EMILY KWONG, HOST:
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MADDIE SOFIA, BYLINE: You're listening to SHORT WAVE from NPR.
(SOUNDBITE OF MUSIC)
KWONG: Hey there, nerds, dreamers and nerdy dreamers. It's Emily Kwong here with producer Thomas Lu.
THOMAS LU, BYLINE: Howdy, howdy, Emily.
KWONG: Hey, hey. So what do you have for us today, T-Lu (ph)?
LU: Emily, you've known me long enough to know that I'm a big fan of ice cream, right?
LU: So chocolate, chocolate peanut butter cups, red bean, vanilla, pistachio, toasted marshmallows...
KWONG: Yep, yep.
LU: ...Green tea. I can go on and on and on. I just love ice cream.
KWONG: Fact check - true. Dear listener, you cannot say enough about how much Thomas Lu likes ice cream. In fact, behind the scenes, when he runs meetings, he tells us to all bring our favorite frozen treat. And the meeting is so much better.
LU: I mean, it just helps. But I think I might have found someone who might be a bigger ice cream person than me.
MAYA WARREN: I told myself when I was about 20 or 21 that I'm going to do what I love and love what I do for the rest of my life. And for me, that involved ice cream.
LU: That's Dr. Maya Warren. She's a food scientist. But more specifically, she's an ice cream, aka frozen aerated dessert, scientist.
KWONG: Amazing - I mean, it isn't work if it's what you love.
LU: I mean, she's living the dream, Emily. And I know what you're thinking. What's so interesting about - and I'm oversimplifying here - frozen milk? Well, let's just say it's art.
WARREN: Ice cream is my canvas waiting for me to put whatever I want to put on it. And, you know, just like you were to go into a museum and look at art, it's the same thing. You go into an ice cream shop instead, and you eat the exhibit, which, of course, is the ice cream.
LU: So today on the show, I scream.
KWONG: You scream.
EMILY KWONG AND THOMAS LU: We all scream for ice cream.
LU: We'll talk about some of the processes that create the texture of the deceptively simple dessert and how that texture plays into our enjoyment - plus a quick recipe to make at home.
KWONG: Lucky me. This is SHORT WAVE, the daily science podcast from NPR.
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KWONG: All right. Thomas Lu, SHORT WAVE producer and resident ice cream aficionado, what did you learn about the science behind your favorite frozen dessert?
LU: So let's start at the beginning. What is ice cream? Dr. Maya Warren tells me that there are legal guidelines.
WARREN: At the core of it, ice cream in the United States has to have 10% or more milk fat.
KWONG: And that comes from the cream, right?
LU: That's exactly right.
LU: So the milk, the cream, basically the dairy in ice cream - but that's not all. Ice cream is a combination of lots of other things.
KWONG: Like ice.
LU: OK. OK, Emily. I'm sensing a little hostility towards this beautiful dessert.
KWONG: No, no, no. No, no - it's just an observation.
LU: Whatever, Emily.
KWONG: That's the other part of the word - ice cream.
LU: Right, uh-huh. You're not wrong. The basic component of ice cream are fat globules from the dairy, ice crystals from the liquids, air, sugar and sweeteners, flavoring and sometimes additional ingredients like eggs or chocolate chips, which can give ice cream its body.
KWONG: So yummy. And that's kind of cool that there are solids, liquids and gas from the air bubbles all in one. You've got all three basic states of matter wrapped in one dessert.
LU: Yes. That's right, Emily. It's the balance and distribution of all these components in a mixture that builds ice cream's microstructure. Maya puts it this way.
WARREN: It's sort of like taking a selfie, and it's like how you looked at that moment. Then we freeze it, so we don't let it move. We make sure that it's frozen in time.
LU: And it is this frozen snapshot of the microstructure, Emily, that gives ice cream its taste and texture.
WARREN: It is such a sensation experience because it's not just about the flavor itself. Some ice cream products might have more air; some might have less air, some might have, you know, a little bit of a coarser texture. Some might have a little bit of a kind of a weaker texture. Some may have a fuller texture.
LU: Put in another way, Emily, the way ice cream is made and the ingredients that we use to create these ice creams add a physical and sensational experience in our mouths.
KWONG: Got you. Got you. That's super awesome.
LU: Isn't it amazing? But speaking of air, Emily, just like milk fats, there are guidelines as to how much air can be incorporated into ice cream. This is called the overrun. For ice cream, the maximum overrun is generally around 100%, meaning that the volume of ice cream doubles with the addition of air. These are typically the store brands or the economy brand. They'll often feel lighter or have a fluffier texture. The premium and super premium brands, on the other hand, tend to have overruns at about 50 percent or lower, which is why these brands feel a little heavier or denser.
KWONG: Got it. Like, when you're holding the carton, you can feel that it's heavier.
LU: That's exactly right. Maya also says that typically more premium brands will also have a higher percentage of milk fats.
WARREN: If you have more fat, you don't want to necessarily add so much air because fat is expensive, and you're paying for the fat. So you want it to be a heavier product. You want not to go against what you're paying for.
KWONG: So with these premium ice creams, they have more fat and less air in them, which also means they're likely to taste creamier and feel denser as a result. Is that right?
LU: Yeah. Generally, that's the idea. And to be labeled ice cream in the U.S., after adding in the ingredients and accounting for the milk fats and the overrun, the finished product must weigh no less than 4.5 pounds per gallon.
KWONG: Delicious. OK. So I think I have a good handle on the science behind ice cream's texture. But Thomas, how does this all happen? Like, what have you found out about the process of making ice cream?
LU: Ooh, get ready for a roller coaster, Emily.
KWONG: I'm ready. I'm ready.
LU: So let's focus on the commercial process. When you're making ice cream in huge volumes, the ingredients - milk, milk solids, sweeteners and other elements - need to be perfectly balanced.
KWONG: Mmm hmm.
LU: Then you take that mix, pasteurize it and homogenize it, which, you know, fancy words for basically killing the bacteria and preparing the fat globules so that they're ready to be transformed.
WARREN: Meaning that it's going to stay in a tank that's at about 4 degrees Celsius. And we're going to cool it and hold it at that temperature so that the ingredients can perform at their highest capability.
KWONG: Got it, following. OK.
LU: Get ready. The next step is the coolest - pun very much intended.
LU: It's called dynamic freezing, where basically the ice cream mixture is getting churned and frozen at the very same time.
LU: The way this is done is that usually the mixture is put into a barrel with a blade inside called the dasher. And this barrel is surrounded by an even larger container.
WARREN: So there's a barrel that the dasher's in. And on the outside of the barrel, there's some type of refrigerant, or some type of solution, that can allow you to lower the temperature at which you're actually going to start freezing.
LU: And the dasher, she says...
WARREN: ...Scrapes the sides of the barrel where the ice is forming. So the ice cream mix is cooled from the outside in...
WARREN: ...Because the ice cream mix that's actually touching the sides of the barrel, that ice is actually going to start freezing before ice in the middle of the ice cream mix. So that rotation of the dasher scrapes larger pieces of ice and turns them into smaller pieces of ice.
KWONG: Oh. So this is how the ice cream gets churned and frozen at the same time. This is the dynamic freezing process.
LU: Look at you, Emily, getting all this knowledge.
KWONG: I try. I try.
LU: (Laughter). So the breakdown and shuffling of ice crystals help give the creaminess we're all used to in ice cream. The smaller the ice crystal, generally, the creamier it will feel. The dasher also begins to incorporate and distributes air into the ice cream. But importantly, the sheer force of the dasher also causes the fat globules throughout the mixture to attach to one another in a phenomenon called partial coalescence.
KWONG: So Thomas, at this stage, is ice cream starting to look like ice cream and getting its dreamy texture?
LU: Dreamy indeed. After this dynamic freezing, the final step is hardening. You want to freeze the ice cream as quickly as possible to prevent the ice crystals from growing any larger. That locks in the microstructure, giving you that beautiful ice cream selfie.
KWONG: Delicious. Also, Thomas, I just got to say, making ice cream sounds like a super involved process.
LU: Yeah. It's amazing, right? There is a lot of chemistry and physics to it. But luckily for us, Emily, this all can be simplified, and you can recreate the dynamic freezing process right at home. One way to do that is the ice cream in a bag method popular in a lot of elementary schools. So, Emily Kwong, did you get all the ingredients that I asked you to?
KWONG: Like a good student, I did. I'm ready for the Thomas Lu School of Ice Cream. Let's go.
LU: Oh, goodness. So all you need are one gallon-size zip bag.
KWONG: Got it.
LU: One quart-size zip bag, one cup of half and half or equal parts milk and cream, one tablespoon of granulated sugar and one and a half teaspoon of pure vanilla extract.
KWONG: Got to start with the perfect formula.
LU: Exactly. First mix your ice cream base together.
LU: Now pour that ice cream base into the smaller bag. Zip it up tightly, and set it aside. In the larger bag, fill it with three cups of ice and a third cup of kosher salt.
LU: This is what's going to help us freeze our ice cream mixture.
KWONG: Wait. How does salt and ice help with that?
LU: Yeah. So remember how we added sugar to our base?
LU: Sugar actually lowers the freezing point of the ice cream mix, meaning it will not freeze at zero degrees Celsius like normal, regular water. Oh, OK. So to freeze this mix, we are recreating a crude form of the barrel and coolant from the commercial ice cream making process.
KWONG: Got you. Got you. So the salt and the ice combined in that bigger bag do that work.
LU: Yes. Salt lowers the freezing temperature of the ice, similar to how we put salt out on snowy days.
KWONG: Oh, OK. Yeah.
LU: But - OK, let's go back to our ice cream, Emily.
LU: Place the small bag fully zipped into the larger salt and ice bag. Zip that up.
LU: Grab oven mitts and/or a towel, and wrap it around this bag.
LU: And then - get ready for it.
LU: I want you to shake.
KWONG: Shake - I am shaking.
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LU: After a bit, check the smaller bag to make sure it's becoming ice cream. If not, keep going. I'd say for about 15 minutes, give or take.
KWONG: Fifteen minutes? Thomas...
KWONG: ...That's a SHORT WAVE.
KWONG: OK. Question though - this shaking, I'm guessing it is distributing air and breaking up these ice crystals?
LU: Yes. This is more or less mimicking the freezing of the ice cream base.
LU: The salt and ice in the outer bag work as the freezing agent. And like the commercial barrel I described earlier, the mixture will start to freeze from the outside of the bag inwards. So the shaking of the bag helps agitate and break up the ice crystal that are forming.
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KWONG: OK. So listeners, it's been about six minutes of shaking, and the ice cream looks ready, or my arms are just sore - you know, one or the other. I think I'm ready to try this.
LU: OK. So tell me how it is.
KWONG: Here we go. Oh, it's a really creamy texture. I noticed that right away. And as for the taste - oh, money. That's really good. I mean, it's not as creamy as the ones in my freezer right now. I will say that. I will say. It's a little icy, you know?
LU: (Laughter) Yep, that sounds about right. Still, you know, yummy snack to snack on, but maybe not the best quality.
KWONG: Oh, for the science lesson alone, it was worth it. Thank you for the Thomas Lu School of Ice Cream Lesson 1. This has been awesome.
LU: Thanks for having me, Emily.
(SOUNDBITE OF SONG, "SUN ALWAYS SHINES")
MARK ROBERTS AND PATRICK STURROCK: (Singing) Eating ice cream, skimming stones - we spend each day on the beach.
KWONG: This episode was reported and produced by Thomas Lu with help from Rebecca Ramirez, edited by Gisele Grayson and fact-checked by Indi Khera. The audio engineer for this episode with Gilly Moon.
LU: Special thanks to Duncan Coltharp (ph) and Dr. Maya Warren's team for making this episode happen.
KWONG: Before we go, quick reminder that you can help us out by completing NPR's podcast survey. It's short, anonymous. It will help us serve you better.
LU: Just go to npr.org/podcastsurvey. Again, that's npr.org/podcastsurvey.
KWONG: I'm Emily Kwong.
LU: And I'm Thomas Lu.
KWONG: Thank you for listening to SHORT WAVE, the daily science podcast from NPR.
(SOUNDBITE OF SONG, "SUN ALWAYS SHINES")
MARK ROBERTS AND PATRICK STURROCK: (Singing) ...In a fight. You lift my feet off the ground. Sun always shines when your around.
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