There are two methods. In the simplest, known as sugar-frosting, the flowers are brushed with egg white and then dusted with caster sugar, and this will preserve them for up to two days.
If you want them to last longer you need to use gum arabic (available from cake decorating shops and some chemists) dissolved in rose or orange-flower water. To use gum arabic as a covering, melt 15 g of gum arabic in 2 tablespoons of flower water. The gum must be thoroughly dissolved, so follow the packet instructions.
Using an artist's paintbrush, coat the flowers, petals or leaves on all sides with the gum arabic mixture, then dust them all over with caster sugar. Place the flowers on a baking tray lined with nonstick baking paper and leave them in a warm place for one to two days until they are dry and hard. Stored in a glass jar with a tight-fitting lid, they will keep for up to a year.
As this method takes quite some time and the flowers tend to lose their shape, it is best-suited for petals and flowers that are strongly scented and flavored and have a good color, such as gardenia, rose petals or violets. They can be used whole as decorations, or can be crushed and stirred into meringues or whipped cream for a delicate flavoring. Frosted flowers are a pretty touch on a cake or mousse.
** Asian Recipes