Why does ice cream melt?

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July is National Ice Cream Month and July 19 is National Ice Cream Day. President Ronald Reagan proclaimed it so in 1984. Whatever flavor or topping you choose, you can be sure if you don't eat it fast enough it will become a sticky mess.

So why does ice cream melt?

To find out, we talked to Sean O'Keefe, a professor and food chemist at Virginia Tech. Dr. O'Keefe remembers playing cards and eating ice cream with his great-aunt when he was a kid. The ice cream at her house was his favorite because her freezer didn't work too well and the ice cream was usually soft. That's one reason ice cream melts: temperature.

Dr. O'Keefe explained that energy is present in both cold and warm objects. But the warmer an object is, the more energy it has. So when ice cream (a cold object) is eaten in a warm place (like your backyard in July), it quickly begins absorbing energy. Once a certain amount of energy has been absorbed, the ice cream begins to melt. It's like boiling water. The water will boil and turn into a gas once it has absorbed enough energy from the heat of the stove.

Ingredients also affect how ice cream melts. Cream, sugar, eggs and things such as chocolate chips, cookie dough and fudge go into ice cream. One of the most important ingredients, however, is fat. It gives ice cream its creamy texture. It also helps determine how fast it melts. O'Keefe said nonfat ice cream will melt more slowly than regular ice cream because it contains more water. More water means the ice cream will have to absorb more energy before it can melt. Also, low-fat ice creams tend to have more air whipped into them, which allows them to keep their shape longer.

Finally, have you put a carton of melted ice cream back into the freezer? You probably noticed that it doesn't taste as good. It is rough and crunchy, right? This is because the water that melted from the ice cream refroze and expanded into large ice crystals.



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