Lecture 5: The Ice Cream that Will Freeze Granny

Creating ice cream that will re-freeze time after time but still remains as tasty as the day it was made, is a major culinary conundrum. New ways of conjuring up this faultless cuisine may come from the most unlikely places - serving up the perfect ice cream may depend on understanding how Arctic fishes stop themselves from freezing in their icy homes. But if we can mimic this seemingly magical feat, could we do far more than make the perfect raspberry ripple? Could we cryogenically freeze your granny and then defrost her back to her radiant self again?

Professor Tony Ryan
[Tony comes down the stairs of the lecture theatre and hands out ice creams.] Pass one down.
Hello.
I bet you all like ice creams as much as I do.

Do you want a napkin to go with it?
No.
Do you want to give it me back?
OK, how about you, do you want an ice cream?
You have a napkin.

You see, the only reason we can make this delicious treat is that we know some fantastic physics and chemistry.

Chemistry that allows us to make ice cream cheaply and in large quantities and most importantly with great taste and today we're going to explore the science behind ice cream and I'm going to show you how the very same science might save lives in the future.

So to find out what ice cream is made from - I need two volunteers.

You sir, you, yeah, you and the person next to you, down you come.
[Boys stand with Tony in front of table.]
So I'd like you to stand here and you come round this side.
OK, so what do you think the ingredients of ice cream are?

Volunteer
Ice cream.

Professor Tony Ryan
Ice cream.
What's in ice cream?

Volunteer
Flavours.

Professor Tony Ryan
Flavours, OK, what else?

Volunteer
Ice.

Professor Tony Ryan
Ice, very good!
And erm, why is it called ice cream? [Laughter.]
Because it's got cream in it, OK.

[Tony assembles ingredients.] So here's some ice, right and here's some cream.
Actually you find that as well as cream it's got some fat, right, there's something else you've missed out, it's got flavour.

Here's some vanilla flavour, because we like vanilla ice cream, yeah.

Now, does it taste sweet?

Volunteer
Um, yes.

Professor Tony Ryan
Right, OK, that's because it's got sugar in it, OK.
So what we're going to do is we're going to put the ingredients in here, if I can.

Right, so there's some ice, there's some sugar.
Here goes some flavour.
Have you ever made ice cream like this before?

Volunteer
No.

Professor Tony Ryan
No.
Just come round here for me, will you.
There you go, and you, well done.
This is a little bit of cream, yeah.
And we'll just put a little bit, because it's not very thick cream this, so we'll just put a little bit extra fat in there, because actually by weight ice cream's ten per cent sugar, thirty per cent fat and sixty per cent ice cream.

And then what we need to do is give it a good old ...

[Shot of blender.] Ooh, have I broke it?
No, it's working again.
OK, so all the ingredients are there.
Are you looking forward to tasting it?

Volunteer
No

Professor Tony Ryan
[Laughs/audience laughter.]
Really?
I mean it looks like ice cream.
[Boy looks at container of 'ice cream'.]
Have a taste, just tell me what it tastes like, because it's got all the right ingredients, hasn't it.
You have a taste.
Do you want me to have a taste as well?
Go on then, oh, go on, I'll have a taste.

What's wrong with it?
It's fantastic!
So thank you very much for being so good at that.
Ugh, it's horrible!
[Applause.]

[Overhead screen shots of icebergs.] Now ice is found all across the planet, even at the Equator and we have icebergs floating on the sea.
Why do they float?

You see, ice shouldn't float, should it?
It's a solid and solids are supposed to be denser than the liquids.
So here's an ice molecule, it's actually a water molecule and when the water molecules pack together, they go from being a beaker of water, like this, to being an ice crystal, like this.

You see the water in the liquid is all mixed up and random.

And the molecules are actually closer together and when it becomes a solid, the molecules go further apart, because they have these things called hydrogen bonds that make them expand a little bit and our friends at St. Wilfred's school helped us to demonstrate this.

[The overhead screen shows footage of children lying down 'acting' as hydrogen atoms and bonds.]
In this demonstration their legs are the hydrogen atoms and their arms are the hydrogen bonds.
So when they crystallise, and arm grabs hold of a leg and you get a lattice of evenly spaced out molecules and this is a clue as to why the ice floats.
So out they come again to make the crystal.

So we have, under the microscope and Mike Hamslow from our chemical and process engineering department's going to come down and run it, a cold chamber.

You see, when the water's in a liquid, it's buzzing around and they're moving and they hit a surface.
Sometimes they stick, sometimes they come off and eventually they stick and the crystal grows.

So Mike, is it crystallising yet?

Mike
Tony, it is.

Professor Tony Ryan
[Overhead screen shows growing crystals.]
Marvellous, let's have a look.
Oh, oh it is just starting to crystallise now.
So you can see the crystals growing.
We have them on the end of a needle so you can see them under this microscope.
We can control the size of an ice crystal by how fast we freeze the water, so the point of this needle was acting as a surface to nucleate, to cause them to be born.

Thanks Mike.

Mike
You're welcome.

Professor Tony Ryan
Cheers. [Applause.]

So if you cool the water very quickly, you form lots of ice crystals on lots of surfaces.
Any speck of dust or bubble will be there to cause crystals to grow and eventually there'll be no more free water molecules buzzing around to add onto the crystal.
So if we want to make perfect ice cream, we need to know how to cool the ingredients quickly, because what we want to happen is that the molecules rattle around and make a lattice.

[Close-up shot on screen of water molecules.]
So here are the water molecules on your screen now.
They're rattling around, we slow them down as they cool and eventually they'll lock into place and we need to lock them into place in lots and lots of small crystals.

So now, to make our own ice cream, I need a couple more volunteers!

OK, we'll take this person here and the one in the pink jumper behind her.

You two are going to form part of a production line with Annie and you're going to mix things together.

[The volunteers stand with Annie by a bowl.]
She'll show you what to mix together and then we'll pass them out into the audience.

So, they're going, they have pre-made ice cream ingredients.
They're going to put them in a big bag along with some water, some ice and some salt and then we're going to pass them out to you and you're just going to squidge them gently, otherwise you'll have cold, wet feet, right and we're going to make ice cream.

So we can't be sure when and where ice cream was invented, but we're going to use a technique that people have used for well over a thousand years.

[Close-up shot of container and thermometer.]
So here I have not quite ice and water and the temperature should read, what?
What should the temperature be?

Shout.
[Minus, what, nought degrees].
Nought degrees and it's not quite nought degrees, because the thermometer's in a little bit of air, right, so what I'm going to do is I'm going to put some of this ice into a bag - right and then I'm going to just add a touch of water, because nought degrees is when you have ice and water together.
So - we can't quite get there, so what I'm going to do now is add some salt.

[Shot shows the temperature falling.]
So I've added the salt and look what happens.
The temperature plummets.
It's already down to minus three.
In fact, if I get the amount of salt and ice right, it'll go down to an amazing minus twenty-one degrees.

So that's down to minus six, it's still falling.

So when you pass those bags around, be careful.
They're going to be maybe minus ten, minus fifteen degrees centigrade.

And they might stick to your hand, OK.
So be careful.

So our production line's just about finished.
Let's make this the last bag.

Now whilst those are being passed round we need to find the pouches and the pouches you're going to get look like this and they're going to help us explain why the temperature went down.

So there's a little button in the corner, a little metal disc.
Those of you who've got a pouch, I want you to click the metal disc now and you can see there's a tree-like structure growing out from the pouch.
Is that right?
Can you see it?
Have you clicked the disc, yeah?
We can see tree-like structures. And what else is happening?

It's getting really, really hot.
You see this tree-like structure, these are crystals forming inside the pouch and when they turn from a liquid to a solid, they give out heat and this gives us a clue as to why the temperature fell when I added so much salt.

[Film footage on screen of lorries gritting a road.] So in the winter you see lorries spraying roads and paths with grit.

And the lorries are actually spreading salt on the road.
And the salt melts the ice.
And it melts the ice because on ice there are water molecules that are buzzing off into the water and in the water there are water molecules that are buzzing in to the ice.
But when the salt goes down, the molecules in the water have someone to play with, so they don't want to go on to the ice, they want to stay there, so molecules stop going on to the ice, but keep on coming into the water and the ice melts.

But it needs heat to melt and it takes that heat from the road.
In the same way that my bag of iced water and salt got cold, it got to minus seven degrees centigrade.
So, now we've seen if you want to make ice cream you need to have exactly the right ingredients and the right conditions.

To get an ideal ice cream, you need the right texture.Small crystals, so it feels good in your mouth and in a minute we'll look at how texture alone doesn't control our perception and our enjoyment of food.