Cakes & Sponges
Royal, Glace And Fondant Icings Set By Three Different Routes
One dries into a rigid protein-bound solid, one crusts as water evaporates from a sugar syrup, and one sets by controlled crystallisation, which is why they behave so differently.
The three common sugar icings look similar in the bowl and behave nothing alike on a cake. Each sets by a distinct physical process.
Royal icing sets through egg white
Icing sugar beaten with egg white forms a paste in which the protein, not the sugar, provides the eventual structure.
As water evaporates the protein network dries and hardens into something genuinely rigid and brittle, capable of holding fine piped detail and of supporting weight.
The set is slow and depends on air movement and humidity. A thick layer can be dry on the surface and soft underneath for a long time.
Glace icing simply loses its water
Icing sugar mixed with water or juice is a supersaturated sugar solution with no protein and no fat in it at all.
It sets by evaporation alone, leaving a thin crust of sugar over a softer interior. It never becomes hard the way royal icing does.
That crust is fragile and re-softens in damp conditions, which is why glace icing suits things eaten soon and works badly on anything stored.
Fondant sets by controlled crystallisation
Poured fondant begins as a cooked sugar syrup that is agitated as it cools, forcing sugar to crystallise in very small crystals rather than large ones.
Those fine crystals are what give fondant its opacity and its smooth, slightly yielding bite. The size of the crystals is the whole quality of the product.
Warming it gently makes it flow for coating and it firms again on cooling. Overheating dissolves the crystals and the fondant sets dull and grainy.
Sugar concentration governs keeping
All three hold very little free water once set, which is why iced goods keep better than uniced ones in some circumstances.
But each will draw water from a moist cake beneath it or from humid air above it, and each fails in its own way when it does.
Royal icing softens at the base, glace icing goes sticky and transparent, and fondant weeps a syrup layer at the surface.
Choosing between them is a structural decision
Piped detail that must hold its shape and survive handling needs the rigid protein set, so royal icing is the only option.
A quick opaque coat on a loaf or a bun wants glace icing, which needs no cooking and no equipment and is finished in a minute.
A smooth mirror surface with a soft bite needs fondant, and the cooking and working it requires is the price of that particular finish.
Questions readers ask
Is a cracked cheesecake ruined?
Not at all, since a crack is a cosmetic fault and the texture is usually still good. Topping it with cream, fruit or a soured cream layer hides it completely.
Can I bake it without a water bath?
Yes, with a lower oven temperature and a longer bake, and many recipes are written that way. You lose some protection against overcooked edges, so watch the wobble more carefully.
