Fat is what gives milk its rounded mouthfeel. The rounded mouthfeel is enhanced by steaming the milk because the fats melt; therefore, the more fat there is in milk, the silver it becomes when steamed and the higher the flavor and velvety smoothness of the complete cup of coffee.
Milk with more protein has a more stable foam structure and are crucial in creating the surface of air bubbles in milk foams.Introducing air into coffee by steaming leads to an unstable interface between air and water, which needs to be stabilized by the adsorption of “surface-active components” made up of proteins. The proteins attach to the surface of the air bubbles and stabilize them. The milk proteins form a stable surface layer, resulting in stable foam.