14-12 Crystal Structure: crystallography of chocolate (expensive & cheap chocolate)


Uploaded by JUSTANEMONERD on 09.12.2012

Transcript:
PROFESSOR CIMA: I want to spend the last minutes on chocolate.
And we're going to do some more with planes. We're not done with that.
But I want to spend a moment talking about triglycerides.
This is important because why are we learning all this
crystallography stuff? Does anybody have any idea?
STUDENT: Chocolate. PROFESSOR CIMA: Yeah.
It's only because of chocolate. All right.
So, quiet please. Glycerol.
I'm going to start with glycerol. Whoops.
H. This is glycerol. It's got three OHs.
And then-- so this is not stuff that particularly tastes
any good. And then there's this stuff.
One, two, three, four. Just because my model.
Five, six. This is called--
so this is glycerol. And this is a fatty acid.
You've heard all about these. This is a saturated fatty acid.
No double bonds here or anything. As we'll learn, I can react this with this
to eliminate water. Oops.
O. To do this. And at that point, this becomes a triglyceride.
And that you've heard of. These are fats.
And they are ubiquitous in nature. These are groups.
All this stuff here can change and it does. A lot.
But the basic structure is the same. We got these, they're called ester bonds.
Three ester bonds. And it's called a triglyceride.
Now I showed you this little model. Here's the model of this thing.
It's really floppy. But if you lay it down and you try to put--
as I've drawn it here-- tried to put them like this, it doesn't pack
very well. About the only way you can get them to lay
down nicely on the desk, which is very interesting, is like this.
Where one of these triglycerides sits off by itself.
Or one of these fatty acids sits off by itself. Oop, I broke it.
[LAUGHTER] PROFESSOR CIMA: I knew something was wrong.
I broke a bond. All right.
So you can see one of them sits off by itself. It naturally wants to do that.
Now there's a couple different ways I can do it.
I can have-- I don't know if you can see this here.
Here's two double bonds. This is the double bonds here on this oxygen.
They can fit like this or they can fit like that.
Surprisingly, it works better on my desk in my office.
But naturally. So imagine trying to stack these things.
And when you stack them, you get stacks of these three sides.
One like this, one like that, one like that. You can see it sits like this.
And then I pack the next one like this. And so there is my unit cell.
And so this particular structure is called the beta form.
Just pack all-- its molecular crystal. The only thing holding this crystal together
are Van der Waals forces. And actually, only Van der Waals forces in
this case. Last minute.
Now if I look down the end, I can see these aliphatics.
There's the beta form. I'm not done.
You can see them here. That's one orientation.
Remember, I switched this. In this form, it turns out I switched it the
other way. I get a completely different crystal structure.
Beta prime. Now why is this important for chocolate?
Well, when you cool chocolate from high temperatures, the fats in it
start to crystallize. And which one of these different crystal structures
is produced determines the melting point.
In fact, here's a bunch of different ways you can pack these things.
Look at the melting points. They vary from 16 degrees C up to 34.
36. What's body temperature?
STUDENT: 37. PROFESSOR CIMA: 37.
You hold this one in your hand, it still stays solid.
It doesn't melt in your hands, it melts in your mouth.
Now the significance of this is it's actually hard to introduce just one
crystal form. How do I know what crystal form is going to
be produced? So if you go to a real professional chocolatier,
what they do is as they cool the chocolate, they take some old chocolate
and throw it in. Why do you think they do that?
STUDENT: [INAUDIBLE]. PROFESSOR CIMA: They're seeds.
They grow the crystal form they grew before. It's very important.
Now, you ever seen old chocolate? Last minute.
If you're going home at Thanksgiving, you can really impress
your parents, right? [LAUGHTER]
PROFESSOR CIMA: You ever seen old chocolate that has that white stuff on
the outside? STUDENT: Yeah.
PROFESSOR CIMA: You know what that is? The cheap chocolate.
They made this and in the package it started to convert to that because
it's more stable. It's a solid state phase transformation.
It's called bloom. It doesn't happen with the expensive chocolates.
[LAUGHTER] PROFESSOR CIMA: And the other thing about
the expensive chocolates is that not only do the melting points change, but
the mechanical properties change. You can tell good chocolate because it snaps.
You get that Hershey Bar, it goes like this and it just, hmmm.
[LAUGHTER]