What’s the Skin on Heated Milk?

Published on Tuesday, February 12, 2008, by Michele Foley / Edit Post

Why does heated milk form a skin?

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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POST A COMMENT |10 Comments

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  • Why didn't I learn this kind of stuff in culinary school? No, I wasn't out that day.

  • When a protein becomes denatures, that means that IT loses its structure, not a cell (a cell is made up of millions of different proteins, among other things). In this case, the whey proteins aren't even inside a cell.

  • I have always known the skin as a "plechta," coming from the Yiddush language.

  • Oops. That was fully my editing mistake, Maxmillion. I've changed the copy back to a much more logical "or."

  • "...stirring constantly, and keeping the pot covered..."

    Just how does one achieve this physical impossibility?!

  • Great info. I love using milk to make hot chocolate, but hate the film.

  • For people who like the skin, try skimming it off when it forms, and then mixing it will a little bit of sugar and eating it...awesome...

  • Sniglets comes from Not Necessarily the News, which was an HBO show in the 80s.

  • Looooove the skin, on that and on pudding.

  • Related trivia... In the 1970's reference book SNIGLETS (it was distributed via the television show REAL PEOPLE) that skin/oily membrane that formed on top of liquids (gravy, milk, coffee, pudding, tea) was called a BLEEMUS.