4.2: Demonstration – Factors affecting diffusion rate

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From the course by The Hong Kong University of Science and Technology

The Science of Gastronomy

195 ratings

The Hong Kong University of Science and Technology

The Science of Gastronomy

195 ratings

This course introduces a number of basic scientific principles underpinning the methodology of cooking, food preparation and the enjoyment of food. All topics covered have a strong basis in biology, chemistry, and physics application. Among others, they include the consumption of cooked food, the physiological and evolutionary implication of the senses, geographic and cultural influences on food, and the rationale behind food preparation. We will also discuss issues such as coupling of senses to improve sense stimulation; altering flavor by chemical means; and modification of the coloration to improve the appearance of dishes. Following the video demonstrations of the scientific principles of cooking, you will learn to recognize the key ingredients and their combinations for preparing good healthy food. At the end of this course, you will be able to: - appreciate the scientific basis of various recipes; - develop your own recipes by integrating some of the scientific principles into new dishes; - recognize the influence of the material world on human perception from the different senses; - appreciate the art of integrating science into cooking and dining. Important Note: This course is not designed for people with special dietary needs such as vegetarian, diabetic, and gluten-free diets. If you feel uncomfortable with any part of the assignments or activities of this course, you can substitute some of the ingredients or ask friends and family members to help with the tasting of your assignments. Alternatively, you may skip that specific assignment provided that you have fulfilled all other qualifying requirement to pass the course. Course Overview video: https://youtu.be/H5vlaR0_X2I

From the lesson

Module 3 and Module 4

This week, we will talk about how flavor and aroma of food affects our perception of taste of food.

4.0: The Sense of Smell - Topic Outline0:27

4.1: What are we smelling?9:57

4.2: Demonstration – Factors affecting diffusion rate4:37

4.3: How do I smell coffee?10:52

4.4: Why do orange and lemon smell different?9:41

4.5: Why can’t I taste the “strawberry”?11:08

4.6: Demonstration – How does aroma affect our taste?4:35

Assignment 5 – Instruction Video2:24

Assignment 5 – Result Sharing1:39

4.7: Smelling good makes it tastes better!14:03

4.8: Demonstration – It is all about congruency!3:25

4.9: The Sense of Smell - Topic Summary1:01

Meet the Instructors

King L. Chow
Professor
Division of Life Science
Lam Lung Yeung
Associate Professor
Department of Chemistry

Remember that in the lecture we talk about diffusion.

Diffusion is how the molecules from one place, they would be able to move over to

another place, based on the concentration difference, and they need to act through

some sort of, like a medium. Now, we want to do a demonstration and

tell you. How the medium, whether the temperature

is high or low or whether its viscosity is high or low is going to affect the

diffusion rate. So we do this by the following setting.

On your left hand side we have a glass of water, which is room temperature.

Relatively cold. The other side you'll see the steam

coming out of it. It is a glass of hot water.

Temperature roughly around like maybe 18, 19 degree.

So we're going to put in some dye, and ask how long does it for the dye to

diffuse throughout the entire glass of water. So let's take a look at it.

This is the hot water. And this is the cold water.

Let's pay attention to it, and see actually how fast they become totally

homogenized, to have the red color fully distributed in the entire glass.

You may notice that on the left hand side where the water is cold, sometimes you

find that the dye they simply they are stagnant, they are not moving at all

based on the very slow Brownian movement, they are simply moving a little bit, but

then they are not really mix very well. But if you look at the right hand side

you may recognize that you can't see the dye at all, basically they are almost

like a mix up in a homogenized solution form.

So what it tells you is that, on the right hand side, because of the high

temperature, in fact the dye molecule, they would be able to move around very

effectively. And so therefore they diffuse very well,

but on the left hand side you find that with cold water at low temperature,

diffusion occur much, much more slowly.

In the second experiment, we're trying to

demonstrate the effect of viscosity. Essentially the experiment is identical,

but let's look at what we do. We have two glasses in front of us,

again. On the right hand side, we are going to

put in some starch solution. They are much more viscous.

Here it goes. You say that they are a little bit

turbid. Okay, done.

And on the left hand side, we are going to put in water.

This is just tap water. So, basically in terms of viscosity, they

are not as viscous. So, we do the same experiment again.

Now, what we do is that we are going to put in one drop of the dye.

Let's look at it, one drop. And let's look at it, one drop.

Now clearly, you can start, see the difference.

On the left hand side, it's just the water with relatively low viscosity, and

you'll find that the dye start to move around a little bit.

As effectively as what you saw a moment ago.

But if you look at your right hand side, look at what happen.

So this is a very viscous starch solution, and you'll find the dye is at the top,

and it's not moving at all, basically, they have a hard time defusing over

distance. Let me remind you, in fact.

Diffusion still occur, but it's much much more slowly if we stay here for long

enough you will find that, the dye would start go into the solution, but then it will

never be at the rate as fast as the one in the water.

So combine both together basically it tells you the same concept again.

High temperature with a very high rate of diffusion.

High viscosity, they block diffusion very effectively.

So in the future, when you're cooking something and you want to have certain

dye being mixed up with the food, think about the viscosity and the temperature.

The same thing when you're thinking about aroma, how something smell.

If you have a medium which is of low viscosity, actually they would be able to

diffuse much faster, and also you can increase the temperature by heating up a

little bit, they always smell better.

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