Meringues are very fragile. They should be made right before they are to be used. If allowed to sit for any length of time, they will begin to break down.
All utensils must be perfectly clean. No residue of fat should remain or the whites will not achieve the necessary volume.
If using copper, clean the bowl with salt and vinegar and wipe it dry with paper towels before adding the egg whites.
When separating the eggs, make sure that no yolk contaminates the whites because the fat of the yolk prevents the whites from achieving the desired volume.
Fresh egg whites are more viscous and produce a more stable meringue. On the other hand, older egg whites give more volume when beaten.
Adding a pinch of salt to (foamy) egg whites reduces viscosity, which increases the volume. However, the addition of salt lessens stability.
Egg whites beaten by hand with a balloon whisk usually result in a more even-textured meringue than those beaten with an electric mixer.
Using room-temperature egg whites also increases the volume. On the other hand, cold egg whites produce a more stable meringue.
When egg whites for a meringue are beaten, air is trapped in the form of bubbles, which serves to lighten other mixtures. The meringue can be folded into mousses, cake batters, pastry creams, and so forth to add airiness. If the meringue is folded into a preparation that is then baked, the air expands in the heat of the oven and causes the preparation to rise, as with a soufflé.
A copper bowl, if available, is a good vessel to use to beat egg whites, as the copper reacts with the proteins in the egg whites, yielding more volume than that produced in a stainless steel bowl. This method produces a great, even consistency.
If using a copper bowl, never add acid to the mixture or a toxic reaction will occur that will turn the whites green.
The more sugar called for in the specific recipe, the longer the beating process.
When making a French or Italian meringue, the sugar should always be added after the soft peaks have formed.
Be gentle when folding meringues into other mixtures, as they are easily deflated.