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MICHAEL CIMA: I'm going to talk about food. Before we get to chocolate, we're going to
talk about ice cream. And I'm a big fan of ice cream.
The thing about ice cream is it starts with cream, which is a derivative of
fat, or a derivative of milk. And it's enriched in the fat components here.
You can imagine centrifuging it and get all the fat sort of thing.
So that's the cream that goes into ice cream. So typical ingredients, though, involve a
number of other things. The most important thing is the whole milk,
the cream, sugar, and water should have been in red there.
You'll notice there's air here, which is kind of interesting, and egg.
Now milk has a bunch of triglycerides. It's an emulsion.
And these can be quite large. Whole milk, the droplets of fat are around
0.4 to sometimes 10 microns in size.
And that's why milk is opaque, because there's these large globules of fat
floating around in it. And they're stabilized by these phospholipids
that are produced in the tissues of the cow to stabilize that oil.
The protein component that's in the milk is a protein called casein.
There's actually four different versions of it.
But one of them is critical for keeping this globular protein in solution.
So there's these globules of fat. And then in the water space are these particles
of casein protein. It's also sort of merged with phospholipids,
so it's actually a complicated particle.
And it's the strands of the kappa-casein that keeps it stabilized
because the amino acids on one arm of the casein molecule are
all positively charged. And they're all sticking out on the surface.
And so now all those particles have a positive charge in solution, so they
repel one another. And so they keep dispersed that way.
At any rate, so that's the milk. And the egg here is to--
well, if you want to make like French custard type ice cream, you know, it's
kind of got a different taste to it, you actually make a custard and add
that to the mixture that you're going to freeze, because what you're doing
there, what a custard is, you just take the protein that's in the egg and
you denature it. And it becomes stiff.
And so that's what gives it that-- I don't know, French vanilla kind of ice cream
had kind of got a cake-like texture to it.
And that's all that is. Now the sugar is-- well, it's sugar, right?
So it's just sweet. And that's what you're starting with is those
basic ingredients. Now I'll get to the air in a second.
So now what happens? So here we go.
It's this highly technical instrument. This is an ice cream maker.
There's this bucket here that you sticking in your fridge.
And inside this is a material that freezes around or about minus 4 or 5.
It is a glycol organic acid mixture. It's actually a eutectic of these things.
So remember eutectic, I've got two pure materials, A and
B. And A melts here. Let's say B melts here.
And it turns out they're immiscible in one another in the solid state.
So this is liquid. This is I'll call that alpha plus liquid.
This is liquid plus beta. And this is alpha plus beta.
And if you do it right, here at this composition, this stuff in here--
and that eutectic temperature is minus 4 degrees C.
And so now of course, if you have this and you freeze it, to melt it, the
heating curve is going to look like-- this is time.
This is temperature. It will, of course, start at the minus 4 degrees
C. And you put this bucket on the machine and
the temperature will rise-- oh no, sorry.
Let's say in the fridge it's minus 5. And then temperature rises as you put it on
this machine. And then it reaches minus 4 degrees C. And
of course, it stays constant until all the alpha and beta solid have melted to
form the eutectic liquid. And then the temperature rises.
And of course, it's during this period that you're going to
make the ice cream. So it's called a phase change material.
The phase change is melting. That's all it's doing.
And of course, the ideal material has a pretty good, large heat effusion, so
that the heat required to go from here to here is large, so it can suck a lot
of heat out the material that goes into the bucket.
OK, so there's some 3.091 right there. And there's more when we do this thing here.
So you see this kind of crazy-looking gizmo here.
It's actually got usually two blades on it. It's hooked to a plastic case that connects
that mechanically to the base.
And then you see in here, there's a little gear and there's a little
receptacle on the bottom of the bucket. And when you turn this thing on, it rotates
the bucket and the blades stay stationary.
So if you look down into this bucket it kind of looks like this.
Well, now I have four blades. Usually it's only two.
But it's four blades. And in the reference frame of the bucket,
even though the bucket is moving, it looks like these blades are doing
this. And the reason why they're doing that is because,
where do the ice crystals form?
STUDENT: [INAUDIBLE] MICHAEL CIMA: Well, where the heat is going
out, right? It's going into this phase change material
here. In order to go up this heating curve-- not
a cooling curve now, a heating curve.
I start at the minus 5. It heats rapidly to minus 4.
And then during this period it takes heat to go from alpha
and beta to the liquid. And so the heat is going out through that
surface. And that's exactly where you form the ice
crystal. Now, if you didn't have this blade here, that
crystal would continue to grow.
And you'd end up with this sort of granular-- it's sort of like, anybody skied out in New
England? Do you know what loose granular is?
Ice, large chunks of ice, right? That's what would be in this.
And so what happens is, before that crystal can grow too large, it comes
by and knocks it off. And that's all that is.
Now, if you go to an industrial ice cream set up, they'd have these.
They can do it on a continuous basis. So they have this jacketed steel tube.
Jacketed means that there's a steel tube, but inside here there's a place
for a refrigerant to flow. So it can suck, basically the refrigerant
is like a phase change material, except it's going from a liquid
to a gas. And it sucks heat out as it does that.
And of course, that's the cold surface. So if you start down here and start pumping
in this sugar and egg and cream mixture, it will freeze as it moves
along here. And of course, I don't want it to just freeze
completely. And I don't want the crystals to grow too
big. So on the inside is this rotating thing that
scrapes the inner wall all the time.
So it's just like the bucket, except that now it's a continuous process.
It's continuously doing that there. And you have a machine down here that just
automatically puts up the tub, the gallon tub or whatever it is, and squirts
in the ice cream and goes on to the next one.