At some point when you were a kid, you probably made homemade Popsicles out of fruit juice. One listener noticed that the homemade ones never taste quite as good as the ones you buy in the store. You'll hear why in this Science Update.
Freezing the perfect Popsicle. I'm Bob Hirshon and this is Science Update.
Making your own Popsicles seems like a good way to save money and keep your kids happy. But Maria Kirsch of Newton, Massachusetts, found that the result can be disappointing.
Why is it when you purchase Popsicles in the grocery store, the flavor goes all the way through, and when you make them at home, there's always ice that fills up the middle, so the flavor seems to be just on the outside?
We asked Douglas Goff, a professor of food science at the University of Guelph in Ontario, Canada. He says when a Popsicle freezes, it doesn't just turn into a solid block of juice. The water comes out and freezes into a network of pure ice crystals.
Then what you have is a mixture of pure ice crystals with this very concentrated sugar juice, if you like, with the flavor and color in it that is surrounding the ice crystals, but not frozen.
Commercially made Popsicles are frozen very fast, which keeps the ice crystals small, and the flavor and the color well-mixed. In a home freezer, though, there's plenty of time for the ice crystals to grow.
They keep pushing that unfrozen phase away, and it just ends up near the surface.
If you've got a science question, don't freeze up. Call us at 1-800-WHY-ISIT. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
First of all, credit where credit is due: “Popsicle” is actually a brand name (for a product invented by an 11-year old, no less), although it has turned into a generic term, just like the brand names Novocaine, Kleenex, and Band-Aid. So we should really be saying "quiescently frozen juice-based dessert." But like everyone else, we won't bother.
Anyway, the essential principle behind this story is that just because something is frozen doesn't mean it's frozen all the way through. If you look at a Popsicle (or homemade imitation) on a microscopic scale, it's really a complex matrix of ice crystals with droplets of sugar and flavoring suspended in it.
As Goff explains, the main difference between a homemade and a factory-made Popsicle is in how big the ice crystals are. Homemade pops freeze more slowly, which means that tiny crystals of ice tend to glom onto other pieces that have already frozen. As the pure ice chunks up, it shoves the droplets of flavor to the outer surface of the pop. On the other hand, if you freeze a Popsicle very quickly (the pros use a brine bath set to minus 40 degrees Celsius), the water droplets freeze more or less simultaneously, which means they freeze in their tracks, leaving the flavor and sugar molecules evenly dispersed throughout.
Another example of the effect of non-uniform freezing is the salt that road crews use in the winter. The salt doesn't exactly melt the ice on the road; rather, it keeps the droplets of water that do melt from re-freezing. You see, even on a cold day, a certain number of water droplets on the surface of the ice are constantly melting and re-freezing due to factors like sunlight, friction from tires, and so forth. When you pour salt on the road, the salt mixes in with the liquid water droplets so they don't re-freeze. This keeps happening until there's no solid ice left.
Now try and answer these questions:
- What separate components make up a Popsicle? Which freeze? Which remain unfrozen?
- What difference does freezing a Popsicle quickly make on the structure of the ice crystals?
- Suppose a homemade and a factory-made Popsicle were left in room temperature for 20 minutes, and each melted about halfway. What would the remainders of each look and taste like?
- Why do you think water droplets tend to stick together when given the chance?
This question on Popsicles from How Stuff Works includes more information and links to other Popsicle-related topics.
The University of Guelph's Ice Cream Science site goes into great detail about the making of another popular frozen dessert.