Italian Meringue is beaten egg whites to which a sugar syrup at the
"soft ball" stage is added. The syrup that is not hot enough will form
an unstable foam: too hot and it will crystallise and won't combine with
the egg whites.
Why use Italian Meringue to make the Macaron Shells?
Since the sugar is incorporated in the form of a syrup, it disperses
into the beaten egg whites well, puffing them up while evaporating some
moisture. The sugar envelops the air bubbles and holds the foam
together, making the meringue smoother and shinier.
Since Italian meringue has a denser consistency, it is easier to
incorporate into almond paste.
More Great Recipes @ www.macarons.org.uk