I Scream, You Scream: The Science of Ice Cream
Ice cream is a staple food found in most cultures and places. If flavored ice counts as ice cream, its written history goes back thousands of years to the Persian Empire and the Roman Empire. It was, however, the Arabs who came up with the brilliant idea to add milk to this concoction, producing the ice cream that we know and love.
The mechanism of the first ice cream machine is credited to an African-American confectioner named Augustus Jackson in 1836. Despite this innovation, the hand-cranked method of ice cream making was patented by a woman, Nancy Johnston, on 9th September 1843. Believe it or not, before the days of the electric ice cream maker, ice cream was created by hand-cranking the mixture of ingredients in a compartment filled with ice and salt.
The salt-in-a-bag method is simply a modification of that! Before walking through how to make ice cream without electricity, let’s talk a bit about the science of ice cream.
Have you ever noticed that, if you just put melted ice cream back into the freezer, the texture is no longer the same? This is because the water molecules of the melted ice cream have crystallized, turning what used to be gloriously creamy ice cream into tiny bits of ice. To ensure that this does not happen during the production process, the ice cream ingredients are constantly being mixed, usually by an electrical mixer.
So how can we get our ice cream to freeze while we mix if we don’t have electricity? One of the things we need to do is lower the temperature of the ice cream mixture enough so it can freeze. We accomplish this by adding salt to lower the freezing point of the ice that we use to freeze the ice cream.
Water molecules on the surface of ice cube can easily go from being in a solid state to a liquid state, because they interact frequently with the heat in the environment. Once they’re in a liquid state, though, they can also refreeze when they come into contact with the surfaces of the unmelted ice cubes. This creates a dynamic equilibrium between the solid ice and liquid water. When salt is added, it dissolves in the liquid water and changes its properties, interfering with the water molecules’ ability to line themselves up into solid ice. Thus, the freezing rate goes down, a phenomenon we call freezing point depression. In order for the water to freeze, it must absorb greater amounts of heat from its surroundings, which, in our case, means that our salted ice cubes absorb heat (or cool) our ice-cream mixture.
Other important factors that affect ice cream production are the amount of air and type of dairy. The amount of air mixed in with the ingredients affects how dense the ice cream is. A lot of added air makes soft-serve ice cream; less might make something more similar a tub of ‘Ben & Jerry’s. Dairy, or the ratio of milk vs. cream, affects how how thick the ice-cream is. This is mainly because the amount of fat present affects the homogenization of the ice cream mixture.
Now to the fun part!
- About five minutes
- For yourself only
- Ice cubes
- 1 cup dairy (either milk or some mixture of milk, half and half, and cream)
- 1/2 cup of salt
- 2 tablespoons sugar
- 1/2 teaspoon vanilla extract
- 1 pint size (small) Ziploc bag
- 1 gallon sized (large) Ziploc bag
- Mittens for your hands— this will get VERY cold!
- Put the dairy, sugar, and vanilla extract into the small bag and stir it together. Seal the bag tightly.
- Place the ice and salt inside the big bag, and then place the sealed small bag containing your ice cream mixture on top of the ice and salt.
- Put your mittens on, seal the big bag, and shake for about 5 minutes, or until you see your ice cream mixture solidify. The shaking will replace hand-cranking or electrical mixing.
- Take the small bag out of the larger one and open the bag. Add any confectionery (e.g. chocolate sauce, coconut flakes, etc.) to your ice cream, and enjoy!
- Note how the texture of the liquid mixture will solidify smoothly without any crunchy ice bits. The more you shake the bag, the smoother your ice-cream should be!
By Asta M, Staff Writer.
Edited by Yi Z.