英语听力文摘 English Digest 543、什么是鲜奶油？！

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　　It adorns¹ ice cream sundaes, pumpkin² pie, gingerbread, and hot chocolate. Its sweet creaminess tantalizes³ taste buds and sends dieters running from the scale. Did you ever wonder why heavy cream whips into soft, light peaks? You can find out by listening today's moment of science!

 

　　When you whip heavy cream, air is forced into the liquid, and a stable mass of bubbles forms. Each bubble is a tiny pocket of air encircled by a thin film of water with proteins and other substances dissolved in it.

 

　　The protein molecules⁴ that are caught in the bubbles are pushed and pulled by different forces, so they lose their normal shape. Once they lose their normal shape, the protein molecules bond together into a thin film, which stabilizes⁶ the bubble walls.

 

　　The proteins that stabilize⁵ whipped cream are the same as milk proteins, so it would seem that milk would whip into soft shiny peaks as well. But you could whip a bowl of milk all day and never get anything more than a light froth that would disappear almost immediately.

 

　　The difference lies in whipping cream’s high fat content. Whipping cream is from thirty to forty percent fat, in contrast to the four percent fat found in whole milk.

 

　　The fat molecules in cream form a strong, stable network that keeps the bubble walls from collapsing⁷. Small spheres of fat molecules gather together in the bubble walls, where surface forces break open the sphere. The exposed fat molecules stick to each other, and the network forms.

 

　　The fat molecules in milk act similarly, but there aren’t enough of them to stabilize the milk foam⁸ for more than a few seconds.

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