This Week's Experiment - #279 The Science of Ice Cream

milk
sugar
vanilla extract
sandwich sized plastic bags (the "zip to close" kind works best)
a large plastic bag
ice
salt
a small dish

From the list above, you may have guessed that we are going to make some ice cream. We will investigate some of the science involved in making good ice cream.

Start with the sandwich sized bag. Into it, put one cup of milk, one tablespoon of sugar and 1/2 teaspoon of vanilla extract. Close the bag and shake it a bit to mix the ingredients. Open the bag and pour a little of the mixture into a small bowl. Reseal the bag.

Put the bowl of mixture into the freezer. We will let it sit undisturbed as it freezes. Put the sealed bag with the rest of the mixture into a larger plastic bag. Add enough ice to fill the larger bag about half full and then sprinkle a handful of salt onto the ice. Seal the large bag. Shake the bag for about 5 minutes. If your hands get cold, you can wrap the bag in a towel, or you can get a friend and play "cold potato" by tossing it back and forth. Of course, then you would have to share the ice cream. Keep going until you can see that the ice cream mixture is frozen.

After it is well frozen, open the large bag and pour out the ice and saltwater. Open the small bag and use a spoon to taste your results. Yum! You now have some homemade ice cream. As you eat it, pay close attention to its texture.

By this time, the mixture in the bowl should be frozen. Try to eat this with the spoon. Not as easy is it? And the texture is much different.

What is the difference? The biggest difference is that the mixture in the freezer sat still as it froze. When the mixture is undisturbed, the ice crystals grow quite large, making the ice cream very hard and icy. By constantly disturbing the ice crystals, you wind up with lots of small crystals instead of a few larger ones. This makes the texture of the ice cream much smoother and more pleasant.

The mixing also traps air bubbles in the mix, making it fluffy and lighter. You can see how much air they mix with store bought ice cream by letting some melt and then refreeze. You will find that it shrinks by almost 1/3 of its size once the air bubbles come out.

If you are a dedicated scientist, you could also test to see whether adding chocolate syrup to the mixture changes the way that it freezes. Of course, you should also test it with fresh strawberries, diced peaches, blueberry jam, .............

From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com

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