So how much energy has to be given to food to heat it up?
If you want to heat up an egg by 10 degrees Celsius,
how many joules does it take to do that?
So the answer to this, it turns out, is contained
in a simple mathematical equation.
A little later on, I'm going to try to convince you
that if you know how to use it properly, you have the opportunity
to cook a perfect egg without investing in a rather expensive immersion heater.
That is, by understanding the equations that we give you,
you really will have the opportunity to use them to invent new recipes,
if you so desire.
Now having said that as introduction, let me go into the equation.
The equation says that Q equals mcp delta T. So Q is the amount of heat
that you've dumped into the food.
And that is a quantity that is measured in joules.
m is the mass of the food.
For the purposes of this equation, m should be measured in kilograms.
So delta T is the temperature difference.
That's the final temperature of the food minus the initial temperature
of the food.
And it should be measured in either degrees Celsius or degree Kelvin.
And finally, cp is a number that we call the specific heat of a material.
It is the number that characterizes how much something heats up when
you apply a specific amount of heat.
And it is measured in the units of joules per kilogram per degree Celsius,
or degree Kelvin.
And this is a number that we will give you,
and that actually is different for different foods.
So it turns out that the specific heat for water
is 4.18 joules per gram degree Kelvin.
That's the specific heat for water.
Now water is an important cooking material, and we will talk about it.
So for eggs, another important cooking material,
their specific heat is about 3.18 joules per gram per degree Kelvin.
For milk, it's about 3.85.
It's closer to water.
For beef, depending on the cut of beef, it's somewhere between 2 and 1/2 and 3
and 1/2 joules per gram degree Kelvin.
For air, the specific heat of air is 1.
The specific heat of olive oil, something
which is also useful for cooking, is about 1.97 joules
per gram per degree Kelvin.
So it's actually a lower specific heat by about a factor of 2,
than that of water.
Now actually those two numbers, if you think
about them, the specific heat of water and the specific heat
of olive oil, the fact that most foods are between those two numbers
is not an accident by any means.
In fact, the main ingredient of all food is water.
This is something that we're going to have to think about constantly
throughout this course.
It will come up again and again in one context or another.
And another important ingredient to food of course,
is fat, which olive oil is a fat.
And fats tend to have a specific heat around 2.
And foods like beef, it's somewhere between 2 to 4.
And that's essentially why.
It's a little more complicated than that.
But that's not a bad explanation to get us started.
Now so to get us started, let's actually do something.
So I want to calculate the energy that it takes to boil one cup of water.
So it turns out that one cup of water is 237 grams.
And so what I want to do is start with water that's at room temperature.
So the temperature in this room is about 23 degrees Celsius.
And I'd like to heat it to the boiling point, which is 100 degrees Celsius.
And I'd like to know how much energy do I have to dump in.
And when you multiply this out, it turns out that the number that you get
is 76,280 joules.
So that's how much energy it takes to boil a cup of water, 76,280 joules.
Now it's a little bit hard to know how to think about that,
because it's just a number in joules.
So one way to think about it, how much wood
would it take to make this much energy?
Now if you take the energy density of wood, which I already
told you is 14 megajoules per kilogram, then you
can calculate for yourself that the amount of wood that it takes
is 76,280 joules divided by 14 megajoules per kilogram.
We find that it takes 5.4 grams of wood, in order to create this much energy.