After hundreds of years, the best ice creams are still made with fairly simple ingredients: milk, cream, sugar, and maybe eggs. You can make ice cream using a 1/2 cup of milk, 1/2 cup of cream (which is higher in fat than milk), 1/4 cup of sugar, and 1/2 teaspoon of vanilla or other flavoring. Stir the ingredients together in a bowl, then pour the mixture into a quart-size freezer ziplock bag. Stick this bag inside a gallon-size ziplock, half-filled with ice and rock salt – about 2 cups of ice and 1/2 cup of salt.
Salt lowers the freezing point of water, which causes the ice to melt at a lower temperature. The lower freezing point provides the temperature difference needed to transfer heat between the freezing ice cream ingredients and the melting ice. Rock salt doesn’t lower the freezing point as much as table salt does (so it results in smoother ice cream, because it freezes more gradually), but for this activity you can try table salt. Use a thermometer to measure the temperature in the outer bag. Next, begin shaking the bag so that the ingredients are whipped together. What do you expect to happen to the cream mixture? After five minutes of shaking, let the bag sit for a few minutes. Now take the temperature inside the gallon bag again. Has it changed? What happens if you don’t shake it?
When the ice cream is thick, get out a spoon and enjoy!
Ice cream is a colloid, an emulsion where two substances are just suspended within each other rather than being chemically bonded together. This is why many ice creams also have an emulsifier to prevent the fat molecules from separating from the rest of the ice cream (this makes the texture of the ice cream smoother). Ice cream also uses a stabilizer (like gelatin or guar gum) to help hold air into the ice cream, which gives it its light texture. To be officially called ice cream, the colloid has to be at least 10% milk fat and 6% non-fat milk solids (such as proteins).