Science Friday
Hosted by Flora Lichtman and Ira Flatow
We’re pulling a few of our favorite stories about the science of ice cream out of the freezer.
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Hey, I'm Flora Lixman, and you're listening to Science Friday.
Today in the podcast, pondering a burning late summer question.
All ice cream will melt, but does all ice cream collapse? That's the real, real question.
It seems like summer just got started. We're not ready to let go of those days at the beach, the backyard cookouts, and the ice cream. Not only do we love ice cream here at SciFry, obviously, we've also done many stories over the years about the science of this frozen treat, so we're pulling a few out of the back of the freezer for you. Back in 2015, Ira sat down with Maya Warren, an ice cream expert who was, at the time, a PhD candidate in food science at the University of Wisconsin-Madison. Her task? Help us understand a summertime food science mystery that made the evening news in Cincinnati.
Christie Watson's kids love ice cream. But one recent morning, she saw an uneaten ice cream sandwich sitting on her patio table.
I noticed that my son had left his ice cream sandwich outside. And I was wondering, why was there still ice cream there?
Yes, why, after sitting out for hours in the summer heat, did the ice cream sandwich appear to be largely intact, just slightly melted, if at all? Was there some unnatural ingredient packed into this frozen treat that helped it withstand 80-degree temperature? What gives? That's what we wanted to know. Did this ice cream sandwich really melt, or not melt?
You know, the ice cream sandwich actually melted. I haven't heard of anyone at high temperatures that they can have ice not melt. So the ice definitely melted, so therefore the ice cream melted. But the structure itself did not collapse, which is really, really, really cool, to say the least.
Tell us, why is that so cool?
So, basically, in ice cream, you have a couple of different parts. You have your ice crystals, of course, which melt. You have your air cells. So picture kind of like a soda with little bubbles around it. That would be kind of like your air cells in ice cream, dispersed throughout your ice cream. And you also have your fat globules. And sometimes, usually those fat globules are partially coalesced. They kind of agglomerate together, but they're not fully coalesced like butter, but they're held together through a partial crystalline network, which causes them to sometimes be really, really large in size. And that, along with the air structure, sometimes causes ice cream not to collapse from natural gravitational force.
And so what that mom probably mostly encountered was the water, basically, the ice that turned into water. But if it was outside, it probably evaporated off at some point. And what was left there was basically kind of like a foam that consisted of your fat globules, your air cells, and some of the, probably a couple of sugars, and some of the other ingredients in ice cream. So it actually did melt.
Huh. So there's no stuff inside of it that tries to make it keep its shape? I mean, like gelatin or any kind of stuff, binders, things like that?
Sometimes in ice cream, depending upon the brand, or you know, how the person's making it, or the company, but sometimes there are things in there that can help ice cream retain its shape. But actually, you know, we can use something that's free and that we're in every day, air. Air really does help ice cream retain its shape. So sometimes, the more overrun, which is the amount of air that's in ice cream, the more overrun in a product, the more it's gonna help retain its shape, generally speaking. And so, I'm assuming that that Walmart type of ice cream probably had quite a bit of air in it.
So you can, you can adjust the amount of air you pump into it?
Definitely. You can inject, you can decide to inject a certain amount of air. So for instance, if we're looking at an ice cream like Haagen-Dazs, that kind of ice cream has about 27% overrun, versus an ice cream like an Edees has almost upwards to 100% overrun. And so when you go to the grocery store, and you're like, why is this pint of ice cream $5, versus this pint of ice cream's $2? You know, usually the $2 pint has a bit more air in it, versus the $5 pint. So you are kind of paying what you, what you get for.
Oh, well, we did our own, actually, not-so-scientific experiment. And we showed you, we have a time lapse of them melting, and they're melting dramatically at different rates.
Indeed, indeed. So different parts of the microstructure really, really do control how your ice cream is gonna melt. So, I looked at the video, and I saw that there was a Breyer's, a Ben & Jerry's, and a Haagen-Dazs. And with that, because of the differences in the microstructures, you'll usually see different rates of melt. But, but one thing that I did notice in the video, was that they, they all started off at different sizes. And so, we kinda threw a different, another variable into the experiment. And so, I, unfortunately, I can't honestly say one melts faster than the other, because we kinda threw in a different variable, especially if they didn't all start at the same time. But, you know, from the research that I do, on the microstructure of ice cream and other frozen air-rated desserts, we do know that there are certain parts of the microstructure
that you can control, which will then assist in controlling the rate of melt, and/or the melt behavior. At the end of the day, all ice cream will melt, but does all ice cream collapse? That's the real, real question. And from the microstructure, the air cells, the, so the amount of overrun and the size of the air cells, as well as the amount of fat, the type of fat, the agglomeration, the partially, partial coalescence of that fat, all come into play to create that one kind of particular ice cream. And those really have to do with our sensorial likings as well. So microstructure is really, really key in ice cream.
That's Maya Warren talking to Ira Back in 2015. At the time, she was a PhD candidate in food science at the University of Wisconsin-Madison. And since that interview, she finished her PhD. And you can now find her at drmayawarren.com. Ice cream scientist.
Coming up after the break, breaking down the many components of your homemade ice cream. It is more complicated than it looks.
Ice crystals and flat globules and air bubbles and all this other stuff and the ratio of the ingredients and how you make it really determined if it's going to fail or work.
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Okay. Now that you know the fundamentals of the structure of ice cream, want to try to make your own? Here's Ira again.
Do you scream for ice cream? Or is it more like you scream when you've just spent hours mixing up a homemade batch of ice cream? And then it melts everywhere? Or is full of fat globules? Or has the consistency of a bag filled with crunchy shards of ice? I hate it when that happens. Jeff Potter, author of the book Cooking for Geeks, is here to help solve your frozen food failure. To understand how the professionals, let's talk about how the professionals make the cold confection. We send producer Becky Fogel to Ample Hills Creamery in Brooklyn for some hands-on research into ice cream science. She met with owner Brian Smith up on the roof of his shop, and he explained how one
essential ingredient can also lead to your ice cream's demise. The biggest obstacle to smooth and creamy ice cream is water.
Water is the number one ingredient in whole milk. It's the number one ingredient in heavy cream. And so we don't want the water in our ice cream to turn icy.
Now Jeff, you're here with us. How do you prevent your ice cream from turning into a frozen milk snowball?
Well, it's a good question. There's really two main things to it. It's how you make the ice cream and then what's in the ice cream. So the first bit of that, how you make it. Ice crystals are all about nucleation and crystal growth. And so when you look at making ice cream, it's all about trying to keep those ice crystals that are in the base from turning into larger, grittier things. And then the second part of all this is what's actually in the ice cream. Because that actually changes how the mixture sets up when it freezes. Ice cream is one of those amazingly complicated, well, a scientist would call it a complex colloid. You know, it's this strange, bizarre texture in a lot of ways.
It's got a lot of stuff in it.
It's got a lot of stuff in it, yeah. It's fundamentally, it's a liquid that actually traps solids in it. So you've got ice crystals and fat globules. And you also have air in there, trapped in that liquid. And as well, that liquid is actually an emulsion. And now, when I say liquid, I mean the actual ice cream in your freezer is technically actually a liquid with ice crystals and flat globules and air bubbles and all this other stuff going on in there. And the ratio of the ingredients and how you make it really determined if it's going to fail or work.
So how do you, how do you make sure then if water is the right, is the most important ingredient? How do you make sure you do that right?
So for handling the water, it's really about interfering with the ice crystals from forming. So when you look at the recipe, that's one part of it. So it'd be fats, the emulsifier, the sugar in there. You know, a good ice cream has got a certain amount of fat in it. It's typically 14 to 22% fat. If you're looking at a recipe and you're not sure how much fat's in it, there's actually a nice little trick. You can go to Bull from Alpha and, online and just plug in the ingredients. So if you go and take, you know, two cups cream plus one cup milk plus three quarter cup sugar, it'll spit out the weight grams and you can figure from that that you're at 16 and a half percent fat. So if you take your recipe, make sure it's about, you know, 14 to 18%. Fat seems to be pretty standard for home stuff. And then the other bit of it is to actually, you know, if you're, if you're at home making
ice cream, there's actually a couple of cheats you can do. One is to use eggs because they have lecithin in them and that lecithin acts as an emulsifier. So if you're making an American style, or sometimes called Philadelphia style, ice cream, it doesn't have the egg, don't have the lecithin, doesn't have the stabilizer. So that's the second part of the ingredients. And then the third thing is the amount of sugar in there. And sugar is actually really important because of freezing point depression. So ice cream with sugar in it, you know, has a lower freezing point and it changes how that, that three dimensional structure, that complex coiled actually sets up in the freezer.
Do you need a special kind of ice cream maker to make it the best? You have these electric ones, you got the hand churning ones?
Yeah, there's hand churning ones. You know, there's, there's of course a geeky approach of using liquid nitrogen, which works really well because it freezes the water really fast, so you get small ice crystals.
Give us a lesson in there.
Liquid nitrogen?
Yeah.
What do you do with it? The liquid nitrogen stuff's actually a lot of fun. It's been done for over a century. You basically take your ice cream base, about equal parts ice cream base to liquid nitrogen, you throw it in a bowl, you should know how to handle liquid nitrogen.
That's pretty, 300 degrees.
You want to be careful with that.
It's cold, but it actually freezes it very quickly and it gives you really creamy texture, at least initially, because the first bit that happens when you make the ice cream is it comes to what's basically like a soft serve stage. And the second thing that happens is you put it into your freezer, it sets up into a more solid state. So in soft serve stage, about 50% of the water's frozen. At a later stage, it's about 75, 80% is frozen. In terms of ice cream makers, you're probably not going to get liquid nitrogen so easily, you know, at your local grocery store.
If you're in a university campus, maybe in the chemistry department.
Right. Salt and ice, the old traditional churning is actually a great way of making it. And a lot of people think about the, the salt and the ice combining, you know, to, to what they would say is freezing point depression. That's not actually really what's going on there. It's actually an endothermic reaction from the salt actually cooling down as it dissolves, cools down this running mixture. And then of course, the ice itself, as it melts, that takes a lot of heat in from the base. So, you know, if you want, salt and ice in the old fashioned churning, actually it's a fun summertime project. You know, about 20 or 30 minutes, you can set up some. Ice cream and, you know, enjoy it. And if you're not worried about ice crystals, you eat it right away. You don't even have to deal with the possibility of not having enough fat or emulsifiers in there.
And if you are worried about the crystals?
If you are worried about the crystals, then you need to make sure that you actually get those fat, sugar, and emulsifier. Emulsifier is not necessary, but certainly it's a cheat. It helps. You have to make sure you get them in the right ratio. Freezing it quickly is important. So not just when you churn it, but when you put it into your freezer. You know, it's not, the freezer is not a time machine. It doesn't stop things. That ice cream has to still set up in the freezer. So set your freezer to as low as possible. Make sure that the container you're using is actually relatively thin and flat as opposed to a kind of a large box, because you need to get that whole thing to freeze up. And if it takes longer, those ice crystals have time to aggregate.
So like a cookie sheet?
Yeah, cookies or just a baking pan. That actually will work better, because you're talking about getting that whole mass to set faster. And the enemy with ice crystals is slow freezes. You want it to set as fast as possible. Another thing, of course, is in your freezer, make sure there's plenty of airflow, so that you're not just putting your container, you know, right next to some other stuff and there's no air to circulate against it in your freezer.
Wow, let's go to the phones. Hi, Jessica, welcome to Science Friday.
Hi, thank you for having me.
You're welcome, go ahead.
So I have an odd question about making ice cream while backpacking. I was wondering if you have any tips on making ice cream with dehydrated milk and snow and kicking around the ball?
If you've got snow to work with on your hiking, I mean, I'm thinking about a warm summertime day here in New York right now with the idea of doing that without snow. But yeah, if you've got, you know, this is very similar to the kind of standard fun physics or chemistry classroom demo you might do in high school, where you take, you know, a ball or even a plastic bag. If you take that snow and you pack it around whatever your container is that holds your ice cream base, and if you can pack some salt with you, that table salt, when you throw it in that snow mixture, is going to, the process of it actually dissolving is going to take a fair amount of heat from the surrounding thing. I actually brought in a little demo of this.
All right. We'd love demos. What have you got there?
So what I've got here is a cup of water and a quarter cup, 50 grams of just standard table salt, and they're at the same temperature. And you'll see that the temperature right now, I have not added them together yet. The temperature of that water is 74 degrees.
Right.
And if I dump this in...
Salt, it's dropping.
It's dropping. So what's it saying?
It's down to 70.
It's about 70, stirring it up.
So you saw almost instantly a four or five degree Fahrenheit just instant drop.
And that happens because?
That happens because the salt crystal itself, when the sodium and the chloride ions are splitting apart from that crystal, that takes a certain amount of heat. Now, to be clear, there's another thing going on here. It's not just the dissolving energy, there's also hydration energy. So as the sodium ions start to float around the solution, that actually also does some stuff. So I actually have a second demo here, just because, you know.
Guess we're Science Friday.
We're Science Friday. And this is calcium chloride. This is the stuff that you'd be sprinkling on your sidewalk out in the middle of winter.
Right. We had a lot of that.
And this stuff, so I'm gonna take, I've got a second cup here. I'm gonna take that probe thermometer, it should go back up too. You can see it's a little bit warm.
It's up, yeah, 73 now.
73, okay. Here's what happens when you add this stuff. It's a salt, it's a different salt, but it's still a salt. And look what happens to that temperature.
Wow, it's going up to 83, 91, 103, 116, 121.
It gets hot.
124, that's all.
So this is just to say that when you're making the ice cream, salt is not just, you know, an incidental ingredient. If you want to go make ice cream while you're hiking, and you've got some snow around, some salt's actually gonna help you pull off that trick.
Lots of stuff, because I want to move on, because a lot of people just don't want to make vanilla ice cream, right? They had candy, they got cookie dough, sometimes fruit. Now, when Becky Fogle went out to see Brian Smith over there at the Apple Hills Creamery, she asked him about adding strawberries to your ice cream. What's the best way to do that?
Anything with fruit in it that you want to pulverize, or smush up and mush up and turn into a puree, is a real challenge, because of course, the number one ingredient in every fruit is water. So when we make strawberry ice cream, one of the tricks that you can do is you macerate the strawberries overnight first, which is to say you take
Your strawberries and you put a bunch of sugar over top of them. And then you leave them in the fridge overnight and that sugar draws out the water from the strawberries. It draws out a lot of the water. And now you have your strawberries, which have a lot less water content. Jeff, how does that sound to you?
That sounds delicious. Strawberries are really tricky because they're like 92% water. And water is the enemy. Now, there's another thing going on here with the sugar is that that fruit, when he macerates it, is actually pulling in sugar. And sugar and water, just like salt and water, has a freezing point depression thing going on. So that fruit will actually set at a lower temperature. So yeah, removing some of the water will certainly help. But the other bit of it is that adding sugar. So when you've got fruits, you usually want about, well, the technical papers say 21% sugar, but I'm assuming it's say 20%. So if you macerate your fruit in a three to four parts fruit to one part sugar, you'll get something that actually has a lower freezing point. It's roughly equal to the same texture as the ice cream.
Last few words, what's the biggest mistake people make and what should they not do? Is it the temperature?
Biggest mistake is probably not actually setting it up quickly enough. You know, get it to be cold.
All right, there you go. Jeff Potter, author of the book Cooking for Geeks. We'll see you next time.
Thanks so much.
That interview was from 2015. Okay, after all of that, you're ready to get churning. Check out the recipe at sciencefriday.com/icecream.
Thanks for listening. Don't forget to rate and review us wherever you listen. It really does help us get the word out and get the show in front of new listeners. I'm Flora Lichtman. Thanks for listening.
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