So how do we apply heat to food to cook it?
We start with fuel, and we then take the fuel
and either burn it or use it in some fashion in order to create heat.
And that heat then transforms food from being
uncooked and rather unappetizing to being cooked and rather delicious.
So let's start to think about the process of transferring heat
to food from fuel for a minute.
So it turns out that wood or any fuel has what we call an energy density.
This is an energy per unit mass-- usually the way
that it's reported-- of how much energy can be liberated by burning the fuel.
So in the case of wood, the energy density is 14 megajoules per kilogram.
That's 14 million joules per kilogram.
Joules is a unit of energy.
It's an SI unit of energy.
It's measured in kilograms meters squared per second squared.
And 14 megajoules per kilogram is the energy density of wood.
Now it turns out that charcoal has actually a much higher energy
than wood.
The energy density of charcoal is about 30 megajoules per kilogram.
So it's almost a factor of two more, which
for those of you who have cooked on piles of charcoal,
this explains why you need less of it, really,
to heat the same amount of food.
So another way to cook food is not to burn fuel at all
but as to use an oven of some sort.
And there are at least two types of ovens.
There are electrical ovens which have hot coils on them, which
actually heat the air within them.
And there are microwave ovens which send radiation in that interacts
with and heats the food,
If you go look at the specs on an oven, it
turns out it doesn't report the energy in terms of the fuel that is used.
It actually reports the energy in what are called watts.
Now watts as a unit of power-- the units of watts are joules per second.
And the point of that, of course, is that if you
put the food in the device for whatever period of time,
then if you simply multiply the power output of the oven times the time
that you put it in, that's how much energy you
could hope to absorb by the food, if it's being done correctly.
I mean, suppose you were to take food and put it in your oven for 10 minutes.
So since every minute has 60 seconds, that's 600 seconds.
If your oven has 1,000 watts, then that means
that the total energy that you can possibly dump into the food
is 1,000 watts times 600 seconds, which is 600,000 joules, which is actually
less than what you would have in a kilogram of wood.
When you apply heat to food, what are you actually doing to it?
What is heat anyway?
One definition is to say that heat is the total energy that
results in the temperature of the system being whatever it is.
Higher temperature, if you have a thing with higher temperature,
that means it has more heat.
So at a molecular level, temperature is a measure
of the motion of the molecules within a material.
So what's actually happening is that the molecules in a material
are jiggling around at some rate.
I mean, you can just imagine them jiggling.
And when you hear them up, they jiggle around faster.
And that's what temperature is.
And it's this jiggling that causes changes
to occur in the material, which is at the heart of what we do when
we cook and try to make food delicious.
So a simple way to demonstrate this to you is to just show you a simulation.
So this is a simulation of a material.
This is a simulation of water molecules essentially in a box.
And initially the water molecules are at a rather low temperature.
And so they're sitting, barely moving at all.
And now what we're going to do is gradually raise
the temperature in the simulation.
And what you see is that when the temperatures is
at a slightly higher level, the molecules start to jiggle a little bit.
And they start to move and bump into each other a little bit.
If we raise the temperature a little more
the molecules start to move by each other.
And when we raise the temperature a whole lot, the whole thing comes apart.
And you see the molecules whizzing around everywhere.
And that whizzing of the molecules, the jiggling and the whizzing,
is the essence of temperature.
And that is what we're actually doing to a material when we dump in heat.