Chemistry
When egg whites are beaten, some of the hydrogen bonds in the proteins break, causing the proteins to unfold ("denature") and to aggregate non-specifically. This change in structure leads to the stiff consistency required for meringues. The use of a copper bowl, or the addition of cream of tartar is required to additionally denature the proteins to create the firm peaks, otherwise the whites will not be firm. Plastic bowls, wet or greasy bowls will likely result in the meringue mix being prevented from becoming peaky. Wiping the bowl with a wedge of lemon to remove any traces of grease can often help the process.
When beating egg whites, they are classified in three stages according to the peaks they form when the beater is lifted: soft, firm, and stiff peaks.
Sugar substitutes are not useful in meringue. The sugar is necessary to the structure.
Egg whites and sugar are both hygroscopic (water-attracting) chemicals. Consequently, meringue becomes soggy when refrigerated or stored in a high-humidity environment. This quality also explains the problem called "weeping" or "sweating", in which beads of moisture form on all surfaces of the meringue. Sweating is a particular problem for French meringues in which the granulated sugar is inadequately dissolved in the egg whites, and for high-moisture pie fillings.
Read more about this topic: Meringue
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