Milk is made up of water, fats, carbohydrates, and different kinds of proteins. If milk is heated above 158 degrees Fahrenheit, its whey proteins begin to denature (meaning the cell structure is altered), says Robert F. Roberts, associate professor of food science at Penn State.

At the same time, the water in the milk starts to evaporate. As this happens, the proteins at the surface are in closer proximity and the denatured whey proteins are “stickier,” which allows them to bind with other milk proteins, such as caseins. The proteins eventually become concentrated enough to create a thin film at the top, says Douglas Goff, professor of dairy science and technology education at the University of Guelph in Canada.

Maintaining a low heat (below 158 degrees Fahrenheit) and either stirring constantly or keeping the pot covered will stop the film from forming.

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