Edible Science: Ice Cream

Shake off the heat with a cool, tasty treat!

You will need:

Ice cream ingredients

  • 1 cup whole milk
  • 2 Tbsp. sugar
  • ½ tsp. vanilla extract

Materials

  • 4 cups ice cubes
  • ½ cup rock salt or ice cream salt
  • 2 zip seal quart-size freezer bags
  • 1 zip seal gallon-size bag
  • Kitchen towel or gloves or oven mitts
  • Measuring spoons
  • Measuring cup
  • Timer or clock

Here’s what to do:

  1. Add your ingredients (milk, sugar, vanilla) to one of the quart-size bags. Remove excess air from the bag and seal it so it is shut tight.
  2. “Double bag” it: Place your bag of ingredients inside a second quart-size bag, remove the air, and seal completely.
  3. Open the gallon-size bag, place your ice cream ingredients bag inside, then add ice and salt, remove any excess air, and seal the gallon bag shut.
  4. This next part can get messy – go outside! Use gloves, oven mitts, or a towel to help you hold the bag. The outer bag will be cold and will get wet and drippy as the ice melts.
  5. Set your timer or note the time and start SHAKING the bag!! Continue shaking for five minutes.
  6. Check your ingredients bag. Is your ice cream ready to eat? If not, keep shaking! Continue checking your ingredients every five minutes. Is your ice cream freezing?
  7. When your ice cream is thick enough, check your timer and note how long it took to freeze, then open your small ice cream bag and … EAT!

Take it further:

  • Why did you add salt to your ice? Do different kinds of salt (table salt, kosher salt, sea salt) have different effects? Which one works best?
  • Try it again without the salt. Does your ice cream freeze?
  • Do different types of milk or cream give different tastes? Does one freeze in less than five minutes? Try the recipe with heavy whipping cream or half & half to compare to whole milk.
  • Try making different flavors using fruit extracts or chocolate syrup. Can you make a custom flavor like strawberry vanilla?

What’s going on?

The freezing point of a liquid depends on how many things are dissolved in the liquid. Milk, for example, is water with a bunch of things dissolved or dispersed in it, like fat, sugar, protein, vitamins, and minerals. Milk is actually an emulsion because the fat and protein molecules are suspended in the water instead of dissolved. Your ice cream is an emulsion too! When you add the sugar and vanilla to your milk, you’ve added more things (solutes) to the water (solvent). Adding all of these solutes makes it harder to freeze the solution, so the freezing point goes down. We now have a solution that freezes at a lower temperature than water. How do we get colder than ice?
To get your ice cold enough to freeze your ice cream, we added salt. The salt (solute) dissolves in the water (solvent) and lowers the freezing point of the water, so your ice gets colder and doesn’t melt as fast. The salt and water solution now has a lower freezing point than your ice cream emulsion, which means the ice gets cold enough to freeze your ice cream!

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