How to fold egg whites into a mixture
Start by folding a third of the egg whites into the other mixture (for the cake or mousse etc). This first portion of egg whites should be added to the mix more rigourously. This helps loosen the mixture and makes folding in the remaining egg whites easier.
Spoon the next third of whisked egg whites carefully on top of the mixture. When folding in egg whites, treat them gently. If the air is knocked out of them, they won’t lighten your dish as they should.
Fold with a large metal spoon, if possible, in order to minimise the crushing of the air bubbles. Cut down vertically into the centre of the mixture. Then fold the bottom of the mixture up and over the top of the egg whites. Give the bowl a quarter turn and repeat this action, folding slowly and carefully.
Repeat with the final third of egg whites. Continue turning and folding until there are no visible streaks or lumps of egg white left in the bowl. Continue to treat this mixture very gently as you pour it out. If you are making a mousse, spoon into a ramekin and allow to set for an hour in the fridge.