Mix together the lemon juice and egg
whites and gradually beat in the icing sugar until the icing
(frosting) is smooth and white and will coat the back of a spoon. A
few drops of glycerine will prevent the icing from becoming too
brittle. Cover with a damp cloth and leave to stand for 20 minutes to
allow any air bubbles to rise to the surface.
Icing of this consistency can be poured on to the cake and smoothed
with a knife dipped in hot water. For piping, mix in extra icing sugar
so that the icing is stiff enough to stand in peaks.