Ice cream is a frozen custard (or milk mixture) with air whiped into it. The custard or mix, especially for light ice cream, is a delicate balance of dairy products, sweetener, flavorings, and sometimes eggs. Each plays an important role in the overall outcome.
In this step, you heat the milk, and combine the egg yolks and sugar separately. Sugar may cause the milk to curdle if heated, and the egg yolks may coagulate if exposed to extremely high temperatures. Gradually add half the hot milk to slowly heat the egg mixture.