Dr. Free-Ride: Do you remember what [Dr. Free-Ride's better half] said we were going to do at some point this summer? Using the machine in our garage that Uncle Fishy and RMD left for us?
Younger offspring: That ice cream machine?
Dr. Free-Ride: Yeah.
Younger offspring: Oh, I love that!
Dr. Free-Ride: Well, what are we going to do with it?
Younger offspring: Make ice cream.
Dr. Free-Ride: Do you know how making ice cream works? Or can you tell me your theory about what's involved in making ice cream?
Younger offspring: Well, I think there's, like, this thing ... in the machine, you put ice into one part, and then you put something that helps the ice cream actually be ice cream, I forget what it's called, into this bucket also. And if you want to make chocolate, you put chocolate, sugar, and milk, I think ... And then you put a lid on and turn it, and after you turn it for awhile, then you open it and it's ice cream.
Dr. Free-Ride: The part that you put in with the ice is rock salt. The rock salt doesn't actually go into the part that you eat, it just goes into the part that makes the part you eat really cold. Do you know why rock salt is useful there?
Younger offspring: No.
Dr. Free-Ride: Rock salt actually helps lower the freezing point of water. What that means is that you can actually get the icy part a little bit colder than the freezing temperature for water.
Younger offspring: You can?
Dr. Free-Ride: Yeah, you can. It's called a freezing point depression. If you get some salt dissolved in the water, that keeps things nice and cold. So, that's one thing, you want to make stuff cold. But then you also know that there's a part that stirs. Do you know why you have to stir when you make ice cream?
Younger offspring: Umm.
Dr. Free-Ride: What would happen if you just mixed the ice cream ingredients, the milk and sugar and eggs --
Younger offspring: Eggs? I didn't know there were eggs in ice cream.
Dr. Free-Ride: Sometimes. But what would happen if you just mixed all that and stuck it in a container in the freezer?
Younger offspring: One, the ice won't get it. Two, the rock salt won't get it. It would be better to freeze it with rock salt.
Dr. Free-Ride: Well, in the freezer it would still get cold, but the stirring is pretty important. If you don't stir it while it freezes ... do you know what milk has in it?
Younger offspring: Calcium.
Dr. Free-Ride: Sure. But what's in milk that helps it quench your thirst?
Younger offspring: Cholesterol?
Dr. Free-Ride: No. What quenches thirst?
Younger offspring: Water?
Dr. Free-Ride: Water. There's water in milk. So if you just stuck it in the freezer, do you know what would happen? I think the water would freeze at a slightly different temperature than the other components of the milk. If you tried to make milk ice cubes, they would be very odd. And besides, what kind of texture do you like your ice cream to have?
Younger offspring: Soft.
Dr. Free-Ride: So frozen solid like an ice cube wouldn't be what you were aiming for in ice cream.
Younger offspring: Can we make coconut ice cream?
Dr. Free-Ride: Hmm, we could try using coconut milk in that.
Younger offspring: Yay!
Dr. Free-Ride: But let's talk some more about why we don't make it like ice cubes. When you're stirring, you're taking the part that's right up next to the edge of the container that has the ice and rock salt right outside of it, and you're stirring that and you're putting a new part of the ice cream mixture in contact with the really cold surface. And then as you stir another part of the ice cream mixture gets moved to the really cold surface. The stirring helps the parts of the ice cream mixture all get cold at the same rate rather than freezing in big chunks.
Younger offspring: So that's why you have to stir.
Dr. Free-Ride: Are there any experiments with making ice cream that you think we ought to do this summer? Besides different flavors.
Younger offspring: Hmmm.
Dr. Free-Ride: Well, what do you think could make a difference in how the ice cream turns out?
Younger offspring: I think we should try with soy milk.
Dr. Free-Ride: OK, what kind of milk we use. We could use soy milk, we could use coconut milk, we could use whole milk, we could use cream -- it is called ice cream, after all. So we could also look at the effect of the fat content on the texture of the ice cream.
Younger offspring: How much fat to how much protein to how much water.
Dr. Free-Ride: Sounds good. What else might make a difference?
Younger offspring: Eggs. Sugar.
Dr. Free-Ride: You know what? The way the rock salt lowers the freezing point of the ice, sugar also depresses the freezing point of the ice cream mixture a little. So how much sugar might make a difference to how the ice cream turns out.
Younger offspring: You don't have to tell me that. We could experiment with it and find that out.
Dr. Free-Ride: Fair enough. How about how long we stir it? Could that make a difference?
Younger offspring: Yes.
Dr. Free-Ride: Can you come up with a little chart before we do this of variables -- that is things we could vary in the ingredients or what we do to make the ingredients into ice cream?
Younger offspring: OK.
Dr. Free-Ride: And are you committed to doing a taste test for every batch we make so you can report on the differences?
Younger offspring: Yes, I can do that.
Dr. Free-Ride, I salute you. This is exceptional parenting on so many levels.
The churning helps it freeze evenly, but importantly it also introduces a whole bunch of air to the mixture. I wonder what would happen if the hole on the top of the mixer was covered to not allow extra air inside.
When you try peaches for flavor, don't get it "too peachy" if you have anyone tasting related to the offsprings' eldest uncle.
Happy summer experimenting.
Dr Free-Ride, I love this! Thank you SO MUCH for posting it. This is exactly the kind of conversations I want to have with my son when he is older (he's 3 weeks old right now). How did you inspire such an inquisitive mind? It's wonderful!
I don't think you have the full explanation for putting the salt in with the ice. I have always understood that the way the kind of ice-cream machine you are talking about works is that when you add the salt to the ice this forces the ice to melt, because of the lowering of the freezing point that you mention. However, it isn't just the lower freezing point of the salt/water mixture that freezes the ice-cream. It's the fact that changing from ice to liquid requires a large amount of heat energy, known as latent heat, and this energy is taken from the ice-cream, causing it to freeze.
It's the same as the way that ice melting in a drink cools it down far more than adding a similar amount of liquid at the same temperature as the ice would.
We just made ziploc ice cream - a great alternative if you don't have an ice cream machine, or a place to store one. You put your ingredients in a small ziploc, seal it up with some extra air, then put it in a larger ziploc filled with ice and rock salt. Then send your munchkins out to jump up and down with it until it gets hard. Works amazingly well. (And Jason, you don't need to add much extra air to get reasonably fluffy ice cream).
I always thought it worked best to add the extra ingredients at the very end....
It rarely snows where I grew up in Texas, but when it did mother would make snow icecream. I hae no idea how she did it but it was a treat.