Why does custard go thick when you heat it up?

When custard is gently heated, the egg proteins (ovalbumin and globulins) start to coagulate or denature, forming bonds that trap water and other compounds, causing the mixture to thicken. At the same time, the sugars in the custard undergo caramelisation, developing a slightly browned color and enhanced flavour.

The specific steps involved in the thickening of custard are as follows:

1. Protein Denaturation: As the custard mixture is heated, the temperature causes the protein molecules to unravel their tightly coiled structures. These uncoiled protein chains can then interact with each other, forming bonds known as disulfide bonds. These covalent bonds crosslink the protein molecules into a network that traps water and other components, resulting in thickening.

2. Gel Formation: As more and more crosslinks form between the protein strands, a network of protein chains develops. This network forms a gel structure that captures the liquid components, giving the custard its characteristic thick and smooth consistency.

3. Coagulation Point: The temperature at which the egg proteins in the custard fully coagulate and form a stable gel is called the coagulation point or setting point. This temperature typically ranges from 175 to 180 degrees Fahrenheit (80 to 82 degrees Celsius) but may vary depending on the specific recipe and ingredients used.

4. Continued Heating: If the custard is heated beyond the coagulation point, the protein network can become tighter, resulting in a firmer custard. Extended heating can also cause curdling or separation of the custard mixture, especially if it is not stirred continuously or if the temperature is too high.

It's important to note that the thickening of custard is a gradual process that occurs over a range of temperatures. Custards can be cooked on the stovetop, in the oven, or over a double boiler, with different methods requiring slightly different temperature control to prevent curdling.