PROFESSOR: There are

four ways that chefs use to control the

viscosity- their reductions, their starch-based thickener,

there’s making a emulsion by adding fat, or using modernist thickeners,

where small amounts of material, small amounts of a polymer,

are added to increase the viscosity by large amounts. In all cases, what we’re doing

is adding something to the fluid. In the case of reduction, we’re

reducing the amount of liquid. And what remains are particles,

are nonvolatile solids. This increases the concentration,

or the concentration of the solids, in the liquid. In the case of starch, we’re

adding starch granules. In the case of fat, or making an

emulsion, we’re adding drops of fat. And the drops of fat

behave just like particles. In the case of the modernist

thickeners, we’re adding polymers. And they also have the effect of

adding something to the fluid. And this is what increases

the viscosity of the fluid. The simplest way to understand this,

is to think of particles in the fluid. When the fluid flows,

the particles also flow. And they have to flow over one another,

in order for the whole fluid to flow. The way they behave depends

on the concentration of the particles in the fluid. It depends on the volume of

the particles, or the volume that the particles occupy in the fluid. This is called volume fraction, or

the volume fraction of particles, in the fluid. We can calculate that easily. We calculate the volume

of each of the particles, and we take the total number of

particles times their individual volume and divide that by the

total volume of the fluid. When there are very few

particles in the fluid, or their volume fraction is very low,

then they have almost no effect at all. When the fluid flows, the

particles just flow with the fluid. However, when the volume fraction

of the particles increases, then they become crowded. It’s more difficult for the

particles to flow over one another. And they collide with each other,

they move on top of each other. They take much more volume,

and the flow is restricted. In this case, the particles really

impact the flow of the fluid and increase the viscosity of the fluid. However, for that to happen,

the volume fraction of particles has to be quite high. It has to be sufficiently high that

they can’t easily flow over one another. This doesn’t happen unless the

volume fraction of the particles gets to be of the order of 50%,

or half the volume of the fluid is occupied by the particles. When the volume fraction is

so high, then the particles can’t easily flow over one another. Now we can understand in a very

simple way why starch is so effective at increasing the viscosity. First, the starch granules expand. They absorb water, and they get larger. As we show in this little

cartoon, the volume gets larger. And so, the volume fraction,

the effective volume fraction of the starch granules, increases. And while you may

start out with only 15% volume fraction, when

they’ve expanded, then they can occupy a much

larger volume fraction. The second way that starch can

increase the viscosity of the fluid is that the granules stick together. In that case, they form these more

tenuous structures, as we show here. The volume of these

tenuous structures is the volume of the structures

themselves plus all the water that they enclose, as shown by this circle. And that’s how you can greatly

increase the viscosity of a fluid, even if you don’t have,

essentially, 50% of the volume. You have 50% of the volume

fraction of particles, because the particles occupy a

much larger effective volume. The last way to increase the viscosity,

is to add these modernist thickeners. What they are, are large

molecules or polymers. And in that case, only a very,

very small amount of material can lead to a large

increase in the viscosity. The way these work is actually

very similar to way the particles work, except that polymers

have a unique feature that they can occupy a huge

volume with very little mass. To understand that, let’s understand

the way a polymer actually behaves. A polymer is a very long,

but very flexible, molecule, and is made up of many, many monomers. And for a polymer,

because it’s so flexible, the bond between each of the

monomers can bend in any direction. It can bend either way. It can bend one way or the other way. And it does that, with

equal probability. This has very much the appearance

of a random [? walk. ?] Here’s an image of a

random walk of an ion. It can go in any direction. It goes randomly in all

different directions. This random walk is exactly what

describes the behavior of a polymer. And, in fact, we can lay the polymer

shape directly onto this random walk. So the same way we describe the random

walk that describes diffusion of ions through a fluid, that’s

the way we describe the shape of a large polymer molecule. What this means is that the

polymer molecule actually takes up a huge amount of volume,

for a very small amount of mass. Yet because the polymer spreads and

fills the water with this polymer, it can increase its viscosity

by a very large amount. Because the polymers occupy so

little actual volume of the fluid, they can entangle. They can come together and

intertwine among each other. That means the effective

volume of the polymers is actually much greater than one. And the viscosity is even larger. Remember that the polymers have

to get out of the way of one another, in order for the fluid to flow. Now they have to disentangle for

them to get out of each other’s way. That’s called reptation, and

that’s a very, very slow process. You can think of it like

a bowl of spaghetti. If you want to take one spaghetti out,

you have to pull along its length. And that takes a long time. And that’s why these thickeners,

these polymer thickeners, are so effective at increasing

the viscosity of a fluid.

Wow, this is really excellent. Thank you

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