Frosty Homemade Ice Cream

• One small Ziploc baggie (quart or sandwich size)
• 1/2 cup of milk (whole milk or cream work best, but any variety will be fine)
• 1 Tbsp. sugar
• 1/4 tsp. vanilla
• One gallon-size Ziploc bag
• 8-10 cups of ice
• 6 Tbsp. salt

 Once we had all of our supplies ready we followed these directions to make our delicious science ice cream in a bag:

• Add milk, sugar, and vanilla to the small baggie. Seal the bag, being careful to release excess air.
• Add ice and salt to the bigger baggie.
• Place the small baggie into the large bag of ice and salt. Seal the large bag.
• Shake for about 5 minutes or until the milk mixture turns into a soft solid.
• CAREFULLY open up the large bag, remove the small bag and rinse it off quickly and thoroughly in cold water (pay special attention to rinse off the opening of the baggie). Without this step salt may get into the ice cream, which completely spoils it! (I know from experience.)
• Either grab a spoon and eat your ice cream right out of the bag, or pour your ice cream into a bowl. Top with sprinkles and toppings of your choice and enjoy!

The Science of Homemade Ice Cream

To make any variety of homemade ice cream, milk needs to be partially frozen. Putting milk directly in the freezer and leaving it there makes the milk freeze solid, which is no good. When you try to eat the ice cream it’s no longer creamy and smooth. It gets all icy and crystal-y and weird.

Water freezes at 32°F, but because milk contains proteins and fat it freezes at a lower temperature. This means that trying to freeze milk with ice cubes won’t work either. You need to add a special ingredient to your ice cubes to make a mixture that is even colder than ice alone. Ready for the secret ingredient? I’ll give you a few clues. It’s white. It’s something almost everyone has in their kitchen. It tastes yummy on popcorn with butter. It’s….SALT!

We talked about the magical science of how salt and ice cubes work when we did our Fishing for Ice experiment and when we made our delicious fruity slush. When salt is added to ice, it lowers the freezing point of the ice, making it melt. This leaves a salty-icy-watery mixture that is much colder than 32°F. The temperature of the salty mixture is close to 0°F! (You can verify this with a thermometer.) This temperature is cold enough to freeze milk into homemade ice cream in less than 10 minutes without freezing it solid.