Dr. Skip's Corner: K-12 Teaching Adventures @ OSU Engineering
July 2004

Ice Cream in a Bag!

Ice cream has been a summer favorite for over a hundred years, especially in the United States. Americans have the highest consumption of ice cream in the world, at over 23 quarts per capita per year! You can celebrate July, National Ice Cream Month, by making your own tasty creation. All it takes is a few simple ingredients and some knowledge of Chemical Engineering!

Ice cream became popular and affordable to Americans after to the invention of a hand cranked ice cream churn by Nancy Johnson, which was patented in 1843. Prior to her device, ice cream was a treat only the wealthy could afford. Ice cream is made when milk or cream is mixed with sugar and flavoring and agitated as it is frozen. Ice cream gets its creamy texture from the way that the fat molecules, air bubbles, and ice crystals are combined within a highly concentrated mixture of sugar-water. Freezing too fast can cause the ice cream to be grainy, too slow and the ice cream will be soft. Testing various techniques until the ice cream is “just right” is a perfect science lesson for a hot summer day.

Materials:

  • Quart-sized zip top plastic bag
  • Gallon-sized zip top plastic bag
  • Ice cubes
  • Salt
  • 1 c. half & half
  • 3 T. sugar
  • 1 t. vanilla

Procedure:
Pour the half &half into the smaller zip top bag. Add sugar and vanilla. Seal the bag, and place it into the larger bag. Fill the larger bag with ice cubes and add about  ½  c. salt. Seal the larger bag. Shake the bag, and toss gently back and forth. It will get cold so you may want to wrap the bag in a towel. Keep it moving for about 15 minutes until it becomes ice cream. Wipe off top of small bag, then open carefully and enjoy!

Why add salt?
We add salt to lower the freezing point of ice. Ice will melt at 32 °F (0°C), but when salt is added, the freezing point is depressed: a 10% salt solution melts at 20 °F (-6°C). Ice must absorb energy in order to melt (melting is an endothermic process*), so heat is transferred from the environment (which includes your ice cream mixture!) to the ice, making the ice cream mixture colder. You can control how fast the ice cream forms, by modifying the amount of salt you use. You could try rock salt which, with larger crystals, takes longer to dissolve and may result in a smoother ice cream.

Find out more about the Science of Ice Cream:
http://www.foodsci.uoguelph.ca/dairyedu/freeztheor.html
http://www.eurekalert.org/pub_releases/2002-02/aaft-tso020602.php

*Thermodynamics of melting and freezing:
- melting is an endothermicprocess (heat is absorbed) while freezing (or crystallization) is an exothermic process (heat is evolved). The “hand warmers” that are sold in which a liquid is crystallized by rubbing the “little disc” or “snapping” something in a pouch to seed the crystallization is an example of an exothermic process we all encounter in daily life.