Whisking egg whites by hand or with a mechanical mixer causes the proteins in the egg white to denature and form weak bonds with each other. This plus the incorporation of significant air causes protein-stabilized bubbles to form, which produces a fine voluminous foam. Incorporating sugar sweetens the foam and stabilizes it by "holding on" to the water in the egg whites and preventing it from seeping out of the structure. Adding a small amount of acid in the form of lemon juice, vinegar, or cream of tartar helps keep the egg white proteins flexible, allowing them to unravel and make bonds without packing so tight that they squeeze out the water. The inclusion of even a small amount of fat will interfere with the protein structure and prevent the formation of a stable foam. Over-whipping the whites will also cause the meringue to become grainy and break down, and leaving the whites to sit for too long will eventually cause it to weep and separate.
In general, meringue is made with one part egg whites per two parts of sugar by weight. Soft meringue, which is used on lemon meringue pie and queen of puddings, for example, has less sugar than hard meringue. Soft meringue may contain equal parts of sugar and egg white by weight, whereas hard meringue may contain up to two parts sugar to one part egg white by weight.
Sometimes a small amount of flour is added to meringue in order to give it extra stability. These are sometimes called sponges but are more like meringue than cake.
In Western patisserie, there are three main types of meringue that have slightly different properties and uses. In English, these are French, Swiss, and Italian meringues.
French meringue is prepared by whipping egg whites to a soft foam, before gradually beating in granulated sugar until it forms firm, glossy peaks. It is very light but not particularly stable compared to the other meringues. As a result, it is often baked to dry out the meringue and make it crisp. Because it is so light, it can also be folded into other preparations to give them volume. Additionally, because French meringue involves beating in granulated sugar it is best to use superfine and/or powdered sugar to make sure it dissolves properly in the egg whites.
Swiss meringue is a partially-cooked meringue that involves warming egg whites and sugar in a double boiler until they are dissolved and the mixture is approximately 130˚F. This mixture is then whipped until it cools and reaches firm peaks. Because the sugar is added to the egg white early in the process, the resulting meringue is soft and dense. It is more stable than French meringue and can be baked or used as a frosting or topping.
Italian meringue is a cooked meringue. First sugar and water are cooked together to the soft-ball stage (about 236-240˚F). The whites are whipped to soft peaks, and the hot syrup is gradually beaten into the whites to form a glossy meringue. This is the most stable meringue, and it is often used as is or for filling and decorating pastries.
Meringue can be used in a variety of pastry applications. Uncooked French meringue can be folded into batters to make soufflés, cakes, and some cookies. It can be baked into meringue cookies or a crisp base for pavlovas and meringues. Meringue can also be used to lighten mousses or poached and served with custard. Swiss and Italian meringues are good for topping desserts and can be made into buttercream. Uncooked meringue can be toasted with a torch or a broiler much like marshmallow.