Custard

Custard is a versatile culinary preparation used in many different dishes. It is made by cooking sweetened milk, cream, or cheese with eggs or egg yolks. These eggs act as a thickener to change the liquid into a soft gel. Some recipes include extra ingredients like flour, cornstarch, or gelatin to change the texture. Depending on the recipe, a custard might be a thin sauce or a thick cream.

People use custard for sweet desserts or savory dishes like quiche.

Preparing custard is a very delicate operation for a cook. If the temperature increases too quickly, the mixture can undergo overcooking and curdling. This process is known as syneresis, where the custard becomes bumpy instead of smooth. A fully cooked custard should generally not exceed 85°C. It typically begins to set at around 80°C. To manage this, cooks often use a double boiler or a water bath. A water bath slows down the heat transfer from the oven. This makes it easier to remove the custard before it curdles.

There are several distinct types of custard based on how they are made. In French cookery, the term custard refers specifically to an egg-thickened preparation. When starch is added to milk, egg yolks, and sugar, it is called pastry cream, or crème pâtissière. This version is a key ingredient in French desserts like éclairs and Napoleon pastries. It is also used in Italian pastry and Boston cream pie.

If gelatin is added to the mixture, the preparation is known as blancmange. In the United Kingdom, some traditional recipes use cornflour or regular flour as the main thickener.

Custard has a deep history that stretches back to the Middle Ages. During this time, custard tarts were very popular. These tarts are actually the origin of the English word "custard." The French term originally referred to the crust of a tart. This term comes from the Italian word "custarda," which comes from the Latin "costarda." A 14th-century English collection called The Forme of Cury contains recipes for these tarts. These early versions used meat, fish, or fruit held together by the custard.

Many different cultures have developed their own unique custard traditions. In Japan, a savory custard called chawanmushi is steamed and served in a small bowl. China also has a similar savory egg dish. In Greece, galaktoboureko is a pastry filled with semolina custard. Peru has a dish called manjar blanco, which is a type of baked milk. In Germany, a thick custard called custard royale is used to garnish wedding soup. These examples show how custard is valued across the globe.

The chemistry of custard explains its unique physical properties. Stirred custard is thickened by the coagulation of egg proteins. These proteins, along with emulsifiers in the yolk, create a gel structure. Egg yolk also contains an enzyme called amylase. This enzyme can break down added starch, which helps the custard thin out in the mouth. Adding cornstarch acts as a heat buffer. As the starch hydrates, it absorbs heat and maintains a constant rate of heat transfer. This helps prevent premature curdling and creates a smoother mouth feel.

Custard also displays interesting scientific behaviors in its texture. Cooked custard is considered a weak gel that is viscous and thixotropic. This means it becomes easier to stir the more it is manipulated. However, unlike some other thixotropic liquids, it does not recover its thickness over time. Interestingly, an imitation custard powder mixed with water shows the opposite property. This is called negative thixotropy, or dilatant behavior. This property is so unique that it allows for the demonstration of "walking on custard."