary in order to bring the temperature of the ingredients in the bowl up high enough to coagulate the protein in the egg yolks and gelatinize the cornstarch so it will thicken. Stir for about 1 minute without stopping. This is another very important step; if you stop even for a second to switch hands because your arm is tired, it may not work out.
Cover the pastry cream with plastic wrap and let it cool down over an ice water bath.