THE SCIENCE OF MERINGUE
Meringues are essentially made of foamed egg whites and sugar, but they can be made in a variety of ways and they can be used in a lot of dishes that need some foamy texture. When used on their own, meringues need to be very stable, so they don't collapse or release water. Since egg white foam alone cannot be stabilized for a long time, it needs that extra ingredient that will do the job. This ingredient is sugar. Meringue doesn't just get its sweet flavor from sugar, it also becomes stable as the sugar makes the liquid of the air bubbles more sticky and so it hinders its leaking from the foam. Another way to stabilize the meringue is the heat. The proteins that hasn't become denatured by whisking become denatured by heat, and thus they contribute to the foamy texture of the meringue. Of course, as the water evaporates, the meringue gains more stability as well. In some dishes, like chocolate mousse, the egg foam is further stabilized by even other ingredients, like chocolate.
What exactly is the role of sugar in an egg white foam? Sugar gives more body and texture to the meringue, as well as more glossiness. It also makes the meringue more crispy when it is heated. The amount of sugar that is normally added to the egg white foam varies between fifty-fifty or 70-30. Addition of sugar should chary, because we cannot dissolve too much of it in an egg white. Another thing to consider is that granulated sugar dissolves slowly and not very effectively, which will give the meringue a coarse texture and beads of syrup. The best ways to use sugar is to get a powdered one, as fine as possible, or to make or buy a sugar syrup by dissolving sugar in low amount of water before adding it to the egg white foam.
The most silky texture in a meringue is obtained by adding sugar after the egg whites are beaten to a firm egg white foam. Then, if the meringue is supposed to be soft, sugar can be gently folded in with a spatula. If a firm meringue is desired, the sugar has to be beaten into the egg whites. The more beating, the more stiff the foam will beccome and so the meringue can be shaped more easily. Your choice depends on your needs: if you want a soft mass, then beat the egg whites first and then fold in the sugar. Firmer, stiffer meringue is made by beating the egg whites together with the sugar. I just repeated this rule because it's worth to have in mind when preparing meringues.
When it comes to the heat, cooked meringues that is, more care has to be taken so that the hot air doesn't unfold the egg proteins too quickly so that the air bubbles will collapse before the water has evaporated. Usually, a plain cooked meringue is baked in low heat for a long time. If we are careful, the heat is actually very helpful. It will unfold the egg proteins that were not unfolded by the beating procedure and make them a part of the threaded, aggregated protein network that stabilizes the foam. It also makes sugar more soluble in water, so more sugar can be added for more body. And, just for the safety reasons, the heat will kill any possible salmonella bacteria in the egg, although nowadays (at least in controlled areas) the eggs are checked for infections prior to their packaging and distribution.