MICHAEL BRENNER: This week, we're going to study the physics of baking.
Baking has arisen already on several occasions in this course.
But so far, we haven't been able to actually go
into the physics of baking, that is, how do recipes for baked goods
work and produce the delicious outcomes that we all so much enjoy?
And the reason for that, the reason that we haven't discussed baking as of yet
is that, as you will see this week, the physics of baking
involves many, in fact, if not most of the ideas
that we've discussed up until this point.
Baking involves elasticity, viscosity, emotions, heat transfer, solubility,
many concepts like that.
And this week, we're going to go through what
is going to be, on one hand, a bit of a review of the concepts
that we've discussed up until this point.
And on the other hand, we're going to show you
how these ideas apply to baking.
So before we begin, I just want to give one caveat.
We are not professional bakers.
We are scientists.
And our intent in this week's baking discussion
is not to basically go through every method and every technique that
is required to basically make a baked good delicious.
What we're going to do is to expose the basic scientific issues that
underlie baking.
And by putting them together in your own mind,
they will hopefully help you make sense of recipes.
And to help us with this, we're very fortunate to be
able to visit the kitchen of Joanne Chang of Flour Bakery.
JOANNE CHANG: Hi, I'm Joanne Chang of Flour Bakery.
I'm going to be showing you today how to make yellow birthday cake
and talking to you about all of the science behind making birthday cake.
So first, let me show you all the ingredients
we have for yellow birthday cake.
We have unsalted butter, granulated sugar, we have eggs, and egg yolks,
and there's a little bit of an element here as well.
We also have some buttermilk, and then our dry ingredients,
which are flour, and then in this container, our combined baking soda,
baking powder, and kosher salt.
OK, the first thing I do is I'm going to put the butter into a mixer.
You could do this by hand, but I promise you after about 10 minutes your arms
will be really tired.
So if you can, do it in a mixer.
Now, one of the things that's important about the butter
is that we want it to be at room temperature.
Now, why is it important that the butter is at room temperature?
Because what we're going to be doing is we're
going to be adding millions and millions of minuscule microscopic air pockets
into the butter, and we're going to do that with our sugar.
If you imagine sugar, it's an actual crystal, right?
Sugar is a crystal.
And the crystalline structure of the sugar, what it does
is it acts as tiny little shovels into the butter,
and it digs all of these little holes, and makes
a little minuscule pockets of air.
If the butter's too cold, then the sugar can't do its work.
The shovels just are strong enough.
So you want to make sure you start off with room temperature butter.
I'm going to slowly start adding the sugar.
And as I'm doing this, the butter is slowly
starting to get more and more worked up.
And I'm going to show this to you once all of the butter gets put in.
Now, why is it so important that I have all of these air pockets?
So if you imagine what you want for the ideal birthday cake,
you want something that's nice and light, and fluffy, and tender,
and velvety.
And the last thing in the world you want is a really tough, hard-to-eat, chewy
cake.
So what we try to do in the making of the cake
is create as many opportunities as possible for science to do its magic,
and take all of these ingredients, and turn them into something
light, and fluffy, and delicious.
So when I talk about this sugar creating air pockets into the butter,
later on when I actually added all of these ingredients to the cake batter,
and I put that batter into the oven, all of those pockets of air
will expand in the oven once they get into the heat of the oven.
And that is what is going to help cause some of the rise of the cake.
And the rise of the cake is what's going to make the cake really light
and fluffy instead of heavy and dense.