Science
Milk science: proteins, fat and foam
Why milk foams and steams the way it does, at a level beneath the practical steaming technique.
Proteins and foam structure
Milk foam is a structure of air bubbles stabilised by milk proteins, primarily whey proteins, which unfold and form a film around air pockets when milk is agitated. This is why steaming technique (introducing air in a controlled way) matters as much as the milk itself.
Fat content and texture
Fat content affects foam stability and mouthfeel: lower-fat milk can foam more voluminously because fat can interfere with protein film formation, but higher-fat milk often produces a smoother, more stable microfoam texture that many baristas prefer for latte art and mouthfeel.
Why plant-based milks behave differently
Plant-based milk alternatives lack the same whey protein structure, so manufacturers typically add stabilisers and emulsifiers to help them foam; results still commonly differ from dairy milk in foam stability and texture, and vary significantly between brands.