Custard is a variety of culinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce (crème anglaise) to a thick pastry cream (French: crème pâtissière) used to fill éclairs. Most common custards are used as desserts or dessert sauces and typically include sugar and vanilla. Sometimes flour and corn starch is added as in pastry cream or crème pâtissière.
Custard is usually cooked in a double boiler (bain-marie), or heated very gently in a saucepan on a stove, though custard can also be steamed, baked in the oven with or without a water bath, or even cooked in a pressure cooker. Custard preparation is a delicate operation, because a temperature increase of 3–6 °C (5–10 °F) leads to overcooking and curdling. Generally, a fully cooked custard should not exceed 80 °C (176 °F); it begins setting at 70 °C (158 °F). A water bath slows heat transfer and makes it easier to remove the custard from the oven before it curdles. A Sous-vide stick may also be used to produce a table top water bath with precisely controlled temperature.
Mixtures of milk and eggs thickened by heat have long been part of European cuisine, since at least Ancient Rome. Custards baked in pastry (custard tarts) were very popular in the Middle Ages, and are the origin of the English word 'custard': the French term 'croustade' originally referred to the crust of a tart, and is derived from the Italian word crostata, and ultimately the Latin crustāre.
Examples include Crustardes of flessh and Crustade, in the 14th century English collection The Forme of Cury. These recipes include solid ingredients such as meat, fish, and fruit bound by the custard. Stirred custards cooked in pots are also found under the names Creme Boylede and Creme boiled.
While custard may refer to a wide variety of thickened dishes, technically (and in French cookery) the word "custard" (crème or more precisely crème moulée, [kʁɛm mule]) refers only to an egg-thickened custard.
When starch is added, the result is called pastry cream (French: crème pâtissière, pronounced [kʁɛm pɑtisjɛːʁ]) or confectioners' custard, made with a combination of milk or cream, egg yolks, fine sugar, flour or some other starch, and usually a flavoring such as vanilla, chocolate, or lemon. Crème pâtissière is a key ingredient in many French desserts including mille-feuille (or Napoleons) and filled tarts. It is also used in Italian pastry and sometimes in Boston cream pie. The thickening of the custard is caused by the combination of egg and cornstarch. Corn flour or flour thicken at 100 °C and as such many recipes instruct the pastry cream to be boiled. In a traditional custard such as a crème anglaise, where egg is used alone as a thickener, boiling results in the over cooking and subsequent 'curdling' of the custard; however, in a pastry cream, starch prevents this. Once cooled, the amount of starch in pastry cream 'sets' the cream and requires it to be beaten or whipped before use.
When gelatin is added, it is known as crème anglaise collée ([kʁɛm ɑ̃ɡlɛz kɔle]). When gelatin is added and whipped cream is folded in, and it sets in a mold, it is bavarois. When starch is used alone as a thickener (without eggs), the result is a blancmange. In the United Kingdom, custard has various traditional recipes some thickened principally with cornflour (cornstarch) rather than the egg component, others involving regular flour; see custard powder.
After the custard has thickened, it may be mixed with other ingredients: mixed with stiffly beaten egg whites and gelatin, it is chiboust cream; mixed with whipped cream, it is crème légère, [kʁɛm leʒɛːʁ]. Beating in softened butter produces German buttercream or crème mousseline.
A quiche is a savoury custard tart. Some kinds of timbale or vegetable loaf are made of a custard base mixed with chopped savoury ingredients. Custard royale is a thick custard cut into decorative shapes and used to garnish soup, stew or broth. In German it is known as Eierstich and is used as a garnish in German Wedding Soup (Hochzeitssuppe). Chawanmushi is a Japanese savoury custard, steamed and served in a small bowl or on a saucer. Chinese steamed egg is a similar but larger savoury egg dish.
Recipes involving sweet custard are listed in the custard dessert category, and include:
Cooked (set) custard is a weak gel, viscous and thixotropic; while it does become easier to stir the more it is manipulated, it does not, unlike many other thixotropic liquids, recover its lost viscosity over time.
A suspension of uncooked imitation custard powder (starch) in water, with the proper proportions, has the opposite rheological property: it is negative thixotropic, or dilatant, allowing the demonstration of "walking on custard"; see the physical properties of custard powder. Starch is often added into custard to serve as a safety net from overcooking and to prevent premature curdling. The starch granules behave as a heat buffer in the mixture. Granules slowly hydrate and absorb heat in order to maintain a constant rate of heat transfer. Along with preventing the formation of curds, starches help improve the smooth texture and thick mouthfeel.
Eggs contain the proteins necessary for the gel structure to form and natural emulsifiers to maintain the structure. The components of a whole egg is the egg yolk and egg white. Egg yolks contains a high amount of high-density lipoproteins, low density lipoproteins, iron, lecithin, thiamin and vitamin A. Yolk also contains an array of enzyme like amylase, which can breakdown any starch that has been added. This enzyme activity contributes to the overall melting/ thinning of custard when it is in your mouth]. Lecithin is another key aspect of the yolk that helps maintain the milk- egg interface. Egg whites are 90% water and contain good amount of protein called albumen protein. The albumin proteins are made up of ovalbumin, ovotransferrin, ovomucoid, globulins, lysozyme, ovomucin, avadin, and a few others. Ovalbumin, ovotransferrin, and lysozyme are all proteins that are observed to set at 80˚C, 60˚C, and 75˚C respectively.
pH is a very important property of this structure. It is observed that the higher the pH(9 or higher) the harder the gel and at a low pH(5 or lower) the gel structure has difficulty forming. This is due to the protonation of proteins that make incapable of forming covalent bonds.
The first step in forming the gel always involves milk and heat-treating it. In another container egg yolk, egg whites, sugars, emulsifiers, and stabilizers are blended together. The egg yolks and egg whites are either fresh or in a powder form. Then the heated milk and egg mixture are mixed together. It is important to note that the hot milk is slowly poured into the egg mixture in order to prevent curdling. If done the other way around the hot milk can overcook the egg protein and ruin the homogeneity of the custard. Once everything is mixed, the milk-egg solution undergoes homogenization in order to ensure all the ingredients are spread throughout. This is done in two stages, the first at 2000 psi and the second at 500 psi. The first stage is done at high pressure to break down any fat globules. The second stage is efficient at creating a homogeneous mixture. At this point starches, in a starch slurry, and flavors are added to prime the liquid for gelatinization. The best way to create a gel in the liquid is to apply a gentle heat towards the egg proteins. The ideal temperature will 85˚C for 5 minutes, this will cause for the egg proteins to slowly unwind and rearrange. The custard is then cooled, which allows for the gel structure to set. Cooling is normally done by running cold water around the container and can be slowly stirred.
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