Why does ice cream melt?
|By Kathy Wollard|
Why does ice cream melt? asks Kim Price, via email.
Whether premium, low-fat, nonfat, or low-carb, all ice cream melts on a hot summer day, dripping down the cone and onto the front of your shirt.
In melting, ice cream is just behaving like any other bit of frozen, icy matter, suddenly exposed to warm air. Matter changes its state, depending on temperature and pressure. Liquid water boils into a gas (water vapor), freezes into a solid (ice), and melts back into a liquid if left out of the fridge.
But ice cream isn’t plain water, and how (and how fast) it melts depends on more than just temperature. The melting qualities of ice cream are actually a favorite focus of some food scientists, since how ice cream melts affects how it tastes.
If you’ve ever made ice cream at home—or idly read the carton label as you dug in with a spoon—you know ice cream’s basic ingredients: cream and/or milk, sugar, and flavoring, plus egg yolks in the custardy varieties. Some brands add chemical stabilizers and emulsifiers. Finally, there’s the fruit, nuts, chocolate chips, candy, cookie dough, and other extras that create hundreds of flavors.
Scientists call ice cream a frozen foam, since ice cream is partly just thin air. Premium, high-fat ice creams contain the least air, while some bargain brands may be more than half air by volume. (Which is why a pint of Haagen-Dazs usually weighs more than a pint of generic.)
After the cream, milk, sugar, flavorings (and egg yolks) are combined or cooked together, the mixture is transferred to an icy-cold blending machine. Air is whipped in, milk proteins and fat droplets surrounding the air in a honeycomb structure. Ice crystals form throughout the mixture as it freezes. Rotating blades break the crystals into tiny pieces, so that the resulting ice cream has a smooth, non-gritty texture. After final ingredients (from coconut to candy canes) are mixed in, the ice cream is put in the deep freeze to harden at about -40 C (-40 F).
Researchers test the melting rates of different ice creams by putting a scoop on a wire screen in a warm room, measuring the fluid that drips through. Ice cream melts as it absorbs heat from the air, with ice crystals on the outside of the scoop melting first.
Scientists say that the melting rate of ice cream depends mainly on the amount of whipped-in air, the size of its ice crystals, and its framework of fat globules. Airier ice creams (and those containing the additive polysorbate 80) tend to keep their shapes longer in the heat.
But leave the carton out on the counter too long, and your ice cream will suffer the dreaded food-science fate called "heat shock." Each time ice cream half-melts and then is shoved back into the freezer, its liquid water refreezes around existing ice crystals instead of forming new ones. Over time, ice crystals get bigger and bigger, and your once-creamy treat becomes lumpy, coarse, and crunchy.
For more on melting ice cream, visit this website.
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Copyright © 1999-2006 by Kathy Wollard & Debra Solomon