cc >> Tom Zinnen: Welcome,
everybody, to Wednesday Nite at the Lab.
I'm Tom Zinnen. I work here at the
UW Biotechnology Center and I also work at the UW Extension
Cooperative Extension. On behalf of our partners, the
Wisconsin Alumni Association, the PLATO Lifelong Learning
Group, and Wisconsin Public Television, thanks to coming to
Wednesday Nite at the Lab. We do this every Wednesday
night, 50 times a year. Tonight I'm delighted to be able
to introduce to you Professor Rich Hartel of the Department of
Food Science here at UW-Madison. He'll be talking about rheology
and tempering of chocolate. It flows and blooms as it grows,
as I like to put it. He was born in Brooklyn,
New York, but I was rather surprised to hear that when he
told me that because he doesn't sound like he's from Brooklyn.
He went to school at upstate New York at Potsdam, and he's
been here a long time. It's been one of the great
pleasures to get to work with him over the years.
He's done science expeditions, he's done Wednesday Nite at the
Lab at least four or five times, and the best part is he brings
experimental samples. [LAUGHTER]
So please join me in welcoming Rich Hartel to Wednesday Nite
at the Lab. [APPLAUSE]
Rich Hartel: Well, it's good to be back.
I remember it was three years ago and I remember that night
because we were over in the other room and I think you were
sitting at my feet and there were people out in the hallways.
So seriously when I wanted to hand out samples I just had to
do this and give the sample. So it was impressive.
I, at that point, didn't know how many sample bags to prepare,
so I'm afraid I was a little short on some of those.
Hopefully I got enough this time.
So how many were at that talk? That was three years ago.
So I recognize several of the faces here.
And so it's for you that I changed everything.
[LAUGHTER] So I'm not going to re-do
that talk. I've decided to go in a
different direction, so I call it 202 instead of 101.
But as with any class you always go back a little bit and cover
what you've done before to bring people back up to speed.
So that's what I'm going to do. I'm going to start out with some
of the same things that we started out with last time.
Primarily, where does chocolate come from and how is it made.
And then we're going to get to a point where we look at a list of
properties that chocolate exhibits, things that we want
from them, and in previous talks I've covered flavor and
polyphenols, that kind of stuff, but this time I'm going to focus
on the rheology of melted chocolate because that's a very
important aspect and you have some samples there that I'm
going to walk you through what the rheological properties are.
We've got to explain a little bit of rheology but eventually
we'll get to a map and I'll put different chocolates on that map
and you'll understand when we get there.
And then another aspect of chocolate that's fairly unique
is if you just take melted chocolate and cool it down, you
get something that you don't want to look at again.
It blooms. So we have to control the cocoa
butter and its crystallization or temper it.
I'm going to talk about that, and I've got some really cool
videos that we take in our lab and show you some things about
cocoa butter and how it crystallizes and stuff like
that. And I'll wind up with bloom,
although I think I'll leave that for 303 in detail.
So that's the plan for today. And so the introduction part,
where does chocolate come from? It grows on trees.
So there's a friend of mine who is down in the plantations, the
cocoa plantations down near the equatorial range, and these pods
grow on trees and then they're harvested and the beans are
removed from the inside. I'll show some more pictures of
that in more detail. The beans are fermented, dried,
and then they're shipped to wherever the chocolate is going
to be made. Where we get cocoa beans these
days from, there's three main sources.
There's west Africa. So Ivory Coast has sort of
become the standard for cocoa beans these days.
Indonesia and Malaysia are trying to build their program.
They tend to make a very different type of cocoa butter,
a hard butter, but they're trying to build their growth.
And Brazil used to be the number one producing country until
witches' broom kind of wiped out the trees.
They're trying to make a comeback, South America in
general trying to make a comeback from some disease
things. So that's where chocolate or
cacao comes from. Here's some, I'll show you some
pictures of these. Here's some cocoa beans.
Sorry, these are the cocoa pods. And I'm supposed to use this and
not my pointer. So here's the cocoa pods growing
on trees. They grow out the side of the
trees. When they get ripe, they turn
this nice color. And then they're harvested still
by hand. So this is a physical process
separating the pods from the trees.
The pods are cut open. Usually a machete is the tool of
choice here. And inside is the cocoa beans.
And you can see here there's half a pod with the beans
sticking out, and there's on the order of 30-35 beans inside
here. It's contained in this white
mucilaginous mass, which just means a bunch of sugars that
hold it all together that provide the growth medium for
the seeds as they sprout to create new cacao trees.
They scoop those out, put them in a pile.
Sometimes it's in a rack like this.
This pile is a fermentation pile.
So the natural enzymes and microorganisms present in the
area start breaking down that mucilaginous, the mass, the
sugars there, and actually stops the seeds from sprouting.
So that's one of the things that happens.
And the fermentation step is where the precursors to
chocolate flavor are developed. So if you don't ferment the
cocoa beans and then try and do all the roasting and stuff like
that, it doesn't give the chocolate flavor.
So the fermentation process breaks down some of the more
complex chemicals that are in the cocoa beans into smaller and
smaller things like smaller proteins and smaller sugars that
then can participate in the Maillard browning reaction that
happens during roasting. So it's an important process to
create the flavors, although the flavors are not created here
themselves. It's the precursors that are
created. So after four or five days of
fermentation, then the cocoa beans are removed from the pile,
set out to dry. So we bring the water content
down to something like a couple percent.
Remember this is done in tropical countries where rain is
a state of life, I think. So this particular plantation
has their beans on trucks or racks and they go underneath the
roof there when it starts to rain, otherwise they come out
and dry. And the principle is to get the
water content down to a couple percent so that we can then
convert it into chocolate. So the article of commerce that
comes from the cacao plantation that we're going to use in
making chocolate is the cocoa bean.
And this is what a bean looks like in large size.
So it's on the order of a quarter in size.
And, again, there's about 25-35 in each of the pods.
So it takes quite a few pods to make a big bar of chocolate.
So this is what would be delivered to a company like ADM
Cocoa in Milwaukee, the old Ambrosia Chocolate Company, that
would turn this into chocolate. And the process for making
chocolate is take the cocoa beans, so once it comes off the
plantation we call it cocoa bean, before that it's cacao.
So there's a fuzzy definition or fuzzy differentiation there.
So we take those cocoa beans. That contains the shells.
And so these shells are separated off.
Sometimes it happens before roasting, sometimes it happens
after roasting, but what we're after are these nibs and these
nibs are again the roasted cocoa beans that provide the chocolate
flavor. So this is the material that has
the smell and the flavor and taste that we think of with
chocolate. But if we just took those cocoa
nibs and ground them down into chocolate liquor, we couldn't
make a good chocolate bar out of it.
Well I take that back, we could take a really good chocolate bar
out of that but it costs too much.
Most of us wouldn't pay for it. So what we do is take some of
the cocoa beans after they're ground up, take some of that
chocolate liquor and press out the cocoa butter.
And then we're going to add some of this cocoa butter back to the
chocolate liquor to give us the right fat content which is
really what governs the viscosity of the melted
chocolate. And all chocolate is sold
specified by a viscosity. So it's a very important
attribute of chocolate, both from the person that makes the
chocolate and sells it to a user, as well as the user who is
going to try and put it into something that we would buy and
eat. The byproduct of the cocoa bean
separation is cocoa powder. This is an important thing if
you're making Oreo cookies or something like that.
So one of the important ingredients for Oreo cookies.
But the two things that we need to make chocolate are chocolate
liquor, which is just ground up cocoa beans, and cocoa butter.
So ground up cocoa beans, we call it chocolate liquor.
There really is no alcohol, although there is an ethanol
step in the fermentation process.
But it goes to ethanol and it quickly goes to acidic acid.
So there's no ethanol left in the chocolate liquor, but we
still call it chocolate liquor. And so it's basically ground up
cocoa beans. It has anywhere from 48% to 57%
fat to 55% average fat content for cocoa beans.
It's got a few percent water, so a couple percent water that's in
there. And then the rest of it is cocoa
solids and that's made up of starch, fibers, gums, sugars,
things like that. So that's what cocoa powder is,
is all the non-cocoa butter stuff and non-water stuff.
So cocoa powder is all of this cocoa solid type material.
What makes chocolate unique are these alkaloids.
And some people talk about caffeine in chocolate.
I had somebody ask me about would it be worthwhile to
decaffeinate cocoa beans. And I scratched my head and said
I'm not sure why because of the alkaloids in there, that's the
really, really minor one. And so the theobromine, and
actually the name for cacao is theobroma cacao and that's based
on the presence of this alkaloid theobromine.
And that's the one that gives us, so if you were here for my
first talk that's the one that gives the pleasant feeling of
happiness. It's part of why people think
chocolate is pleasurable to eat. It gives a good feeling.
And that's actually a physical reaction, but it's not addictive
like caffeine is. So if you think you're addicted
to chocolate, that's up here. [LAUGHTER]
So that's chocolate liquor. Again, we're going to use that
as the base for making chocolate.
So to the chocolate liquor here we're going to add sucrose.
So this is just table sugar that's going to sweeten it.
Bulk it up a little bit. We might add milk powder if
we're making a milk chocolate. Most of what I have here today
is dark chocolates. And then we would mix these
together. The sugar is in the form of
table sugar so you know that it's pretty coarse.
You wouldn't want to put that in your mouth, it feels like sand.
So we have to grind it down to a smaller and smaller particle
size. That's where this refining
operation is, and that's done by passing the chocolate mix here
through a series of rollers and the rollers, so there's a series
of gaps between five rollers and each of the gaps decreases in
size as it passes through to go from, say, 200 microns,
initially, down to 25 microns at the end.
So that's the refining process. Then it goes into a big mixing
tank, a big sheering vessel called the conch.
And that serves several purposes including smoothing out the
flavors, it helps break any aggregates of the particles that
are reformed during the refining operation.
And this is where we add the extra cocoa butter, and then
this is where we add lecithin as well.
So when you look at the ingredients of a chocolate bar,
usually it says sugar, chocolate liquor, cocoa butter, lecithin,
and maybe some flavor like vanilla, something like that.
So that's the smoothing out part of the flavors.
And then we have melted chocolate.
So for many companies then that chocolate, it's in molten form.
The conch is at high enough temperature that the cocoa
butter is melted. It goes into a tank farm.
So many companies have these big tanks, thousands of gallons of
melted chocolate and they even keep the room, at ADM Cocoa they
have a room where it's 105 degrees.
That's above the melting point of cocoa butter, and so they
don't have to heat the tank, they just heat the room, which
is a nice place in the wintertime, not so good in the
summertime. But it keeps the chocolate
melted, and then it would be offered whatever application
they were going to use it for. Some companies actually ship
melted chocolate in tanker trucks.
So if you can imagine these big tanker, like a milk truck, going
down the road but it's insulated and contains melted chocolate.
How about that for an accident? [LAUGHTER]
Everybody would be looking for...
Better than molasses. >> Yeah, better than molasses,
right. So that's the end point of the
manufacturing process of manufacturing chocolate.
So if a company like ADM Cocoa is selling chocolate to a
company like General Mills, then they may send them tanker trucks
of melted chocolate. They may actually form the
chocolate into bars. So we sometimes get 10-pound
bars of chocolate. That's a really cool thing to
have. [LAUGHTER]
I actually have one that says world's largest chocolate bar.
[LAUGHTER] Sometimes they distribute it
in the form of wafers. Sometimes they make, it's mostly
bars or wafers, that's usually what is sent or the melted
chocolate. But in order to get chocolate
into a molded bar, we have to temper it.
So let's work on the liquid chocolate first.
So we'll go to-- Wait a minute, you're supposed
to taste the chocolate liquor. Sorry, I get carried away.
You have a bag in your bag that says CL on it for chocolate
liquor. If you've never tasted chocolate
liquor before, take a nibble of it.
Do you have any left? >> Tom, is there an extra bag
here? I'll give you mine.
Do you have an extra bag? Great.
So this is chocolate liquor. It's just ground up cocoa beans.
What does it taste like? Bitter is the key word.
This is also called baking chocolate.
So it's unsweetened baking chocolate.
And that's the base from which we start, and actually as you
look at different calibers of chocolates, the amount of the
chocolate liquor that's in there usually correlates with the
quality of that chocolate. So if you're buying a Lindt,
70% cacao chocolate, it's got 70% of that in the bar.
We'll talk about that and we'll taste that here in just a
second. A less expensive chocolate may
only have 15% to 20% of that chocolate liquor in it to
provide some flavor, and then there's a lot of sugar to
provide bulking. Okay, so that's chocolate
liquor. It's got about 50% fat and then
all of other what is really cocoa powder mixed in with the
cocoa butter. Okay?
Sorry I skipped over the first taste.
But that leads us right to the next one which is a comparison
of several different dark chocolates.
And so you have, what do we have here?
You have two bags. Neither of them are labeled.
One's got a double set of disks. So disks or wafers are sometimes
what's used to ship chocolate around from a supplier to a
manufacturer. You have a half of square of
Lindt and you have, I thought we'd try something a little bit
different, a dark chocolate Hershey Kiss.
So these have very different qualities of chocolate.
The Hershey dark, which is what's in the Kiss, and then we
picked out some generic, I have boxes of chocolate in my lab so
I just picked out a few things that I had laying around.
And this is a low to medium quality chocolate.
And then, of course, the Lindt is a pretty high quality
chocolate with 70% cacao. Which means that it's got 70%
chocolate liquor. Because it's got 70% chocolate
liquor, it's got plenty of fat. Even if there's 50% fat in the
cocoa butter, then that chocolate has 35% fat.
And typically we're looking for between 28% and 35% fat to give
us the right rheological properties, to give us the right
flow of the melted chocolate. So it's got a little bit less
sugar than the Hershey dark. It doesn't have to add any extra
cocoa butter because it's got enough.
They both have lecithin and vanilla as a flavoring.
In the minimum semisweet, which is probably more like the disks.
I really don't even know what the Hershey's Dark Kiss is like
because that's a fairly new product, but these disks have
probably the minimum amount of cocoa chocolate liquor that they
can get by with. And 35% is about the lowest you,
I think I misspoke before, but that's about the lowest you want
to go to still give a good chocolate taste.
But because that's 35% chocolate liquor, half of it is fat,
that's only 12.5% fat. That's not good enough to give
us the rheological flow properties that we want in
melted chocolate so we've got to add 14% cocoa butter.
And that was when the start of chocolate manufacture came
about was when people realized that we can make a good eating
chocolate if we separate the cocoa butter from the chocolate
liquor and add it back and make a good chocolate.
So what do you think of these disks?
And let's do it in comparison to the Lindt.
Chocolate flavor? The Lindt has more.
And that's a pretty obvious distinction.
Sweetness? The cheaper one, the disks have
more sweetness. Again, 20% more sugar in there.
Some chocolates have maybe as high as 60% sugar.
So one of my friends at ADM calls it bunny chocolate.
So buy the cheapest bunny that you can find but make sure it's
real chocolate. And that's probably got a much
lower chocolate liquor content. So he calls that bunny
chocolate, and it's pretty inexpensive stuff.
What he says is that if the bunny doesn't sell--
No, what he says is if the Santa doesn't sell at Christmastime,
they just melt it back down and make bunnies out of it.
[LAUGHTER] So very distinct difference
in chocolate flavor, chocolate taste.
And again it comes from the amount of chocolate, primarily
comes from the amount of chocolate liquor in those two
products. So let me get your sense.
Where does the Hershey's Kiss fall?
Is it between those two or on one end or the other?
This is the sweeter of them? Sweeter?
The chocolate taste? Better than the disk or not as
good as the disk? So it's a mix.
Some people say better, some people say not so much.
So, again, I don't know off hand what the composition of this
Hershey's Kiss is. I think it's more like the
Hershey's Bittersweet Special Dark, which I think is actually
this composition. So very different flavor based
on how much chocolate liquor is there.
But that's where we've been before is comparing these
tastes. Now I want to talk about what's
different between these in terms of their rheology.
So let's turn to the first topic that we have is how does the
melted chocolate flow? So to think about that, the top
picture here is melted chocolate in my cartoonish sort of way.
So here's all the particles. The big brown blobs are the
cocoa powder particles. The sharper brown/black ones are
the sugar particles. And so when we send that
chocolate through the refining operation, we're breaking those
particles down and down into smaller and smaller size they're
on the average of 20 to 25 microns or so in size.
A really good chocolate would be down around 10 to 15 microns in
size. A much coarser chocolate would
be closer to 30 microns. And so in the melted chocolate
those particles, which constitute somewhere between 60%
and 70% of the mass of that melted chocolate, so if you took
any of these chocolates that you just ate and melted them in your
mouth, what you would have is two-thirds of it in particles,
sugar and cocoa solids, and the rest of it is the liquid cocoa
butter. So the liquid cocoa butter,
we've all seen liquid fats, you just pour them out of container
and they flow really nicely. But if you pack them with all
those particles it's like taking water that you can pour really
easily and packing it with sand or corn starch.
You ever try that? That's a weird one.
And pack it with that many particles.
In fact, if you packed water with corn starch, you'd get a
really weird texture, really weird consistency and very
different rheological properties than chocolate.
It's a different beast all by itself.
But if you pack the water with sand and then try and pour it,
well it's not going to go anywhere.
Maybe not. It depends on how much water
there is, how big the sand particles are.
If you packed the water with 50% sand particles, maybe you could
pour it. It would be like quick sand, it
would pour a little bit. But it would some flow
properties. So the more and more particles
we pack into this liquid, the more and more solid like it
gets. And that's what we're dealing
with chocolate is a liquid, melted fat with these particles,
cocoa butter and sugar particles, packed in a fairly
close proximity. So if these are 20 microns in
size, 60% of the mass is that then we must have bazillions of
particles in there. And as you can imagine if you
try and pour that, those particles have to kind of roll
across each other to get out of the cup and have a very
different flow property, rheology, compared to the liquid
oil itself. The water in there is
particularly dangerous because the water is, well, water is
hydrophilic, it is water, but what part of chocolate is
hydrophilic? Well the sugar particles.
So if you're a water molecule in chocolate, you would go right to
the sugar crystal surface there and hang out there.
That's why the lecithin is there.
The lecithin is a surfactant or an emulsifier, and it has both
hydrophobic and hydrophilic groups.
The hydrophilic group sits on the surface of the sugar
particle, where the water is, with the alkyl chains, the
hydrophobic part, that stick out into the liquid fat.
And it's actually those lecithin molecules that allow the sugar
particles to flow across each other.
Because if you're two hydrophilic surfaces in a
hydrophobic space, you go like this.
You stick together and you don't move.
And that's what happens when you add a little bit of water to
chocolate. It turns into Tom's concrete.
I was going to answer a little bit differently but that's what
happens when you add a little bit of water to chocolate is it
turns into concrete because the surface of the sugar crystals
becomes syrupy, and then when those particles try and move
past each other they just stick together and forms concrete.
That's what it is. So obviously the characteristics
of those particles play an important role in how easy it is
to flow, how easy it is to pump the chocolate and what its flow
properties are for specific applications.
Of course, when we solidify the cocoa butter, we get all of
these little cocoa butter crystals in there.
And that's a very important aspect of creating a nice
looking chocolate is to get just the right shape and size and
polymorph of those cocoa butter crystals.
So we'll go there next after we talk about rheology.
So here's, I like this picture, it's a three-phase contrast
image from a confocal microscope of chocolate particles taken by
a colleague of mine in Ireland. And this is one that has, it's a
milk chocolate. So it's got, the green particles
in there are milk powder. The sharper black ones, like
this one here, is a sugar particle that's been ground down
into a sharper space. There are some other brown ones
or green ones or black ones that look round that are properly the
cocoa powder particles that have been ground down in the refining
step too. But this is really cool because
you can see very clearly how dense those particles are.
So, again, the surrounding matrix, the blue part, is the
liquid cocoa butter. So imagine taking that in a cup
or a beaker and pouring it or trying to fill a mold with that
chocolate. Well, all of those particles are
going to influence how that system flows and are going have
a fairly large impact on the rheology.
So I talk about rheology like it's something everybody
understands. Let me go back and explain a
little bit what viscosity and rheology mean.
So it's the science of the flow of fluids.
One way to look at it. Here's melted chocolate.
That's what it looks like when you pour it from my beaker into
my melting tank. One way to think about viscosity
is the resistance of a material to flow.
So thick things when you try to pour them out of a cup take a
long time to flow or they don't flow.
Really thick things will stay there.
Water is something that has a low viscosity so it pours out
really quickly. Corn syrup, which is made up of
starch molecules that have been broken down, so corn syrup is a
sweetener made from starch but broken into those various
glucose polymers that come from starch.
But different corn syrups have different chain lengths.
And the higher the chain length of the glucose polymer, the more
viscous the material is, not surprisingly.
So corn syrup, something that when you pour it takes a while
to come out and flow. So for us it's important because
we want to know how easy it is to pour into a mold, for
example, or pump it from here to there as we go from the silo
tanks where we're holding our chocolate to where we're going
to turn it into a disk or a molded piece.
We often have candy bars that have chocolate layers on it, and
that's actually a function of the rheology of the chocolate.
I'll come back to that in a minute.
And that actually has some impact in how we swallow it.
In chocolate there's a combination of all the things
that are going on from the melting of the cocoa butter to
dissolving things with your saliva to sliding it down your
throat. Actually I was giving this talk
with a bunch of speech pathologists who study dysphagia
or swallowing problems, and so I was connecting chocolate
rheology with dysphagic fluids. ( chuckles )
But the bottom line is if you make chocolate and sell it to a
user, a manufacturer of a candy bar or something like that, then
they specify what their rheology properties are for that
chocolate based on what the application is going to be.
So in every lot of chocolate that gets sent out, whether it's
in a tanker truck or in wafers, there's a specification that
includes viscosity. So it's really, really critical
to both user and manufacturer to get it right.
[INAUDIBLE]
No, I don't think so. But I don't know enough about
that to say for real. So what's the root of the word
rheology? I don't know that.
So we break rheological fluids, we break fluids into two
rheological classes. Actually there's more than this,
but we can break them down into two.
There's things that their flow properties, it doesn't matter
how fast you pour or how fast you stir.
Water is that way. No matter how fast you beat it,
it always looks like water. It always feels like water.
It always reacts to your spoon with the same force.
And so these things, water, oil, corn syrup, even chocolate milk
are basically Newtonian fluids, and that means that there's a
direct correlation between shear rate, which is the velocity
profile, and the shear stress, which is the force that's
applied to generate that velocity profile.
So a linear relationship that gives us Newtonian fluids, we're
not interested in those because chocolate fits in the
non-Newtonian shear thinning fluid.
So before I go there, let's talk about that shear thickening
fluid. So remember I talked about
packing water with 60%-70% starch.
What happens is it becomes shear thickening.
So everybody go home and do this tonight.
Don't blame me. [LAUGHTER]
But go home and do this if you haven't done it before.
Take corn starch, approximately 70% corn starch, 30% water,
mix them together, and then play with it.
Like take it out and roll it in your hands, and because it's
shear thickening, when you roll it, you're applying a force,
and it acts like a solid. So you roll it in a ball.
You can roll that stuff in a ball, and as soon as you stop
rolling it, it flows. It's shear thickening.
The faster you stir it, the thicker it appears.
And as soon as you stop stirring it, it turns into a liquid.
That's a shear thickening fluid. Seriously try it at home.
It's cool. It makes a mess but that's okay.
But that's not what we're dealing with.
So chocolate is a shear thinning fluid.
It's the type of material, just like ketchup, the more you stir
it, the thinner it appears. So there's many fluid foods
like this that have that characteristic.
Even blood has that characteristic.
Toothpaste, think of toothpaste. It sits in the tube.
You apply a force. That force allows it to squeeze
out because applying that force reduces its apparent viscosity
so it can flow out. But as soon as you stop, it
holds it shape, mostly it holds it shape as a cylinder.
So it reacts to force and flows but as soon as you stop applying
a force it stops flowing. So that's a shear thinning type
of fluid. You apply force, it looks
thinner, it can flow. When you stop applying the
force, it looks thicker again. Ketchup is exactly the same way.
Varibar, that's the fluid that people use to test dysphagic
patients, people with swallowing problems.
So they have these kinds of things, so my colleague over in
the medical school here has that.
And we did a Wednesday Nite at the Lab together a while back
too. If you remember tasting that
chocolate pudding drink, that was our test here.
So chocolate fits in this shear thinning type of behavior.
Which means that the more you stir it, the faster you stir it,
the thinner it appears, the less viscous it gets.
That's a pretty cool property. And so we're going to play on
that here. What affects that viscosity?
Well fat content is number one, but the particle structure,
particle shape, and the effect of the surface, whether it's
moisture or surfactants, also affect that.
So as a chocolate manufacturer, I have several things in my tool
bag that I can use to change the rheological properties.
Mostly they relate to how much fat I add, but they also relate
to how fine I grind those particles, how I grind those
particles, and then what type of surfactants I have, what type of
moisture content I leave in my chocolate.
So all of those things are things that, as a manufacturer,
I can play with. Number one the fat content.
So now I've got to talk a little bit of science here.
This is rheology 101. When we talk about shear
thinning fluids, we have three factors that we're interested
in. And so again we have the
relationship between shear stress and shear rate.
And these are important because chocolate fits this model, and
so we need to understand these because I'm going to draw a map
of these parameters that tell us something about the chocolates
that you're going to eat here. So we apply a force, that's a
shear stress, so it's a force per unit area, to make the fluid
flow. That flow can be characterized
by a shear rate. Or another way to say it is we
stir it at a certain shear rate and the chocolate applies a
force back on our stirring mechanism.
So that's one way to say it. And that response between the
shearing strain, or shear rate, and the stress is governed by a
relationship that's like this. And so there's a consistency
coefficient, K. If this is a Newtonian fluid,
this K would just be the viscosity that we characterize
the fluid with. Here this N value is a shear
thinning index. It tells you how strongly shear
thinning that fluid is. So if N is one, it's a Newtonian
fluid, then you have a linear relationship between shear
stress and shear strain. The lower the N value goes, the
more shear thinning it is meaning the more you stir it,
the lower the viscosity seems to be.
And then there's this third parameter called the yield
stress which I like to use mash potatoes as my example of that.
So you've all had mashed potatoes that have a high yield
stress. Those are the one that is you
scoop it up and a mound stays on your spoon, it doesn't anywhere.
That's because the yield stress, the amount of force it takes
that material move, is larger than the force of gravity.
So the force of gravity is not enough to make that stuff flow.
We've had other mash potatoes where maybe we've added a little
too much water to the powder mix or milk to the potatoes, and it
turns into this gooey consistency and you pick it up
on the spoon and it runs off. Low yield stress.
So the force of gravity is enough to make that stuff flow.
So yield stress, while some people, the rheologists, argue
what that really means, for us what it means is does that
potato, mashed potato, stand up on the spoon or not.
What it means for chocolate is when I put a dollop on to a
conveyor, does it stay up or does it flow out?
I'll get to where you think I'm going here in just a second.
Okay, so those are the three parameters that we're interested
in shear thinning behavior. It turns out it's even more
simple for chocolate because we use this model.
It's a slightly different model but similar.
And so we only have two, and I didn't change that equation.
I'm not sorry. I meant to change that equation.
It's the same one. So this equation should look
like, so sigma to the one half, sigma zero to the one half, K
gamma dot to the one half. So everything is to the one
half. We take out of the N value and
we're left with two parameters. The yield stress, sigma zero,
similar to this, it's a similar principle as before.
And then the plastic viscosity or consistency coefficient is
similar to that. But in this case you need to put
each of those terms up to the one half to make the model that
works for chocolate. It's called the Casson model of
rheology. For whatever reason that's what
chocolate people have grabbed onto as the constitutive
equation that describes chocolate rheology.
Whether it's fundamental or not, it doesn't matter, that's what
everybody uses. And so what we're going to do is
take the two parameters, the yield value, again that's how
much force it takes to make the fluid to start flowing, against
the plastic viscosity. And I'm going to plot here three
different types of chocolates that have very different
characteristics. So it's the combination that
determine the application of that chocolate.
So a chocolate supplier doesn't just get viscosity, they get
these two parameters and they sell those chocolates based on
those parameters. So let's look at the Hershey's
Kiss. And you have a regular Hershey's
Kiss now in milk chocolate. And you never have to eat
anything in my classes but I always want you to at least
explore it, which means poke at it and look at it.
Smell it, eat it. >> If you're not going to eat
yours... >> Tom's always willing.
So look at this cone of chocolate, right.
Hershey's Kisses are made by simply taking melted chocolate
and putting a dollop on a conveyor.
So drop a dollop of chocolate onto a conveyor and this is what
you get. So you can see the depositor
nozzles dropping a dollop of chocolate onto a conveyor.
There's a whole row of, it's pretty impressive to see all of
these Hershey's Kisses or chocolate chips.
Chocolate chips are made in exactly the same way with
exactly the same properties. So that chocolate, if it didn't
have the right rheological properties, would turn into a
puddle. That's kind of like where those
wafers were going. Imagine that.
So this has a slightly lower rheological requirement than the
Hershey's Kiss so it flows more. And if we were to make it even
lower, rheologically, it would just flow out into a puddle.
[INAUDIBLE] >> Temperature dependent,
but we're going to talk about temperature because we
have to have the cocoa butter melted which means it has to be
on the order of 35C. And we're going to cool it down
below that to get the chocolate cocoa butter tempered.
So this is done at about 35C. And this is tempered chocolate
being deposited onto the depositing conveyor.
So if we didn't want the chocolate to flow off, what
rheological characteristic would we want from it?
We would want it to have a high yield stress.
So we want it to have a high yield value so that when it
drops, it stops flowing right away.
We stop applying a force, it stops flowing and just holds its
shape. So there's a little bit of flow
at the start, that's what gives a flat bottom, but then it stops
flowing. We also want it to have a fairly
high viscosity. So viscosity and yield stress
are slightly different things even though we just look at
melted chocolate and it looks like it's thick or thin.
But really there's those two parameters that are important to
us. How much force it takes to make
it move, that's the yield value, and then once it's moving, how
thick is it, and that's the plastic viscosity.
And so to make a chocolate Kiss or a cookie drop, a chocolate
chip, or a wafer that has some stand up properties, then we
want to have chocolate that's got a high yield value, high
plastic viscosity. What do you think we might do
with the variables that we can control to get that?
Lower the fat content. That's usually the way that we
do it. Lower the fat content makes it
thicker, builds up the yield stress because now those
particles can't flow across each other as well.
So it builds up a yield stress, higher viscosity.
But there's an upper limit too. So here's a picture of if you
have too high a yield value then you get all these strings coming
back off. That's not good for your
depositor either. So you can look at the Hershey's
Kiss, it's got a little bit of a string, that's part of the
appeal to it, and I think they put this little thingy on there
to even make it stronger. But you can imagine that if the
yield value is too high that's just going to stick out into
space and cause problems if you want to put a wrapper on it.
So the yield value has to be just right to get it in the
right shape, the right characteristics for the
Hershey's Kiss. So that's one end of the
spectrum, something that gives us a high yield value, high
viscosity, and stands up when we drop a dollop onto a conveyor.
Candy bar? Of course, because it's right
there in your bag, right? That's like asking a give-away
question on an exam. But actually if anybody likes
this kind of stuff, there's a whole bunch of these
cross-sections on the Minnesota Science Museum.
They have some really cool quiz games, I could give you some
that you wouldn't get that are cross-sections of candy bars.
So I want you to go ahead and take a cross-section of that
candy bar, preferably by biting it in half with your mouth, and
try not to smear it though, that's bad.
And then look at that cross-section, and you should
see something like that, right? And so what you see is a nougat
layer on the bottom. There's a caramel layer on the
top with a whole bunch of peanuts in there.
I guess that's what gives us the energy, right?
It used to be a health bar. And then it's coated in
chocolate. So how does a chocolate candy
bar like this get coated in chocolate?
It goes through an enrober, and this is a really cool piece of
thing. We have one in our lab.
We're doing a chocolate social with the Food Science Club, and
we'll crank up this enrober. And basically all it is a
hopper that brings melted chocolate up to the top.
And then there's a louver, this one has just holes in it to
allow the liquid chocolate to flow, and it forms a curtain of
liquid chocolate through which these pieces are conveyed.
So on the far right there, sorry, on the left-hand picture,
on the far right there there's uncoated bars.
These look more like cookies than Snickers bars but the
principle is the same. You've got a bunch of these
uncoated Snickers bars packed together on a conveyor.
They go through this curtain of chocolate, and the curtain of
chocolate covers those pieces so when they come out the other
side they're completely coated. Usually before they go through
this curtain they've gone through a bottomer.
It's a little puddle it goes through to get the bottom done
as well. And then it goes through the
curtain and it's all coated around with chocolate.
Well suppose you had a really, really thin, low viscosity
chocolate, what would happen to that chocolate?
Just flow right off around there, and you'd have a Snickers
bar with a really, really thin layer.
None of us would be happy about that, much less M&M Mars.
So we've got to get the rheological properties just
right to give us the right thickness of that coating.
Also, if we want to put decorations on the side, on the
top, I don't if you spent some time looking at the top before
you wolfed it down, but there's a little swirl on there, and
that swirl is, the property of giving that swirl relates back
to the rheology again. So you have to have the right
rheological properties to make that swirl stand up.
You could put a swirl there but if it's a thin, low viscosity,
it just flows right back in. Like any liquid flows to meet
its levels. So it has to be thick enough and
high enough yield stress so that you form a shape like that and
it doesn't flow back down. So that's exactly what our
interest is. And my tabs got screwed up here.
So enrobing leads to coverage, leads to thickness and
uniformity, leads to side coating, and if you don't do it
right you get feet and tails. So this thing here, I don't know
if you still have any of that Snickers bar left, whether
there's a tail down here at the bottom.
So it's sitting on a conveyor, chocolate comes down and if it's
still flowing after it gets on, so it goes through a conveyor
with holes in it, a screen type conveyor and the chocolate flows
through the stuff that doesn't stay on there.
But then it eventually goes on to a solid conveyor and if the
chocolate is still flowing you're going to get these feet.
So finding the right viscosity is important to eliminating that
feet as well as giving the decorations on the surface of
it, as well as giving thickness of the candy bar, the thickness
of the chocolate on that candy bar.
So what do we want in terms of its rheological properties,
yield stress versus plastic viscosity?
We want something in the middle. It's a little bit down here.
But basically we want something that has enough stand up
properties to give us the thickness that we want on the
side without giving us the feet problem.
So sort of in the middle of the yield stress plastic viscosity
range. How do we do that?
Add more fat. Usually that's the way we do it
to get a thinner viscosity, lower yield value, to give us
the right rheological properties.
Okay? We got one more.
Well, there's one more and then a test.
And that's the Dove. So this is a deposited piece,
which means it's deposited into a mold.
And so if you can imagine a depression in a shape, and
inside that depression it has this shape.
The flat part is the top. So we're going to pour chocolate
into that shape. The liquid chocolate is going to
find its level. Often we have to shake it to
make it do that, but it's going to find its level and then form
that flat surface. And so this part, the part that
says Dove on it, Dove, is in the mold and the mold actually has
that inverse Dove printed inside the mold.
And so when we pour the chocolate in it has to be thin
enough to be able to flow into those fine cracks and crevices.
You want to see something that's really, really fine?
Go buy a Ghirardelli square. So Ghirardelli square has this
really intricate pattern and then it's got a ribbon across
it. And inside that ribbon it says
made in San Francisco. >> Really?
Yes. It's incredible how much detail.
[PHONE RINGING] It can't be mine.
[LAUGHTER] It's incredible how much detail
you can get out of a low viscosity chocolate,
something that will flow into any crack and crevice anywhere.
But seriously, go buy a Ghirardelli bar.
They're a little too expensive for me to buy for you guys
so I bought these Dove pieces. They're pretty good chocolate
too. And actually I was just at their
plant, the Ghirardelli plant, in December, and it's interesting
how they make those filled squares.
So I wish I could demonstrate this in some way, but they pour
the chocolate into the mold, and it's a square mold with the
thickness that you would expect from the shape of the thickness
of that square. So they pour the chocolate into
the mold, and then they immediately turn it up side down
and shake it. And so all the remaining liquid
chocolate drips out leaving a shell of chocolate that's filled
up that mold with the fancy incredible printing that's on
the inside of that mold. And then they turn it back right
side up and then they deposit whatever filling they want in
there and then they shake that to make the filling flatten out
and then they put a dollop of chocolate on the top of each of
those and scrape them off into the cooling tunnel and that's
how they make those squares of filled chocolate.
But again the shape, the intricacy of the printing on
that puts this to shame. There's some intricate design in
making the Dove with the little flourishes, but the Ghirardelli
squares are pretty cool. So go look at that.
So what rheological properties do we need here?
Well I've basically given it away.
We've moved to the other end of the spectrum.
We want something that's really low viscosity, low yield stress
so we pour that dollop into the mold and it just flows into
every little crack and crevice that we have in there.
And so we want it to be down here in the thinner than thin
type of chocolate. What do we have to do to get
there? >> Cocoa butter.
More fat. Basically that's right.
So those Ghirardelli bars, that's how they make the
chocolate so thin that it fills them, is increase the fat level.
They wouldn't tell me what it was and probably if they did
I couldn't tell you anyway. But that's where we're going.
So very simple rheology. It's thickness and yield value
but it determines what type of chocolate we would use.
If we wanted to make a Hershey's Kiss with this chocolate, we'd
go that puddle. And if we wanted to make a fine
Ghirardelli chocolate mold square with a Hershey's Kiss
chocolate, we wouldn't see any shapes.
It would just flow, stop flowing and it would never flow into the
cracks and crevices. So getting the rheology is
really critical to the application for the chocolate
that we're after. And now for the test.
We've all seen chocolate fountains, right?
What do you think they do to make a chocolate that's that
fluid? >> Lots of fat.
Raise the cocoa butter. Yep, that's exactly what they
do. Interesting.
And that's warm enough that it's not solidifying.
It's probably 35 to 40, so it's always liquid, never going to
set up. [INAUDIBLE]
Actually, often when you see a fountain like that,
it's not real chocolate, it's fake chocolate.
And so then it's made with some other fat that might have a
better rheological parameter. So you're right, it might not
be. [INAUDIBLE]
We call it compound coating. So it looks like chocolate,
tastes like chocolate, smells like chocolate, but it's not
made with cocoa butter. >> Man made?
Well, chocolate's man made. Well, machine made.
Okay. So again we have these as our
influences, but it's primarily fat content that we deal with.
So everybody good with the rheology part?
So I have a few more minutes, I'm going to talk about
tempering and show some pretty interesting videos.
And all I have left for you is some lousy chocolate.
[LAUGHTER] So here we are in this process
which is we've got that liquid chocolate.
In order to get it into a molded bar or an enrobed bar, we've got
to crystallize the cocoa butter in just the right form,
otherwise we're not going to get something that we like.
So we need to learn a little bit about the solidification of
cocoa butter, its crystallization process.
So we have to get the cocoa butter below its melting point.
So that's a cooling process. Its melting point is about 34
degrees. It depends on the cocoa butter.
So we have to bring it down below that temperature.
It nucleates and forms a crystalline phase from the
liquid material. That means that these
triglyceride molecules, which look sort of like that, have to
come together to form a crystal lattice.
And then they grow out until we relieve the supersaturation
driving force and everything is at equilibrium, phase
equilibrium. So that's what has to happen.
But of course life's not that simple because the way these
molecules can come together is pretty complex.
So let me show you. So this is the melting process,
lower temperature you form a crystal lattice.
But let me show it to you in pictures.
So here's a video of, I've got cocoa butter on my microscope at
I think we're about 40 degrees at this point.
So it's melted, it looks black. This is cross-polarized
microscopy. So we've set the polarizers
crossed. So almost completely crossed
like if you take two Polaroid glasses and turn them 90
degrees. Nothing gets through.
No light gets through. We're going to back it off just
a little bit. So as the light passes through
the crystals, we can actually see the crystalline light come
through on our microscope. So that's cross polarizers.
So I'm going to cool this now at a certain rate, and we'll watch
the crystallization happen. So the stars come out.
So that's cocoa butter crystal from the cocoa butter liquid
itself. I couldn't do it in chocolate,
couldn't show it to you in chocolate because there's too
many particles. So they're in the way.
I've got to show it just on cocoa butter.
So we cooled it down. It turns out that it solidified
at about 21 degrees. So we were about 40, maybe 35,
and cooled it down, cooled it down.
It wasn't until 21 that the lights came on and the stars
came out. Now, I'm going to heat that back
up again, and watch what happens to the stars.
So I'm heating it, I'm heating it, and eventually the stars go
out. So I melted that fat.
And it melted at 27 to 28 degrees.
So it's supercooling a little bit, and that's typical of
crystallization processes. But what I got here is a
polymorph that I don't want. So then that's the problem with
making chocolate without tempering it is it forms the
wrong crystal structure. And I know it's the wrong
crystal structure based on its melting point.
I could use fancy x-ray equipment to do the same thing,
but we know enough about cocoa butter so say that.
So what I mean by polymorphism is those same molecules can
form different crystal structures just by how they pack
together and organize. So a phenomenon of certain types
of molecules. Fats happen to do it.
A lot of drugs are polymorphic materials, and there are
pharmacists over there making sure that the drug we take,
drugs we take, are all in the right polymorph because the
right polymorph cures us, the wrong polymorph can kill us
because of the way it's bio-available.
So it's really important in the pharmaceutical world.
They live by polymorphs. In chocolate, it's just the
difference between a nice looking piece and a crappy
looking piece. But it's still important for us
to make sure that we've controlled that polymorphic
structure. So for us it's just enough to
say that these triglycerides that make up the cocoa butter
that form the crystals can be packed in different ways to give
us different crystal structures of different equilibrium that
can stay and remain intact for a long enough time.
So we think of cocoa butter as a monotropic polymorph.
That means when we cool it, it forms a less stable polymorph.
And gradually over time it forms a more stable polymorph.
So like most things it's trying to reach an equilibrium.
That equilibrium in this case is the ability of those cocoa
butter triglycerides to pack into the tightest crystal
lattice. That's the lowest energy state,
the equilibrium state. And that's a beta polymorph.
What I crystallized in that previous video was a beta prime.
And I know it because I know what the melting point of the
beta prime is. It's about 28 degrees.
And that's been known, so this is that same video again.
So that's been known for many years.
If we don't do it right, we get this.
This is ugly bloomed chocolate because we haven't crystallized
the cocoa butter in the right way.
But we've known for many years that cocoa butter has six
different polymorphic forms. We're only concerned with three
of them. Because when we cool chocolate
down like I did, or cocoa butter down like I did, it forms the
beta prime, this beta 1-2 formed four which has a melting
temperature of 27.5. This is some work that was done
in 1966, so we've known this stuff for a really long time.
And that's why I can say from where it melted, what polymorph
it was. But that's what happens when we
don't temper the chocolate. It just nucleates by itself into
what's an unstable form. And that unstable form, now I
will show that video again, that unstable form when this melts is
the beta prime form. And that's what gives us this
bloomed chocolate piece. So this is what happens if
anybody has tried to make chocolates at home and didn't
understand the principle of tempering, this is what you got
the next day or that night. And that's not a pleasant sight.
It's got this whitish hazy stuff on there that doesn't look very
good at all. And I'll show you a piece in
just a second. It's the last one in your bag.
So what we're trying to do in chocolate is create this formed
five, a beta formed five according to the Roman numeral,
and it's got a melting point of 34 which is perfect for our
mouth temperature because then it's all melted in our mouth.
Our body temperature is 37.8, but our mouth is a little bit
colder because it's open. Some more than others.
[LAUGHTER] And so we want it to melt
at or around 34 to give us a melted chocolate.
And it turns out that there's another more stable polymorph,
the form six, that's associated with bloom.
So this structure here is actually related to forming the
beta prime, and it quickly transforms to the beta six which
gives us this ugly stuff here on the surface.
[INAUDIBLE] >> Chocolate sauce and syrup
depends on how much cocoa butter is in there, but you can have
that same problem. It could also be mold growing,
though. [LAUGHTER]
This looks like mold, but it's not.
It's actually, we scrape this stuff off and the white stuff is
really just sugar and cocoa solids.
The fat actually contracts and moves away from it.
The brown part is where the cocoa butter is.
So in this particular type of bloom, those brown spots there
are cocoa solids and sugar particles.
That's not going to hurt you. So what we have to do is control
how that cocoa butter crystallizes.
We can't just let it crystallize by itself.
It's going to go into an unstable form.
We've got to turn it into something that is the right,
more stable form, that gives us all the right properties.
And that's called tempering. So the process of tempering on
the left here is taking a fully liquid chocolate, treating that
liquid chocolate in such a way that they we get these seeds of
the right polymorph, and then when this seeded or tempered
chocolate solidifies, it turns into a good solid chocolate that
has all the right properties that we want.
So tempering typically involves heating to melt the chocolate.
So remember if we get a tanker truck of melted chocolate, we
don't have to do that part. We just pump the melted
chocolate into our tempering chamber.
But if we get bars or wafers then we've got to first melt it,
and then cool it down to a reasonably low temperature,
26-27C, which is below the crystallization point of the
beta prime polymorph. And so we get both stable and
unstable, mostly unstable, beta prime polymorphs.
And then we warm it back up above the melting point of the
beta prime leaving behind beta crystals.
And that's what we're looking for is 1% to 3% beta crystals in
our cocoa butter so that when that chocolate sets, those seed
crystals overwhelm the rest of the cocoa butter and it sets up
all into the right stable beta five polymorph.
So that's our aim. If we do it right, we get good
appearance, good snap, good contraction from the mold, and
resistance to bloom. And this is how we test whether
we've got tempered chocolate. Seriously, how it snaps, did it
come out of the mold, and is it resistant to bloom.
If we didn't do it right, we're going to see bloom in a couple
of days. And that's pretty obvious.
So it's pretty important to get the cocoa butter to crystallize
right. If we don't get it quite right
and it's slightly undertempered, then we've got too few of those
crystals we wind up with chocolate that's got specks on
the surface. Not quite as bad as that first
disk that I showed you but still not quite as acceptable as a
good nicely shiny chocolate. But how much of those cocoa
butter seeds? Actually had a fellow from
Morinaga Chocolate in my lab last year and we were playing
with this. So these are his videos.
We can also temper chocolate with seeds.
So these are little spheres, like a millimeter size sphere of
cocoa butter seeds that are used to temper chocolate.
And you can see this little halo in the middle of that slide.
Can you see it in the back? So it's a little halo in there.
It's going to appear really cool in just a minute.
I'm going to do the same thing. I'm going to cool that cocoa
butter, melted cocoa butter down on the slide and watch how it
crystallizes. And so nominally, if I have a
seed crystal in there, it's going to take over and
crystallize the rest of the cocoa butter in the right
polymorph. That seed is in the right
polymorph. So I want to be able to seed the
rest of this cocoa butter. So we thought, well, let's try
this and see what happens. So here's what happens.
That seed crystal grows as we cool.
But the lights come on again. So that seed, the beta seed
crystal grew but then all around it created these beta prime,
unstable crystal polymorphs. So one single seed like that,
not enough to give us tempered chocolate to overwhelm the
crystallization of the cocoa butter.
Now I'm going to show you something really cool.
I call this my supernova video. So I'm going to show you melted,
and then I'm going to explain what you see.
But watch it carefully. So I'm going to raise the
temperature. You're going to see the lights
go out, but there's a period of time when we're between the
melting points of the two polymorphs.
That means the more stable polymorph is going to grow.
But then I'm going to continue to raise it so it's going to
melt. So you're going to see a
supernova. Isn't that really cool?
That's just cocoa butter. [LAUGHTER]
I'll play it again. It's really cool.
[LAUGHTER] So what happened there
is that first we cooled and so we had a stable beta
polymorph at 34 centigrade. We cooled it at 21 or so that's
when the background stuff crystallized out.
But when they heated it back up again, so the beta prime
crystallized on there, when we heated it back up again the beta
prime melted at about 27 just like in the previous one.
But remember that beta crystal has a melting point of 34.
So all in that time period between when it was between 27
and 34, that beta was growing. But then I reached the
temperature where now the beta was going to melt and it
collapsed back in. It looks kind of like a
supernova. [INAUDIBLE]
So once we exceeded the melting point of the beta,
it's gone. And so when we cooled it back
down we just see all the lights come back on.
Good question. And so we were looking at how
many seeds do we really need to get a nice shiny surface.
So he's good at marble tempering chocolate, and that gives you
that nice shiny brown surface. And depending on how many seeds
we added determined whether we got close to that or not.
Whether we were back over here where that ugly disk was.
So I'm going to show a couple more videos.
This is .00055% seeds. They're so small you can't even
see them in here. But now you can kind of see that
there are some bigger blobs there surrounded by the smaller
ones. So again it's not enough for the
stable seeds to take over and dominate the cocoa butter
crystallization. There's still enough of those
seeds there that we wind up with a surface that looks like this.
So it's a bloomed surface. Not as bad as the first disk but
fully bloomed. Here's what happens when I melt
it. Again you can see the beta
primes are going to disappear first.
Beta grows and then it melts too.
So same principle. Now let's do it again
with .137. So this is in the range now
where we have enough seeds that you don't see much.
Those seeds dominate and they take over the whole entire cocoa
butter crystallization. And then when you melt it,
it shows the same thing. They all melt at about the same
condition. If you looked carefully there
were a few beta prime crystals nucleating in the background,
but they were just overwhelmed by the stable polymorph and
that's what gives you the nice shiny surface.
So if you look at this last set of disks that I have in the bag
there, and you're welcome to taste it.
I did this talk once and some guy had put this in his pocket.
So how can you tell this is fat on the surface of the chocolate?
So this particular one is fat on the surface of the chocolate.
You can tell it by putting your finger on it, and hopefully your
finger is warm enough to actually melt it away.
So this guy had put it in his pocket, and it was enough to
melt the chocolate. So you're welcome to taste this.
This is from 2001. I have a reputation of saving
things in my storage room and then feeding it to my students.
[LAUGHTER] Because this kind of thing
is really cool. So we talked about improper
tempering caused by melting and then re-cooling.
So if you've had a chocolate bar in your car that melts during
the day and it solidifies again at night during the summertime,
you open it up and chances are it's going to look like that one
really, really bloomed disk that I showed first.
And that's because you've melted, you've broke temper, and
then you cooled it back down and unstable polymorphs crystallized
and you've got really ugly stuff.
We have this gradual bloom during storage and that's what
happened with this. So this has been sitting in my
office or my lab now for years, and over time it's gradually
grown these cocoa butter crystals that are sticking out
of the surface of this. And then in another version of
this talk, maybe chocolate 303, we'll explore Reese's Peanut
Butter Cups and how come they bloom faster than other
chocolates. So with that, just to summarize
there, so the surface discoloration on chocolate is
either these mounds of sugar in cocoa solids like we do in
untempered bloom or these crystals which are cocoa butter
crystals that grow out of the surface over time.
But again that's something for another talk.
So what I tried to do, I've run a little bit over here, no, it
was Tom that ran over... [LAUGHTER]
Was, again, go back and talk a little bit about where
chocolate came from. We talked about how it's made,
spent a little bit of time going back through chocolate liquor
and different candy bars, chocolate bars, but then spent
some time on rheology of melted chocolate and tempering.
So I hope you understand something about why controlling
viscosity is important. It just looks like chocolate but
really there's a lot of science that goes into making this
chocolate the way it is, and these are just two of the
properties, the rheology and the tempering.
And then I'll leave bloom as a chocolate 303 for some time down
the road. We've done some interesting
work. Actually, let me leave you with
a question. So Reese's Peanut Butter Cups
has peanut butter center and chocolate outside.
The peanut oil migrates into the chocolate, causing bloom.
So if you look at a Reese's Peanut Butter Cup that's old,
and I have some from 2008 in my lab.
[LAUGHTER] They'll come out for class
next week, so I use them. Actually I have an agreement
with Hershey, and they send me a big box of stuff every year.
I put half aside for next and use half this year with the half
from last, and we compare side by side the shelf life of
Reese's Peanut Butter Cups. So they can see the bloom, but
it's fat migration that causes it.
But here's a question: if you bake chocolate chip cookies--
You know where I'm going with this, if you bake chocolate chip
cookies, you have chocolate chips and a cookie dough, cookie
dough contains shortening, some fat migrates from the shortening
into the chocolate chip, have you ever seen a chocolate chip
bloom? In a chocolate chip cookie?
You have, Tom? >> If it's been around for a
while. >> Maybe a long time.
Maybe. >> They usually don't last that
long. >> That's the standard answer is
they usually don't last that long.
I challenge you to find a chocolate chip on a cookie
that's bloomed, because it's rare.
But that's fat migration. In that case the fat migration
stops bloom, and that's chocolate 303.
So that's all I've got. No, wait a minute.
One more quiz. [LAUGHTER]
What rheology is needed to make my Bucky chocolates?
Right, that's it. Thanks.
[APPLAUSE]