Marshmallows are just actually made up of gelatin, sugar, salt, water, flavorings and/or corn syrup. Sometimes they add eggs in it. Usually, there are no artificial preservatives used in marshmallows. Generally marshmallows are preserved because of the sugar contents in it. The thing that can change the quality of marshmallows is their shape and water content. As time passes by, the water molecules might go out of the mallows thus making it dry-looking. To attend to that situation, store the marshmallows in a container with so much less air.