Can I make cookie dough for meringues ahead of time, and put them in oven later on?
No, the air in the whipped egg whites will either deflate or dry out. Once baked, they keep for quite a while.
I wouldn't recommend it because whipped egg whites will lose the air after awhile. You could possibly freeze it, but I'm not 100 percent sure on that one.
No. They need to be baked right away. If you're in a dry climate, they can be stored in an airtight container. If you make the meringue batter and let it sit, it'll deflate and/or dry out to the extent that you will not be able to pipe them.
Egg whites (also known as the egg's albumen) have the ability to hold voluminous amounts of air when whipped. An egg white contains no fat and is composed of mostly protein, water and water-soluble vitamins such as riboflavin. Whipping the albumen stretches the protein causing them to unwind and join together loosely. This makes the proteins unstable.
Whipping egg whites is similar to blowing air into a balloon. Done correctly you get a fully blown balloon; overfill the balloon and it will pop. And, just like a balloon, if left to sit too long the air leaks out. If you over-whip the egg whites, the egg whites will have a thick and grainy texture. If you let them sit too long they will deflate. Unfortunately, if either happens, you have to start again with a new batch of room temperature egg whites.
Raw egg whites can be refrigerated in a tightly sealed container for up to 5 days. Or you can freeze egg whites in an ice cube tray, adding 1 egg white (about 2 tablespoons) to each section. When frozen, transfer to a zipper-lock plastic bag. Thaw overnight in a bowl in the refrigerator or in a tightly sealed zipper-lock plastic bag set into a bowl of cool tap water.