Royal icing is a hard white icing

Icing (food)

Icing, also called frosting in the United States, is a sweet often creamy glaze made of sugar with a liquid such as water or milk, that is often enriched with ingredients such as butter, egg whites, cream cheese, or flavorings and is used to cover or decorate baked goods, such as cakes or cookies...

, made from softly beaten egg whites, icing sugar (powdered sugar), and sometimes lemon or lime juice. It is used on Christmas cake

Christmas cake

Christmas cake is a type of fruitcake served at Christmas time in the UK, Ireland, Japan, Philippines and many Commonwealth countries.A Christmas cake may be light or dark, crumbly-moist to sticky-wet, spongy to heavy, leavened or unleavened, shaped round, square or oblong as whole cakes, fairy...

s, wedding cakes, gingerbread houses

Gingerbread

Gingerbread is a term used to describe a variety of sweet food products, which can range from a soft, moist loaf cake to something close to a ginger biscuit. What they have in common are the predominant flavors of ginger and a tendency to use honey or molasses rather than just sugar...

 and many other cakes and biscuits, either as a smooth covering like marzipan

Marzipan

Marzipan is a confection consisting primarily of sugar and almond meal. Persipan is a similar, yet less expensive product, in which the almonds are replaced by apricot or peach kernels...

, or in sharp peaks. Glycerine is occasionally added to prevent the icing from setting too hard.

As well as coating cakes and biscuits, royal icing is usually considered a decorative icing since it can be used to create many decorative effects

Cake decorating

Cake decorating is one of the sugar arts that uses icing or frosting and other edible decorative elements to make otherwise plain cakes more visually interesting...

, such as flowers and figures. Royal icing is often piped into shapes which are allowed to harden on a non-stick surface. These can then be arranged to create edible decorative effects on a variety of sweet foods.

Health risk

Although royal icing has traditionally been prepared with fresh egg whites, many people substitute meringue

Meringue

Meringue is a type of dessert made from whipped egg whites and sugar, occasionally some recipes may call for adding an acid such as cream of tartar or a small amount of vinegar and a binding agent such as cornstarch found in icing sugar which may be added in addition to the corn starch which...

 powder, given the risk of salmonella

Salmonella

Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

 that using raw egg whites presents. Alternatively, ready-to-use, pasteurized

Pasteurization

Pasteurization is a process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food...

, refrigerated

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

egg whites can be purchased in most grocery stores which should also provide a safe alternative to fresh egg whites.