Aging egg whites mean separating the whites from the yolks, and storing the whites in an airtight container in the fridge for 1 to 2 days before using them to make macarons. Why do you need to age egg whites? “The reason behind it is to reduce the moisture content as much as possible while keeping the protein bonds from the egg whites the same,” says Hélène Dujardin.
Macaron master Pierre Hermé also says that the aging process increases the whites’ elasticity. If you skip this step, you might end up with a runny or watery batter, which will not yield great results. So please age your egg whites, and take them out of the fridge a few hours before making macarons to bring them to room temperature before beating.