Casein interactions The melt and stretch properties of cheese are based on the number of interactions between casein molecules, the major milk protein. The fewer the interactions, the greater the melt. Stretch requires an intact, interconnected casein network and is lost as the interactions between casein molecules (or aggregates) decrease. Stretch is the result of casein–casein interactions that are broken easily but also readily reform at different locations in the casein network. Think of holding a piece of warm Mozzarella, take one end in each hand and gently pull it apart. The casein molecules are grabbing and releasing each other while sliding past as you pull the cheese.