J. Kenji López-Alt Breaks Down the Science of Stir-Fry

Really, this should be called a toss fry

more than a stir fry.

Stir fry's a misnomer for it,

[Announcer] In a wok, fire, oil and metal

combine in an explosive chemical reaction

to create stir fry.

When you order it at the restaurant,

even at the cheapest hole in the wall place,

the meat comes out super tender.

Everything has a really nice savory, intense flavor to it.

[Announcer] But is that taste due to extreme temperatures?

Perhaps some secret alloy wok?

We ask chef and food science writer, Kenji Lopez-Alt,

why does restaurant-style stir fry just hit different?

Some of it involves the way we're going to treat the meat

before we cook it.

And then some of it involves

the way we transfer heat to the food

during the actual cooking process.

[Announcer] Let's cook alongside the guy

who literally wrote the book on woks,

and explore the science behind perfect stir fry.

[Kenji] The important part of a stir fry,

have everything ready before you start cooking.

[Announcer] Our experiment needs protein, alkalines,

vegetable, aromatics, sauce, thickener, oil,

and of course a very hot wok.

I'm using a carbon steel wok.

Carbon steel is what I recommend people use at home.

Carbon steel gets this layer of what's called black oxide

when you heat it up.

So, that's what makes the steel all black.

That black oxide,

not only aids in giving it non-stick properties,

but more importantly,

it actually transfers flavor to your food.

Som if you were to cook a stir fry

in a carbon steel seasoned wok like this

versus a stainless steel wok,

and feed them to people side by side,

the difference in flavor is really immediate and obvious.

So, I'm gonna start by preheating my wok

on the highest heat.

The goal here, is to get the bottom of the wok

really, really ripping hot.

There's this expression,

that a hot wok and cold oil prevents sicking.

I tested this a number of times.

The temperature of the oil actually doesn't matter that much

but what you do wanna do is add the oil

right before adding your food.

The idea is that when you get your wok hot enough

to the point that you're gonna be properly stir frying,

that is going to be well beyond the smoke point of an oil.

Even on neutral, so this is peanut oil,

it has a very high smoke point.

It's gonna be getting up to around 600 degrees or so,

maybe even hotter.

That's well beyond the smoke point of peanut oil.

So, if you leave the oil in there

and let it sit for too long before you start cooking,

it's gonna start developing

these really acrid burnt flavors.

So, preheat your wok really well, add your oil,

and as soon as you add your oil,

swirl it around and get your food in there

so that the oil doesn't get a chance to burn.

[Announcer] Having the right wok technique is key.

Let's get a closeup look at the mechanics of those moves.

So, you might notice that this is called a stir fry

but I'm actually doing relatively little stirring.

In fact, some chefs don't even like to use a spatula,

they'll just toss like this.

So, really this should be called a toss fry

more than a stir fry.

So, there's two motions in the wok,

there's tilting and then there's translational motion,

forward and back.

And so, what you're doing is,

the first part you're tilting the wok forward

and its back towards you,

then while it's still tilted forward

you're pushing it translationally away from you,

and then you're tilting it towards you

and then pulling it back towards you.

And that's the basic motion.

And that's how you get the food to blow over itself

in a kind of wave.

[Announcer] The rotational motion

and rapid translational acceleration,

will cause the food to leap in the air

until mostly cooked through,

while becoming lightly browned in spots.

[Kenji] So the hottest spot of the wok

is gonna be this area down here.

And you'll see, as I'm tossing the food,

that's actually where it spends the most amount of time.

That heats up on the bottom here

through conduction with the metal.

And then as you're tossing it through the air,

you're really encouraging the evaporation

of moisture from it.

That's what the whole tossing motion is for.

[Announcer] Chinese kitchen burners

are 20 times hotter than home burners.

So, what's the at home workaround

to keep the meat tender and the veggies crisp?

So, I'm cooking this stuff

in approximate half pound batches.

That is reasonable for this relatively high output stove.

You know, I lived in a New York city apartment for a while,

that had a really, really crappy stove.

And when I cooked there,

I found that if I tried to do a half pound at a time,

things would end up stewing and simmering

instead of properly stir frying.

So, if you're finding that your food

is coming out wet or bubbly,

cook in even smaller batches.

All right, that broccoli is done.

[Announcer] One of the most crucial steps

is adding alkalinity to the beef.

So, let's take a step back

and see the chemistry behind the prep.

The vast majority of the work

you're gonna do in a stir fry,

comes before you actually put the pan on the heat.

The most important parts are getting all of your food

into nice uniform pieces.

So, with meat, you wanna cut it into pieces

that go against the grain, that are relatively thin.

This will help them cook really fast.

So now we're gonna move into washing the meat,

which is the part

where a lot of home cooks don't do this,

but it is essential.

So what we're gonna do is put the meat into a bowl

and just cover it with cold water like this.

And what you wanna do is really get in there and massage it.

Imagine that you're working a tight muscle

that you really wanna relax,

because that's basically exactly what you're doing.

You wanna squeeze out as much of the red pigment.

the myoglobin that's in them, as possible,

because what that's gonna do is allow room

for marinades and sauces to penetrate,

so your meat's gonna come out more flavorful.

And you might think to yourself,

Oh, I'm just sticking my hand into a bowl of blood.

This is not actually blood in here.

This is the color of myoglobin.

Myoglobin is the pigment inside muscles

and used for similar things in the body,

oxygens for transport and management,

but this is not actually blood.

Now we're gonna put it in a strainer

and we're gonna give it a really good squeeze.

Treat it like you're wringing out wet clothes.

So the next step in the process is marinating.

So, I'm adding about a half teaspoon of baking soda

per pound of beef.

Baking soda, it's alkaline, so it has a relatively high pH.

And what happens is, it prevents muscle proteins

from tightening up overly as they cook,

which means that when your beef cooks

it doesn't expel as much moisture

so it stays juicier.

Don't be afraid of breaking it up.

There's really no way to do this too vigorously.

All right, so now that we have the baking soda in there,

I'm gonna add the other marinade ingredients.

So, that's gonna be a half teaspoon of kosher salts

and a half teaspoon of light soy sauce.

Now, salt is also important,

because what it does is it breaks down some of the proteins,

so that they, again, they don't squeeze this tightly.

Soy sauce also contains enzymes called proteases,

which actively break down proteins

and make them more tender as well.

So that, we're gonna do some Shaoxing wine.

This is mainly for flavor.

Sesame oil and some sugar as well, again for flavor.

And then finally, a little bit of corn starch.

As we mix it together and as we marinate this meat,

the corn starch is gonna form a slurry that coats the meat

in a very, very loose layer.

And what that does is,

that any moisture that does get expelled from the meat

as it's stir frying,

is gonna basically get caught up in that corn starch slurry,

which is going to bind it.

So, it's gonna help the meat retain that moisture

right next to it,

so it's juicy as you bite into it.

All right, so we're just gonna set this aside.

The baking soda, the salt and the soy sauce,

they're gonna continue to act

and they're gonna make the meat more and more tender.

So, the longer you set it aside

the more tender the meat's gonna come out.

But you can get away with about 15 minutes or so.

Really the most important thing with the meat,

is the washing.

That's gonna get you 90% of the way to the tenderness.

Now, the broccoli, it's important that you spread it out

into a single layer like this,

so that all the moisture has a chance to evaporate.

When you're stir frying,

your main goal is to evaporate moisture as fast as possible.

The reason for this is that you want your vegetables

and your meat and everything,

to stay bright and crunchy,

and moisture is really the enemy of heat.

It's a huge energy sink.

So, the less moisture you have on your food to begin with,

the better it's gonna cook during the stir fry.

Evaporating moisture is the biggest energy consumption

when you're cooking food.

Whether you're searing a steak,

whether you're searing chicken breasts,

whether you're stir frying.

So, the drier you get your food to start with,

the better it's going to sear

or the better it's going to stir fry.

[Announcer] Let's rewind to the prep and see a key step

that will lead to crunchy, yet cooked, broccoli.

I've got this pot of blanching water going.

So, this is basically just lightly salted water

that I've got at a full boil.

I like to do this whenever I'm working

with green vegetables,

because it helps them.

It helps make sure that they cook evenly, first of all,

but also gives them a brighter green color.

So we get our broccoli in there.

We're just gonna let it simmer for less than a minute,

basically until you see it turn really bright green,

and we're gonna take it right out.

Now, we're gonna stir fry it.

So again, a little bit of oil.

As soon as that oil is in there, we swirl it around

and we're gonna get half of our broccoli in.

[pan sizzles]

All right, let's just get the broccoli outta there.

The final element in wok hei,

has to do with the way you add the sauce to it.

Chinese American food typically has a lot more sauce

than stir fries you would find in China.

So, the flavor profile in here is mainly going to

be umami, salty, and sweet.

So the umami, that's our sensation of savoriness.

It's something that we taste in the back of our mouths.

It makes us salivate.

The umami in this comes from glutamic acid,

which comes when you've long ferment sauces.

That's the main part of MSG,

what makes MSG tastes so savory.

And the other thing that's going to be in here

is inosinic acid,

which is something that we get out of fish and shellfish.

So, the oyster sauce in here is gonna be rich

in inosinic acid.

That acts basically as a glutamic acid amplifier.

So when you have 'em together

they have this synergistic effect,

where one of them helps the other

and they make each other stronger

than either one of them would be separately.

[Announcer] So what do you think Kenji?

Can homemade stir fry compete with takeout?

It smells really good. It looks really good, I think.

I don't know if it's coming across on camera,

but it looks great in person.

So the secrets here?

The really important part was the washing the meat

and marinating it with alkaline.

And then finally,

making sure that everything was in uniform sizes,

cooking in small batches so that we don't overload our wok,

and saucing at the very end.

So let's see how it is.

Yep. I mean, silky smooth.

Incredibly tender meat. Bright green broccoli.

A nice savory sweet umami-balanced flavor.

That's good. I'd eat it.