Two descriptions are usually used to describe egg white form; both refer to its appearance when the whisk is slowly lifted vertically from the bowl. If it leaves behind a mound with gently drooping tip, then the whites are at the soft peak stage. If the mound tapers to a sharp, upright tip, they are at the stiff peak stage. Beating beyond this point makes the protein network contract and squeeze out water, producing a deflated, curdled looking mass oozing clear liquid. To avoid this, stop beating as soon as the whites are stiffly peaking. They should slide around a little bit when the bowl is tilted. Overbeaten egg whites will stick, not slide.
When you beat air into egg whites, the proteins they contain link up loosely to form thin, elastic walls surrounding the air bubbles. In the oven's heat, beating the bubbles expand and the protein set; this is what gives souffles, meringue, macaroon, chiffon and cake to puff and lightness.
To produce an extremely stable form that results in higher, lighter baked goods adding 1/4 tsp cream of tartar for every 2 egg whites helps to stabilize the form. Sugar in quantity produces a shiny, stable foam, while salt has slight destabilizing properties. One thing is lethal to an egg white foam; Fat, even the smallest trace of fat on bowl or beater will disrupt egg whites, foaming ability, so make sure oil, butter or egg yolk never go near them. Fresh egg whites yield a more stable foam but less volume and older whites, vice versa.
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