The protein substance derived from collagen and used to make marshmallows is gelatin.

What gelatin is

Gelatin is a protein obtained by partially breaking down collagen from animal connective tissues such as skin, bones, and cartilage. It is colorless, flavorless, and forms a gel when dissolved in hot water and then cooled.

Role in marshmallows

In marshmallows, gelatin acts as the main gelling and foaming agent, giving them their characteristic soft, springy, and chewy texture. The gelatin protein network traps air and water in the whipped sugar mixture, allowing marshmallows to hold their shape while remaining light and fluffy.