What ingredient is used to lighten mousse?

Whipped cream or Italian meringue is the correct ingredient used to lighten mousse. Mousse is a light and airy dessert that achieves its texture primarily from the incorporation of air, and whipped cream or meringue serves this purpose exceptionally well. When whipped cream is folded into the mousse base, it adds volume and a creamy texture without making the mixture heavy. Similarly, Italian meringue, which is made from whipped egg whites and sugar, provides both lightness and stability to the mousse.

In contrast, gelatin serves to stabilize a mousse, rather than lighten it, while milk adds moisture and flavor but does not contribute to the airy texture. Egg whites can provide some lift but are typically used in their whipped form as meringue; if not handled correctly in the mousse preparation, they may not yield the desired lightness. Thus, the combination of whipped cream or Italian meringue is essential for achieving the characteristic fluffiness of mousse.