Food Science Experts On Perfecting At-Home Ice Cream

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. 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 SciFri– 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.

SPEAKER: Christie Watson’s kids love ice cream, but one recent morning, she saw an uneaten ice cream sandwich sitting on her patio table.

CHRISTIE WATSON: I noticed that my son had left his ice cream sandwich outside, and I was wondering why was there still ice cream there.

IRA FLATOW: 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?

MAYA WARREN: 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.

IRA FLATOW: Tell us why is that so cool.

MAYA WARREN: So basically, in ice cream, you have a couple different parts. You have your ice crystals, of course, which melt. You have your air cells. So picture like a soda with little bubbles around it. That would be like your air cells in ice cream, dispersed throughout your ice cream. And you also have your fat globules. And usually those fat globules are partially coalesced. They 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 probably a couple of sugars and some of the other ingredients in ice cream. So it actually did melt.

IRA FLATOW: 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?

MAYA WARREN: Sometimes in ice cream, depending upon the brand or 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, we can use something that’s free and that we’re in everyday– 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 going to help retain its shape, generally speaking. And so I’m assuming that Walmart type of ice cream probably had quite a bit of air in it.

IRA FLATOW: So you can adjust the amount of air you pump into it.

MAYA WARREN: Definitely. You can decide to inject a certain amount of air. So for instance, if we’re looking at an ice cream like Haagen-Dasz, that kind of ice cream has about 27% overrun versus an ice cream like an Edy’s 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 is $2, usually, the $2 pint has a bit more air in it versus the $5 pint. So you are kind of paying what you get for.

IRA FLATOW: We did our own, actually, not-so-scientific experiment. And we showed you– we have a time lapse of them melting and melting dramatically at different rates.

MAYA WARREN: Indeed, indeed. So different parts of the microstructure really, really do control how your ice cream is going to melt. So I looked at the video, and I saw that there was a Briar’s, a Ben and 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 one thing that I did notice in the video was that they all started off at different sizes. And so we kind of threw in another variable into the experiment. And so unfortunately, I can’t honestly say one melts faster than the other because we threw in a different variable, especially if they didn’t all start at the same time.

But from the research that I do on the microstructure of ice cream and other frozen aerated desserts, we do know that there are certain parts of the microstructure that you can control, which will then assist in controlling the rate melt and/or the melt behavior, because 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, 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 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.

FLORA LICHTMAN: 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. OK, now that you know the fundamentals of the structure of ice cream, want to try to make your own? Here’s Ira again.

IRA FLATOW: 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 it full of fat globules, or it 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 sent 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.

BRIAN SMITH: 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.

IRA FLATOW: Now, Jeff, you’re here with us. How do you prevent your ice cream from turning into a frozen milk snowball?

JEFF POTTER: Well, that’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 of 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. It’s this strange, bizarre texture in a lot of ways.

IRA FLATOW: It’s got a lot of stuff in it.

JEFF POTTER: It’s got a lot of stuff in it, yeah. And 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. Now, when I say liquid, I mean the actual ice cream in your freezer is technically actually a liquid with ice crystals and fat 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 determine if it’s going to fail or work.

IRA FLATOW: So how do you make sure then, if water is the most important ingredient, how do you make sure you do that right?

JEFF POTTER: 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 would be fats, the emulsifier, the sugar in there. A good ice cream has got a certain amount of fat in it. It’s typically 14% to 22% fat. And if you’re looking at a recipe and you’re not sure how much fat is in it, there’s actually a nice little trick. You can go to WolframAlpha online and just plug in the ingredients.

So if you go and take 2 cups cream plus 1 cup milk plus 3/4 cup sugar, it’ll spit out the weight grams. And you can figure from that that you’re at 16.5% fat. So if you take your recipe, make sure it’s about 14% to 18% fat seems to be pretty standard for home stuff. And then the other bit of it is to actually– 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. It doesn’t have lecithin. It 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 has a lower freezing point, and it changes how that three-dimensional structure, that complex colloid actually sets up in the freezer.

IRA FLATOW: 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.

JEFF POTTER: Yeah, there’s hand churning ones. 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.

IRA FLATOW: And give us a lesson in that.

JEFF POTTER: Liquid nitrogen?

IRA FLATOW: Yeah.

JEFF POTTER: Oh.

IRA FLATOW: What do you do with it?

JEFF POTTER: The liquid nitrogen stuff is 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 how to handle liquid nitrogen.

IRA FLATOW: It’s 300 degrees below 0.

JEFF POTTER: Yeah, it’s–

IRA FLATOW: So you ought to be careful.

JEFF POTTER: It’s cold, but it actually freezes it very quickly, and it gives you a 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. Then the second thing that happens is when you put it into your freezer, it sets up into a more solid state. So in soft serve stage, about 50% of the water is frozen. At a later stage, about 75% to 80% is frozen. In terms of ice cream makers, you’re probably not going to get liquid nitrogen so easily at your local grocery store.

IRA FLATOW: If you’re on a university campus, maybe in the chemistry department.

JEFF POTTER: Maybe. Right. Salt and ice, the old traditional churning is actually a great way of making it. And a lot of people think about the salt and the ice combining 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 the surrounding mixture. And then of course, the ice itself as it melts, that takes a lot of heat in from the base.

So if you want, salt and ice and the old-fashioned churning actually is a fun summertime project. About 20 or 30 minutes, you can set up some ice cream and enjoy it. And if you’re not worried about ice crystals, you can eat it right away. You don’t even have to deal with the possibility of not having enough fat or emulsifiers in there.

IRA FLATOW: And if you are worried about the crystals?

JEFF POTTER: If you are worried about the crystals, then you need to make sure that you actually get those fat, sugar, and emulsifiers– emulsifier is not necessary, but certainly, it’s a cheat that 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, 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.

IRA FLATOW: So like a cookie sheet?

JEFF POTTER: Yeah, a cookie 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. The other thing, of course, is in your freezer, make sure there’s plenty of air flow so that you’re not just putting your container right next to some other stuff, and there’s no air to circulate against it in your freezer.

IRA FLATOW: Wow. Let’s go to the phones. Hi, Jessica. Welcome to Science Friday.

JESSICA: Hi. Thank you for having me.

IRA FLATOW: You’re welcome. Go ahead.

JESSICA: 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. You know.

JEFF POTTER: 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 where the idea of doing that without snow. But yeah, if you’ve got– this is very similar to the kind of standard fun physics or chemistry classroom demo you might do in high school, where you take 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.

IRA FLATOW: All right. We love demos. What have you got there?

JEFF POTTER: 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 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.

IRA FLATOW: Right.

JEFF POTTER: And if I dump this in–

IRA FLATOW: Salt. It’s dropping.

JEFF POTTER: It’s dropping. So what’s it saying?

IRA FLATOW: It’s down to 70. It’s about 70. Stirring it up.

JEFF POTTER: So you saw almost instantly a 4 or 5-degree Fahrenheit just instant drop.

IRA FLATOW: And that happens because?

JEFF POTTER: 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, it’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–

IRA FLATOW: ‘Cause we’re Science Friday.

JEFF POTTER: 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.

IRA FLATOW: Right. We’ve had a lot of that stuff.

JEFF POTTER: So I’m going to take– I’ve got a second cup here. I’m going to take that probe thermometer, and it should go back up to– you can see it’s a little bit warm.

IRA FLATOW: It’s up. Yeah, 73 now.

JEFF POTTER: 73, OK. 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.

IRA FLATOW: Wow, it’s going up to 83, 91, 103, 116, 121.

JEFF POTTER: It gets hot.

IRA FLATOW: 124.

JEFF POTTER: This is just to say that when you’re making the ice cream, salt is not just an incidental ingredient. If you want to go make ice cream while you’re hiking, and you got some snow around, some salt is actually going to help you pull off that trick.

IRA FLATOW: Lots of stuff. Because I want to move on because a lot of people just don’t want to make vanilla ice cream. They add candy. They got cookie dough, sometimes fruit. Now, when Becky Fogel went out to see Brian Smith over there at the Ample Hills creamery, she asked him about adding strawberries to your ice cream. What’s the best way to do that?

BRIAN SMITH: Anything with fruit in it that you want to pulverize or smush up and mush up and turn into a purée 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.

IRA FLATOW: Jeff, how does that sound to you?

JEFF POTTER: That sounds delicious.

IRA FLATOW: [LAUGHS]

JEFF POTTER: 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 just going to say 20%. So if you macerate your fruit in three to four parts fruit to one part sugar, you’ll get something that actually has a lower freezing point that’s roughly equal to the same texture as the ice cream.

IRA FLATOW: Last few words– what’s the biggest mistake people make? And what should they not do? Is it the temperature?

JEFF POTTER: Biggest mistake is probably not actually setting it up quickly enough.

IRA FLATOW: Oh.

JEFF POTTER: You know, get it to be cold.

IRA FLATOW: All right. There you go. Jeff Potter, author of the book Cooking for Geeks. We’ll see you next time.

JEFF POTTER: Thanks so much.

FLORA LICHTMAN: That interview was from 2015. OK, after all of that, you’re ready to get churning. Check out the recipe at sciencefriday.com/icecream.

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