- This egg is cooked.

This egg is very salty.

And these two eggs are disgusting.

Not eating either of those.

But all of these eggs are making me wonder, what does it mean chemically for something to be cooked?

Like ceviche.

What is ceviche, besides being delicious?

At its most basic, it is thin slices of fish tossed in citrus, which turns translucent, floppy fish like this into something more firm and opaque like this.

The fish is cooked in acid.

I dunno if cooked is the right word.

Let's figure that out.

(ambient music) Oh, I should make a molecular model of an amino acid.

I'll do that in a later take.

Let's get into the biochemistry of it.

Meat, including fish, is made up of one or more muscles and muscle is made up of long muscle fibers, and muscle fibers are made up of proteins, and proteins are made up of amino acids.

Each amino acid that makes up a protein has the same backbone structure: an amino group, a central carbon, and a carboxyl group.

What makes the amino acids different is the R group that extends from that central carbon.

Some R groups are small.

Glycine's R group is just a hydrogen, which makes it the smallest and most boring amino acid.

On the other end of the spectrum, you have tryptophan which is this massive unwieldy R group.

Tryptophan is an incredibly misunderstood amino acid that has gotten tied up in all kinds of Thanksgiving turkey conspiracy theories, which to me is just personally super annoying.

But it does make it easy to remember how to draw the structure of it by just drawing a Turkey.

I'll show you.

(cheerful music) Okay, there's the beak, just like turn your brain this way.

And then we have the beak and the feet and the tail and a little wing.

It's a turkey, right?

You can see it.

I'm not crazy.

Now there are more effective ways to group amino acids than by the very real and not at all weird personalities that my brain has assigned to them based on things like their polarity and their charge and their other properties.

But it has bothered me for years that depending on which textbook you're looking at, they will group them in different ways.

Some group them by polar versus non-polar.

Some are acidic versus basic.

Some are aromatic or hydroxyl.

But what I think you should know for today's discussion is not the specific categories, but rather that the different side chains can have different charges, and that they react with water differently.

This affects how they interact with each other and also the cellular environment, which influences the final structure of the protein.

Amino acids are all strung along in one big row like beads on a string, amino group to carboxyl group.

So each amino acid is added through a condensation reaction where the carbon over here loses an OH group and the N over here loses an H group.

You get a water molecule and you get something like this plus H2O from these.

And now this C is bound to this N here.

This is the protein's primary structure, the order of all its amino acids.

The properties of those amino acids will affect how they'll interact with one another.

Some might be attracted to each other, some might be repelled.

And those interactions start to form a secondary structure most commonly made up of what's called alpha helices and beta sheets.

These are formed out of interactions between the carbonyl O of one amino acid and the amine H of another.

But then those can fold up on top of each other even more, giving the protein a three dimensional tertiary structure.

Hydrophobic amino acids typically turn inwards and hydrophilic migrate towards the outside of the structure.

Van Der Waal's forces help to stabilize the protein on the inside, while ionic and hydrogen bonds stabilize the outside, and links between sulfur atoms on amino acids called cystines form disulfide bonds.

And I've always kinda loved that those get their own special category.

The point is proteins are basically long strings of amino acids that fold up in complicated ways and that resulting structure lets them do a specific job.

Now, once it's got its tertiary structure the protein can start doing stuff, moving things around in a cell, chomping up other molecules, building new cells.

They can also make quaternary structures where more than one tertiary structure comes together to form an even bigger structure.

Anyways, these proteins typically have to keep their shapes to do their jobs.

Some really cool ones can change shape but they still usually stay in one of only a few confirmations.

But those shapes are held together by bonds between the amino acids holding them into these big twisted blobs.

But when you cook food, you start breaking down all of those bonds.

The energy from the heat vibrates the molecules, breaking them apart.

The structures break down and they can't do their jobs anymore.

And as the inside parts of the protein start to get exposed to the outside, they start to coagulate together.

This is really easy to see when you fry an egg.

The white of the egg is made up of lots of proteins.

Ovalbumin, ovotransferrin, ovomucoid, ovomucin.

You might be noticing a trend here.

When you apply heat to the egg, some of the proteins start to denature and coagulate together, turning the slimy clear egg white into a thick, stiff protein that's actually white.

Same thing happens if I put this thin translucent piece of fish onto the pan.

Proteins denature, they stick together, and then they give me a firm little fish slab.

Fish slab.

Now we don't need to denature proteins in order to eat them, but for some foods it can make the texture more appealing.

And the process of cooking adds some other unrelated flavor benefits.

On top of that, it has a big benefit in making the food safer, because the process also denatures proteins and other components of bacteria on the food, killing that bacteria.

That is key to why cooking food makes it safer to eat.

Also, does anyone get my Dakota Johnson joke?

I really like limes, I like them so much, I like to display them in my house like this.

Look at all my limes.

So what's happening in ceviche?

We're not adding heat, but somehow we're getting a really similar result.

(upbeat music) Citric acid from the lime juice seeps into the cells and starts to interact with the proteins inside.

It dissociates into anions and cations which disrupt the ionic bonds or salt bridges on the outer shell of the protein.

The hydrogen ions bind to negatively charged amino acids while the citrate ions bind to the positively charged amino acids, severing the bonds between them.

The proteins start to denature, they coagulate, and you get something with a really similar end result to cooking with heat.

Recipe times vary, though.

Some say you should marinate for three minutes, others three hours, and they give different results based on how long you let it sit.

You need time for the acid to diffuse into the meat and also time for the bonds to break and then form again in new ways.

So these pieces of fish I marinated for a couple different times, and you can see that they have different textures.

This one was marinated for a few hours and it's really pretty stiff, this one for just a few minutes, and it's still really floppy, and still kind of translucent.

Like heat, the acid can denature proteins in many bacteria as well, killing them and making the fish safe to eat.

But it's not quite as bulletproof as heat is.

One study that looked at multiple listeria species found no significant decrease in viability of the bacterial cells in the face of a preparation that included multiple potential antimicrobials including lime juice, red onions, and even some jalapeno.

It's starting to look good.

Another study by the USDA showed only about a 2% to 10% reduction in viable bacterial cells across a number of species when preparing ceviche.

That's too low.

And bacteria aren't the only potentially sickness-inducing organisms in fish.

Fish can also contain parasites including small worms, and the lime juice is unlikely to kill those larger organisms.

So for those of you following along at home, make sure you are buying sushi grade fish, which has typically been flash frozen at a very specific temperature and time ratio to kill off those worms.

You cannot do this in your freezer at home, it is not safe.

So really the secret to great ceviche is good fish.

To be safest, don't make it with anything you wouldn't feel comfortable eating raw.

But heat and acid aren't the only way to denature proteins.

Let's go back to the eggs.

So we can cook eggs with heat.

We can cook eggs by letting them sit in, this is lemon juice or lime juice.

This one is alcohol.

It's so gross.

Similar to acid, it can soak into cells and the hydroxyl groups from the alcohol can break bonds between amino acids and then form new bonds themselves, denaturing the proteins.

They can additionally disrupt hydrophobic interactions inside the protein.

And high proof alcohol can also kill bacteria.

But there's a trade-off.

If you denature and coagulate the membrane of the bacteria too quickly with really high proof alcohol, it can actually prevent the alcohol from going deeper into the bacteria and getting at all the proteins inside.

This is why if you get like spray hand sanitizer 70% alcohol is actually a better disinfectant than 99% alcohol.

Also, this looks disgusting.

I thought maybe I'd try it, but I hate eggs and I don't like vodka, so I don't know that I could do this.

Finally, this one's a little different.

This is an egg yolk that I let sit in salt for a couple of weeks.

Salt curing meats, eggs, and fish has long been an effective preservation method.

Salt mostly works by drawing liquid out of the egg or fish or meat and then dehydrating it.

This potentially kills the bacteria in the process, as they get all of their life-giving water sucked outta them.

But some studies have shown that it's not perfect.

In one test, salt curing alone wasn't enough to decrease salmonella populations on infected eggs.

Heat was also needed.

Oh, it's like very firm.

Imagine like a gum drop.

I made like an egg salt gum drop.

Would I eat it?

Yeah, if I liked eggs.

I don't, it's the one food I don't like, and I thought I could do it, but I'm just looking at all of these and I'm just not, ugh, no.

Don't put that in, I don't wanna yuck anybody else's yum.

But like personally, I can't do this.

Ugh.

So are these methods safe ways to cook food?

Look, I love ceviche.

I don't wanna be a ceviche hater.

A 2008 statement from the National Advisory Committee for Microbial Contamination for Foods said that although the use of acids for the production of ceviche may reduce microbial numbers, it cannot be relied upon to result in a safe product.

But as someone with a pretty tough stomach and a good immune system, I'm still gonna keep eating it and sushi and the occasional oyster, even though I know they all come with risks.

And as for the semantic question, is ceviche cooked?

Did you really think I was gonna answer that?

This is a chemistry channel.

That is a semantic question.

You're already gonna be angry enough with us that we even brought it up.

Now I get to try it.

(upbeat music) Yeah, that's delicious.

I made it with hamachi and it tastes so good.

Wait.

Before you go, I wanna tell you about a new PBS Digital Studios show that I've been working on with an incredible team called "Hungry Planet."

"Hungry Planet" looks for the intersection of food, science, culture, and community.

So if you liked this episode of "Reactions" hopefully you'll like that too.

I'm so grateful that I've been able to work on this idea that I've had for a while with just an incredible, incredible team.

And you can find the show out now on "PBS Terra."

It would mean so much to me if you went and checked out episode one, and episodes two and three are coming soon.

So check it out, subscribe, and I'll see you over there.

(upbeat music) Floppy fish like this into...

I just got fish juice in my hair.

(water running)