MOLLY WEBSTER: This is Science Friday. I’m Molly Webster. Ira Flatow is away. Later in the hour, we’ll be talking about how online communication has changed the way we write. It’s a topic of a new book, Because Internet. 

But first, summer means it’s time for ice cream. Have you ever had one favorite flavor you keep ordering, and then you thought to yourself, you know what? I could probably make this. Admittedly, I’ve never had that thought. But if you were the person that had that thought, you got home, you mixed all the ingredients together, and then you kind of got, like, a frozen vanilla ice cube or a chocolate chunk that was just like a chunk of chocolate. 

Ice cream and all frozen desserts are not just delicious, they’re very complicated chemically. They’re a mix of ice crystals, and emulsifiers, and a lot of air bubbles. So my next guests are here to tell us about the science behind these frozen treats and to help you get the perfect homemade scoop each time. 

So I would like to welcome to the table Matt Hartings, a professor of chemistry at American University in Washington, DC. He’s a home ice cream maker and author of the book, Chemistry in Your Kitchen. And Ben Van Leeuwen, co-founder of Van Leeuwen Artisan Ice Cream based here in New York. 

And if you have had a frozen dessert fail, we want to hear from you. What are your science questions about getting the perfect ice cream or frozen custard? You can give us a call. Our number is 844-724-8255. That’s 844-SCI-TALK. Or tweet us at @scifri. 

So Matt, I’m going to start with you down there in DC. You call ice cream a mesh network, something I’ve never heard ice cream described as before. What do you mean by that, and what do you mean by that scientifically speaking? 

MATT HARTINGS: Right. Well, think about just a sponge, right? And when a sponge is dry, right, it’s sort of dry and crumply. But when you put it in water, it soaks all that water up. And it’s squishy, and it has a completely different texture. And not only does the sponge change in texture, but the water changes in texture, too, right? You go from liquid water that sloshes around, and now it’s stuck inside of that sponge. 

And that’s kind of what ice cream is, is we kind of want to stick that water into place and keep it from moving around so much. And it makes a really nice, smooth, rich taste on your tongue. 

MOLLY WEBSTER: Matt, at what point did you start thinking about ice cream scientifically? 

MATT HARTINGS: [LAUGHS] Well, it’s one of the burdens of being a chemist, right, is I’m always thinking about things scientifically. Probably when I really started making my own ice cream, when I wanted to start doing it better. Again, I think of things in terms of science. And so when I started making ice cream and wanted to do better, my immediate thing was to start thinking a little bit more about what was happening. And that helped me a lot. 

MOLLY WEBSTER: Hm. So Ben, what’s the difference between– this is really my question– is what is the difference between ice cream soft serve and frozen custard? Because when I go to the ice cream truck, it definitely doesn’t taste like ice cream I get out of my freezer. 

BEN VAN LEEUWEN: So frozen custard and soft serve would be ice cream. Soft serve tends to be lower in fat and highly stabilized, meaning there’s a lot of gums and sometimes natural, sometimes unnatural compounds that are used. And the reason you need to do that with soft serve is because it’s a sort of on-demand frozen product. It’s constantly spinning around in this partially frozen barrel. 

And if you don’t have a lot of stabilizers in there, the fat’s actually going to churn out, and you’re going to get butter. So the only way to really do soft serve well is if you have really high velocity, if a lot of it’s coming out and you’re selling a lot of it. So we actually tried to do soft serve years ago, but it didn’t work because we weren’t selling enough. So it just sat in the barrel and overchurned. 

And frozen custard is defined, at least by the New York State Department of Agriculture, as having more than 1.3% egg yolks. So if you have more than that in your product, you have to either call it frozen custard or French ice cream. Our ice cream, depending on the flavor, is 5% to 8% organic egg yolks. So we call it French ice cream. 

And then ice cream is the sort of definition of all of those things, including people often ask me what the difference between gelato and ice cream is. And gelato means ice cream in Italian. You can make it in all sorts of different ways. 

MOLLY WEBSTER: [LAUGHS] Matt, I think this has been a controversial statement on your end, something about ice cream and gelato– are they different? Are they the same? Is it just the Italians being awesome? 

MATT HARTINGS: [LAUGHS] Well, the Italians are awesome, and gelato is awesome. And I think there’s a lot of bickering back and forth between people who make this. But I think the biggest difference between ice cream and gelato is the temperature that you serve it at, all right? So gelato is served at a higher temperature than ice cream is. And so you can make it differently. 

But I have seen recipes from Italian cookbooks that look just like ice cream recipes. And I have seen recipes from non-Italian cookbooks for ice cream that look just like gelato. So really, the biggest difference is the temperature that you serve it at. And that temperature difference gives you a little leeway in what you can do with bringing out flavors and making something creamy, or smooth, or whatever you’re looking for. 

BEN VAN LEEUWEN: And the only way you can serve it at a higher temperature is if it’s pretty stabilized. So if it has a lot of gums– natural ones would be guar gum, carrageenan, locust bean gum. So those sort of old fashioned gelato recipe– or Italian ice cream recipes we can call them that Matt was referring to, which is it’s such a good point. They’ll have more egg yolks than any recipe you’ve ever seen and a super high butter fat. Those served at a higher temperature wouldn’t work as well because they might collapse without the stabilization. 

MOLLY WEBSTER: Hm. So much to know here. So we’ve been collecting listener questions all week through our VoxPop app. And so here’s one on crystallization from Ronnie in Pennsylvania. 

RONNIE: Why does ice cream sometimes crystallize? 

MOLLY WEBSTER: Pretty cut and dry. Ben, do you want to start with that one? 

BEN VAN LEEUWEN: Sure. So the crystallization you’re tasting is usually going to be from water freezing. And the bigger the ice crystals are in the frozen water, the more detectable they’ll be on your palate. The way you prevent that sort of ice crystally taste is by freezing the ice cream really, really quickly. 

So the example I always use is if it’s snowing on a warm day, the snowflakes are really big because it takes them a long time to turn from water to solid to ice. So the crystals sort of slowly build and become very large. If they rapidly cool on a freezing cold day, the snowflakes are super small. And the same thing happens with ice cream. 

So in the production facility we use– and it would be the same for a home ice cream maker– when you spin the ice cream from your mix, your sort of liquid base, into a soft serve consistency, after that, you want to do what’s called hardening, and you put it into a freezer. And the faster you harden it, the smaller the ice crystals are going to be. 

But the crystallization you’re tasting at home is usually happening with the failure in what’s called a cold chain, which is like a supply chain that’s cold. So if your ice cream is being delivered to the local grocery store, and the delivery trucks arrive and puts the pallet of ice cream onto the loading dock, and it sits in 90 degree heat for even 20 minutes, it might melt. And then when it re-freezes, it’s going to freeze really slowly. And you’re going to get some ice crystals. 

MOLLY WEBSTER: And you get that weird, like, film on the top that’s, like, ice crystally? 

BEN VAN LEEUWEN: Yeah, exactly. 

MOLLY WEBSTER: Matt, what would you say to Ronnie? 

MATT HARTINGS: Yeah, that’s exactly right. I mean, a lot of times, when, again, as a chemist, I think of molecules moving around all the time, right? And so the warmer things are, the faster things move around. And so those water molecules are still moving around in your ice cream when they warm up just a little bit. 

And so those water molecules can go from liquidy water, and they move around a bunch, and then they find an ice crystal to stick to. And it just gets bigger, and bigger, and bigger. And so you can stop that motion by freezing it down really quickly. 

MOLLY WEBSTER: Ben, when you’re making ice cream, is there a magic frozen point that you’re going for or a speed in which you want to freeze it by? 

BEN VAN LEEUWEN: Well, you want to freeze it as quickly as possible. 

MOLLY WEBSTER: But is that seconds, or minutes, or hours? 

BEN VAN LEEUWEN: Hours. So theoretically, the faster you do it, the smoother it’s going to be. But there’s sort of a point where the value, you kind of top out at how much better it’s going to taste. So one example is some ice cream makers are using liquid nitrogen to freeze the ice cream. And theoretically, it makes it creamier, but to me, you’re going to be able to achieve the same creaminess with a really good formula. 

But exact time– so we like to pull our ice cream out of the ice cream making machine at around 21 degrees, which is fairly low. And then we put it in what’s called the hardening freezer. And depending on the size of container we’re putting it in, it takes anywhere from four hours to 24 hours to harden. 

Because our ice cream is so high in fat– we’re 18% butter fat– and super high in egg yolks, we can take longer to harden it and still get a really awesome texture. If we were a super low fat ice cream, you would have to harden it really, really rapidly and put it through what’s called a hardening tunnel. 

MOLLY WEBSTER: Mm. Matt, I think you’re actually one of those people that experiment with liquid nitrogen. 

MATT HARTINGS: I am. 

MOLLY WEBSTER: And I was going to ask you anyway, I don’t know a ton about how all this stuff folds together, so what does an egg actually do in ice cream? 

MATT HARTINGS: Right, so this gets back to that mesh I was talking about earlier, right? When you make any kind of custard, all right, whether it’s creme brulée, or ice cream, or scrambled eggs, you’re taking the coiled up proteins in that egg, and as you heat them, they sort of unravel and start to stick together. And that’s where they make this mesh, right? This sort of network inside of your whatever liquid it is you’re cooking. 

And so that’s really what’s going on with those eggs, is you really want them to sort of set up into a custard. And in effect, that’s what helps to slow the water molecules down as you’re freezing it, so that they can’t find one another. They have a hard time finding one another. Because they keep running into these little pockets of egg protein. 

MOLLY WEBSTER: Hm. Ben, I think your ice cream has a lot of eggs in it. It’s something like five times the usual amount. Can you talk about that? 

BEN VAN LEEUWEN: So, as Matt just described, using tons of eggs combats the potential iciness. But when we formulated our ice cream– we started 12 years ago– we actually had a basic understanding of that. But the reason we used more eggs is because we like the taste more. And so using a ton of egg yolks gives it this amazing chewiness that we’ve not been able to achieve without using a lot of egg yolks. And we really, really love that. 

MATT HARTINGS: It’s such a good flavor. It really is. 

BEN VAN LEEUWEN: Yeah. 

MOLLY WEBSTER: That’s great. So we’re here talking about ice cream. After the break, we’ll be talking more about the science behind ice cream and taking your questions. So call us, stay with us. 

This is Science Friday. I’m Molly Webster. We’re talking this hour about the science of ice cream and other frozen desserts. My guests are Matt Hartings, a professor of chemistry at American University in Washington, DC, and author of the book Chemistry in Your Kitchen, and Ben Van Leeuwen, co-founder of Van Leeuwen Artisan Ice Cream here in New York. 

And if you’ve had a frozen dessert fail and have questions, we want to hear from you. Give us a call. Our number is 844-724-8255. That’s 844-SCI-TALK. Or tweet us at @scifri. Matt, before we went to the break, we were talking about eggs, using eggs in ice cream. Are there alternatives that we can use in ice cream? 

MATT HARTINGS: Yeah, absolutely. And Ben, I was looking at all your recipes earlier before I came in. And I’m curious at how close in texture your vegan options are, right? There’s lots of different options that you can use for stabilizers. Many of them come from different plants. Corn starch is one of them. And I believe Ben uses locust bean gum. So you have a lot of options, right? Gelatin is another one. Lots of different options that you can find in your regular grocery store for replacing egg. 

MOLLY WEBSTER: So it’s great that you actually brought up all the different types of flavors of Ben’s ice cream because we happen to have a box here in the studio. 

MATT HARTINGS: Oh, I hate you people. 

MOLLY WEBSTER: And Matt, I’m very sorry we couldn’t ship any to you down in DC. We should have said the bring your own bowl should have been a requirement. So I’m opening something now. It’s beautiful. It’s called Earl Grey. This is my first cooking demonstration ever. I’m tasting it. And then Ben, you’re going to tell me a little bit about how you make Earl Gray happen. Maybe you can say that while I’m eating it. 

BEN VAN LEEUWEN: So the Earl Grey is a dairy flavor. So we start with milk, cream, sugar, and organic egg yolks, and we heat that up in a big 200 gallon kettle. And then we use mesh nylon bags and fill them with an amazing organic black tea with bergamot citrus oil, which is what makes it Earl Grey from Rishi Tea. And we steep that tea as we’re cooking the custard. And we pull the teabags out, we homogenize it, we cool it, and then we put it through the ice cream freezer. 

MOLLY WEBSTER: So you actually cook it– like, you steep the tea in the mess of the making it, like in the middle of everything? 

BEN VAN LEEUWEN: Yeah, in the big custard making cauldron. 

MOLLY WEBSTER: Mm. And Earl Grey, I mean, it is like a very pungent flavor as a tea. But you do have to steep it for a long time. How do you get a subtle flavor like that to actually come through in ice cream and not get drowned in eggs? 

BEN VAN LEEUWEN: So we slightly oversteep it. We don’t oversteep it to a point where you’re getting too many tannins. But we steep it more, and we use more per volume than if you were just making a cup of tea. Because as you pointed out, there’s 8% egg yolks, 18% butter fat, really muting a lot of the flavors. 

MOLLY WEBSTER: Yeah. Matt, now that we’re talking about flavors, I just wonder once you start mixing flavor into the chemistry, how that affects things, or if there are flavors that are really hard to do. 

MATT HARTINGS: Right. So one of the things that you need to think about when you’re making ice cream is that it’s cold, right? The ice cream is cold, and that low temperature really affects how you taste the flavors, right? And so they’re not as strong when we taste them cold. So one of the reasons why Ben can oversteep is because those flavors don’t come through as strong when we’re eating cold ice cream versus a hot tea. And so sometimes you need to over flavor to really have something that you’re going to get on your palate. 

Another thing is it’s fun to play with different flavor combinations, right? And there are all sorts of things that you can do, all sorts of crazy things that you can try. And I love the idea of playing around with flavors and finding new things that no one’s ever thought of before. 

MOLLY WEBSTER: So we’re going to bring in a caller who has a flavor question. This is Deborah from Ohio. Deborah from Ohio, do you want to tell us what your question is? 

DEBORAH: Yes, my question is I make a custard with egg, and cream, and all the good stuff– Madagascar vanilla, et cetera. My problem is that– 

MOLLY WEBSTER: You’re doing it. 

DEBORAH: –I like to fold in fresh fruit. I use the old fashioned ice cream freezer. But my problem is when I’m hardening the ice cream, my chunks of fruit seem harder than the ice cream around it. 

BEN VAN LEEUWEN: So yeah, 100%. And if you’re using fresh fruit, it’s going to be impossible to avoid that. I mean, Matt probably has even more data on percentages, but the fruit is going to get really icy because of the water and sort of the lack of sugar in it. So what you could do is you could lightly compote the fruit. But really, what works even better is sort of macerating it with a lot of sugar, cooking it a little bit to get the water out, and then you’re going to get a sort of smooth texture on the fruit, instead of a hard, icy texture. 

MOLLY WEBSTER: And Matt, what would you say in your data driven– 

MATT HARTINGS: Absolutely. I would echo what Ben said. That is exactly what you want to do, is you want to pull some of the water out of the fresh fruit. And it’s sad because you want that flavor of fresh fruit, and when you macerate it, you lose that a little bit. But it’s the playoff you have to take when you’re trying to make ice cream. 

BEN VAN LEEUWEN: What I would do on that is do the vanilla ice cream, and then just put the fresh fruit on top. So then you’re getting the perfect texture on the fresh fruit and the perfect vanilla. 

MOLLY WEBSTER: I feel like this is a good segue way for me to have more ice cream, which is the Cookie Crumble Strawberry Jam. We were talking about fruit, so it felt appropriate. While I’m sampling this, Ben, you said that strawberry might have been one of your hardest flavors to make good. 

BEN VAN LEEUWEN: Strawberry was a really hard flavor to master. We were adding strawberries, which is increasing the overall volume of the mix. But you’re adding no fat, so the fat drops. And I’m not sure why it took us so long to get to this point, but five years in, we said we shouldn’t be using any milk in this product. In order to as much strawberry taste as we wanted, we had to add so many strawberries that the butter fat would come down. So now we use only cream, strawberries, and sugar. 

And the lady who just called in from Ohio, if you want to do a fruit flavored ice cream, better to incorporate the fruit fully, and then you have sort of complete control over your fat and sugar level of the entire mix because it’s uniform. So you can get the fruit flavor. And for us, because we’re not using stabilizers, we always want to stay above 16% butter fat and around 12% sugar. 

MOLLY WEBSTER: And this strawberry ice cream I just ate is vegan. 

BEN VAN LEEUWEN: Yes, so– 

MOLLY WEBSTER: Tell us about how challenging it is to make vegan ice cream or not, I guess. 

BEN VAN LEEUWEN: I mean for us, it’s pretty easy. We started making vegan ice cream seven years ago, which was five years into our business. And the goal was to make delicious ice cream that happened to be vegan, not delicious vegan ice cream. So we tried to match the solids, fat, and sugar levels to our classic ice cream. All of the ingredients behave differently, so we had to tweak it. 

But we’re making the vegan ice cream with cashew milk, coconut milk, raw cocoa butter, which is the fat from chocolate, organic coconut oil, and a little bit of locust bean gum, or we call it carob gum, which is an organic stabilizer. And we use that not as a way to make it creamier, but a way to sort of give it more body, because we’re not using eggs in the vegan. 

MOLLY WEBSTER: We’re going to take a call from Jordan in Phoenix, Arizona. Jordan, what’s your question? 

JORDAN: Is it possible to overturn sorbet? And if so, how can you do it? 

BEN VAN LEEUWEN: No. If it’s a fat free sorbet, it’s impossible. And no sorbets I’ve ever seen have dairy in them, so yeah, impossible. 

MOLLY WEBSTER: Matt, for someone who cooks at home– I know that you do this– talk to me about getting ingredients from stores. Are you just looking at the ingredients you’re throwing into the bowl, or are you paying attention to what’s in those ingredients? 

MATT HARTINGS: Right. So that’s a great question. And Ben alluded to this just a minute ago, when he started talking about making vegan ice cream. Right? So I have a little thing up online with Science Friday talking about how I look at changing ice cream recipes. And you want to try to get your fats the same, and your proteins, your thickeners the same, your sugar amounts the same. And so really paying attention to what’s on the nutritional labels is a great way to start with understanding how to change a recipe. 

MOLLY WEBSTER: Hm. Interesting. OK, one more call, Dustin from Salt Lake City. 

DUSTIN: Hi. 

MOLLY WEBSTER: Dustin, what’s your call? 

DUSTIN: Hi, I have a question about sugar and corn syrup. I’ve made ice cream from recipes in America’s Test Kitchen cookbooks, and they suggest using part corn syrup to make it smoother. And I want to know how the chemistry works behind that. 

MOLLY WEBSTER: Matt, do you want to lead? 

MATT HARTINGS: Yeah, so corn syrup has a lot of fructose or invert sugar in it. And so that doesn’t crystallize as much in terms of the sugar crystallizing out. But it also prevents, in general, the ice cream from crystallizing as well. And so you can dampen that a little bit. I haven’t used that just because the recipes that I use, I don’t see much crystallization from the regular sugar. But that is definitely one way to try to get its smoothness. 

MOLLY WEBSTER: Ben, any final thoughts? You’re nodding. 

BEN VAN LEEUWEN: I just don’t think it’s necessary for home use. If you’re going through temperature shock, that can help, but when you’re making a few pints of ice cream at home, I don’t think it’s necessary to use the corn syrup. Because it’s harder to find, and it’s certainly harder to find an organic version of, too. 

MOLLY WEBSTER: All right. Thank you so much. I would like to thank my guests and thank everyone for the ice cream. It was awesome. So we have Ben Van Leeuwen, co-founder of Van Leeuwen Artisan Ice Cream based here in New York. And Matt Hardings is a professor of chemistry at American University and author of the book Chemistry in Your Kitchen. And Matt wrote up an article about chemistry tips for perfecting your ice cream. You can read it on our website at ScienceFriday.com/icecream.

Copyright © 2019 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/