Marshmallow: syrup becomes an air structure

Useful to know

King Arthur’s homemade marshmallow combines soaked gelatin with hot sugar syrup, then whips the mixture into a thick, airy mass. After setting, it can be cut. [1]

This differs from simply cooking hard sweets. The thermometer and order of additions belong to the method; colour alone is not an equivalent check. Hot syrup needs a suitable vessel and careful pouring. Different thickeners do not create identical structures, so gelatin cannot automatically be replaced one-for-one with an arbitrary substitute.