So in order to cook a food, it is necessary to heat
the inside of the food up to some critical temperature.
And we discussed previously that that critical temperature for most foods,
be it steak or tofu or whatever it is that you like to eat,
happens to be in the range of 60 degrees Celsius or thereabouts
because that's where the proteins in the food
unfold and interact with each other to cause the food to become cooked.
We pointed out that there is a paradox, which
is that, if you look at the recipes for cooking foods,
they tend to require temperatures that are much
above that critical temperature so that, when you cook a steak in the oven,
for example, you use a temperature which is-- I don't know-- 180 degrees Celsius
or so even though you only want the inside of the steak to get to around 60
degrees Celsius.
And then we, basically, explained or started to hint at the reason why.
And the reason why is that it turns out that there
are two chemical reactions that need to occur in order for food to be cooked.
One is the texture on the inside has to change,
and the other is that you need there to be browning reactions on the outside.
And we discussed that those occur at around 120 degrees Celsius.
So now we want to try to put all of this information
together and try to think through why it's hard to cook a steak.
So that is the subject of this discussion.
If you don't like steak-- I myself don't eat steak-- then
you can really think about your favorite food-- why it is hard to cook a potato,
why it is hard to cook tofu, why it is hard to cook fish.
And whatever it is that you like to eat, what I'm about to say applies.
So you might, for example, choose to cook your steak at 180 or 190 degrees
Celsius.
So we want the internal temperature, which starts out at room temperature,
because your steak might have been resting on the countertop
before you put it in your oven.
So maybe the initial temperature is, say, 23 degrees Celsius.
You want to raise that up to about 60 degrees Celsius because that's
when the steak is cooked.
And you also want the outside to get to a temperature of 120 degrees Celsius.
Now, the first problem that you have when you think about this--
and the problem is that we want to get the outside to 120 degrees
Celsius-- is that you'll all recall that the boiling point of water
is at 100 degrees Celsius, and the major ingredient of steak is water.
And so that means that, initially, you can't get the steak above 100 degrees
Celsius because it's made mostly of water,
and water-- the maximum temperature it can
get to is 100 degrees Celsius before it changes phases and actually becomes
a gas.
So how are we going to heat the steak up to 120 degrees Celsius?
The way we're going to do that is we're going
to have to evaporate off some amount of the water
around the outside of the steak.
If we do that, and there's no water on the outside,
so you imagine the whole edge of the steak has no water,
and only the inside of a steak actually has the water, then the outside,
once it's got no water, you can actually heat it up
to close to the temperature of the oven or whatever you want.
So you can get it up to 120 or 180 degrees or whatever.
So if you imagine the temperature profile across the steak--
and here we'll draw it-- so at the outside of the steak, right outside,
where the steak is, the temperature is the temperature in the oven, which
is, say, 180 degrees Celsius.
At the surface of the steak, you'd like to get it to 120 degrees
or so, maybe 130, so that you get the browning reactions to be
how you like them to taste.
There's then going to be a very thin layer where the temperature goes down
from the 120, 130 degrees to the 100 degrees, which
is the boiling point of water, which is how hot the outermost water
layer of the steak will be.
And then what you would like is for it to get to be 60 degrees
or whatever your temperature is in the middle.
And so you can imagine a curve, which basically goes down
to 60 degrees in the middle.
Now, the problem is, if you achieve this target with the way that one typically
achieves it when one is cooking is one puts a thermometer in the center
and says, hello, the temperature is at 60 degrees Celsius,
the steak is cooked, and you see that that's true in the center of the steak,
but the whole outside of the steak is necessarily
at a temperature which is higher than 60 degrees
because, otherwise, you won't hit the browning
reactions that you need on the outside to create the flavor molecules that we
want.
And that, actually, in its essence, is one of the reasons
why it's so hard to cook a steak.