- Here's a chemistry life hack that you might've seen on the internet.

If your coffee is too bitter, add a pinch of salt to magically cover up the bitterness.

Some of the biggest food and coffee creators on the internet advise this, from James Hoffmann, - That is better.

- To Alton Brown, - [Alton] A mere 1/4 teaspoon of kosher salt will help to take the bitterness out of your brew.

- To this very channel, yes, the very first piece of information in our very first video is to add a pinch of salt to bitter coffee.

Add a tiny little pinch of salt and taste a world of difference.

So let's do it.

(playful music continues) This is a controlled experiment, so I have to mix both of these the same way.

(coffee slurping) (playful music continues) A little bitter.

(coffee slurping) Mm, I don't taste that and I go, "Oh, less bitter," but I do taste it and go, "Oh, that's a bit smoother.

That's a little less rough than this one."

This experiment got me thinking.

Does this only work for coffee or does salt also reduce the bitterness of other stuff?

To find out, let's try this again with tonic water, which contains quinine, or as Americans call it, quinine, a bitter chemical.

(playful music continues) (can hissing) (soda fizzing) All right.

(water slurping) Yeah, kinda bitter, actually.

(water slurping) Wow, that actually is kind of magical.

It tastes sweeter, it tastes less bitter, it tastes less sharp.

So as we were shooting this, Andrew mentioned that he doesn't like tonic water, and that reminded me that I don't like IPAs because IPAs suck.

But they are bitter, so let's see if salt can reduce the bitterness.

(beer fizzing) Just a pinch of salt.

(playful music continues) (beer slurping) Mm hm, yep, that's gross.

(beer slurping) Mm hm, yeah.

That's not bad.

I would even drink this just for enjoyment.

(beer slurping) For everyone wondering, it's 10:45 a.m. on a Tuesday.

Chemically, it's pretty weird that salt can change other tastes.

I mean, you'd never look at sodium chloride, one of the simplest compounds there is, and suspect that it could dramatically modify the taste of chemicals that look like quinine or caffeine or this.

This is the bitterest molecule known to humans, N-benzyl-2-2, 6-dimethylanilino-N, N-diethyl-2-oxoethan- 1-aminium benzoate, also known as denatonium benzoate.

Now, as a responsible chemist, I do have to point out that the complexity of a molecule's structure or name tells you exactly nothing about how that molecule will interact with the human body.

Some of the simplest molecules are some of the deadliest.

For example, cyanide.

But as a person, I'm looking at these structures and thinking, "Really, salt can change this?"

So I decided to dig into it and spoiler alert, it did not go well for me.

(playful music continues) (bag thumping) Denatonium benzoate.

(liquid slurping) Ech, ew.

(spitting) (coughing) To try and figure out how salt covers up bitterness, I started looking through Google Scholar, which is like Google search but for science.

And I found this diagram in a 2003 paper.

Now, this diagram is a result of experiments that are similar to the experiments that I, myself, just did.

You give people mixtures of different flavors and you see what they taste.

And what you find is that not only does salt suppress bitterness, it also enhances sweetness and enhances sourness.

This is super useful if you are life hacking your way through flavor.

For example, salads.

But I wanted to know, what's the actual chemistry here?

How does such a simple ion, sodium, affect so many different tastes?

And to be specific, how exactly does salt reduce bitterness?

This very same paper contained my first clue.

A reference to what are called split tongue experiments, which I of course thought meant this.

(liquid sloshing) But it's not.

It's actually a legitimate scientific technique.

Now, the classic split tongue experiment on salt and bitterness was published in a 1984 paper.

And the key part of the paper is this thing.

Now, the way this thing works is that you stick your tongue in here where the arrow is and then these four springs clamp down, effectively splitting your tongue into two sides, left and right.

Then, you can add salt and quinine, which is bitter, to different sides of the tongue or to the same side of the tongue.

Now, why is this important?

Well, it lets you directly compare two conditions.

In condition one, salt and quinine are mixed and applied to one side of your tongue.

Condition two, salt and quinine are not mixed.

Instead, they are simultaneously and separately applied to each side of your tongue.

Now, what a person tastes in these two conditions helps us figure out whether the reduction in bitterness is happening on your tongue or in your brain.

In other words, is the salt chemically interacting with the bitter molecule or the bitter taste receptor while the two molecules are actually on your tongue?

Or is it a brain effect?

Is your brain taking these two independent stimuli and mixing them?

This is really beautiful experimental design.

All you need is a tongue torture device to tell you whether you're dealing with chemistry or neuroscience.

And it turns out the answer is, a little bit of both.

In condition two, when you add quinine and salt to opposite sides of the tongue, the salt reduces the bitterness by about 20%.

When you add a salt and quinine mixture to the same side of the tongue, the salt reduces the bitterness by about 70%.

What that means is that neuroscience is responsible for 20% of the reduction and chemistry is responsible for the remaining 50%.

So, bitterness reduction by salt seems to be happening both at the receptor level and in your brain.

And I was trying to figure out the chemistry component, I stumbled across, honestly, one of the craziest experimental results I have ever read.

Okay, here's the setup.

Researchers took a couple of students and had them stick out their tongues, like this.

Then they set up a flow system, probably something like this (fingers snapping) to deliver different concentrations of sodium chloride dissolved in distilled water to the tips of their tongues.

First, a 15-second stream of one concentration of sodium chloride, like this.

Then, a much shorter pulse of a different concentration.

Like that.

The students did not know what was being dripped on their tongue.

They were just asked to describe the taste.

And they described every taste in the book.

Saltiness, yes, but also sweetness, bitterness, and sourness.

So depending on how you administer sodium chloride, you can get it to taste sweet, sour, or bitter.

Okay, and here's the craziest part.

After the experiment, the students were asked to guess what chemicals they had just tasted.

And they said sugar, acid, quinine, and salt.

They were shocked to discover that only salt was added to their tongues.

So, not only can salt modify other tastes, but salt on its own can taste bitter, sour, or sweet.

When I read that, I, I mean, I pretty much lost it.

I mean, I just lost it.

(melancholy violin music) I called up Gary Beauchamp, who directed the Monell Chemical Senses Center for years.

Hey, Gary?

Hey, it's George.

To ask him to explain to me.

Is now still a good time?

What is going on with salt.

Okay, great.

Yeah, I wanted to know, what the (beep) is going on with salt?

The explanation here is, there isn't one.

We don't know how salt works.

We have no idea how sodium chloride interacts with the bitterness receptor or bitter compounds to make them less bitter.

And I'm not gonna lie, that was a little disappointing, but not totally surprising, especially since some of the effect is happening in the brain, also known as the part of your body that science knows the least about.

So I asked Gary, okay, so then, how do we taste salty stuff?

Like not salt mixed with anything, just plain old salt.

And he said, "We don't really know that either."

(melancholy violin music) (head thumping) How?

How do we not know that?

Now at this point, I'm not gonna lie, I thought, why isn't salt taste science further along?

We've been doing this for decades.

And it turns out, salt is so hard to figure out because it is everywhere.

So it's really hard to come up with salt-free controls for experiments.

Your bodily fluids are .9% sodium chloride.

Your tongue is constantly bathed in saliva, which has enough salt in it that you should be able to taste it, but you don't.

Other flagship flavor chemicals, why did I write flagship flavor chemicals?

Who wrote that?

I wrote that.

Other flagship flavor chemicals, (bell ringing) like MSG, contain sodium, so deconvoluting them from pure saltiness is hard.

And then, there's the fundamental chemical truth that you can't isolate ions.

I can't just sit here and make a beaker full of only sodium ions without the chloride or only chloride ions without the sodium to see how it would taste.

Bottom line, it is not easy being a salt scientist.

Despite all those challenges, I don't actually know of a single science where no progress has been made after decades of solid work.

So it was back to Google Scholar again to figure out what we do know.

And that led me to this paper.

(paper rustling) In the 1930s, people discovered that salt solutions made the nerves that lead from your tongue to your brain fire (fingers snapping) pretty immediately within 50 milliseconds.

This was interesting because sugar makes those nerves fire differently, with a much longer delay.

The speed of the salt nerve response suggested that a membrane channel might be at work.

Now, a membrane channel is essentially a protein pipe that crosses a cellular membrane with a door that can open or close depending on conditions.

Your brain uses these things all the time.

It's how neurons communicate with each other, hence the speed.

Now, certain chemicals can temporarily hold the door open or shut.

A chemical called amiloride, for example, holds the door of sodium channels shut.

So, in 1984, a research group did some more experiments with salt solutions and nerves, but this time, they added amiloride.

And here's what they found.

The left signal, this signal right here, this is the normal nerve response.

And this one right here, this is the response with amiloride present.

So amiloride blocks some, but not all, of the signal.

Now, this strongly suggests that some sort of sodium channel is partly responsible for how we taste salty things.

And that is more or less where our understanding of salt taste is today.

We're pretty sure there is a sodium channel receptor that responds to sodium ions and makes them taste salty.

We also know, by the way, that it responds to lithium ions.

Those taste salty as well.

The weird thing is, even when we block that channel, nerves still fire in the presence of salt.

So, there must be some other receptor, right?

I asked Gary about this.

So then there must be some other receptor, right Gary?

And he said, "Yeah, most scientists in the taste community believe that there is another receptor involved, but no one has actually identified it yet."

This mystery receptor, which I'm gonna go ahead and name salt receptor two, (air thumping) this mystery receptor might explain some weird observations about saltiness, specifically the observation that this stuff (bag thumping) tastes salty.

This is pure potassium chloride, KCl, and it does have a salty taste, though some people, maybe most people, also report that it has a bitter or metallic taste.

So, I'm gonna taste it.

I've got 1/2 gram of potassium chloride, 1/2 a gram of sodium chloride, some water, and we'll taste away.

(playful music) There we go.

Stir it up.

Sodium chloride.

(water slurping) Mm hm, that is the usual salty taste.

Now, potassium chloride.

(water slurping) Mm, so to me, these are almost indistinguishable.

If you gave me these two things and didn't tell me what they were, I would think both of them were just salt water.

KCl is a mystery because we know that the sodium channel receptor is super specific.

It only responds to sodium and lithium.

So, if there were just one salty receptor, potassium chloride shouldn't taste salty at all, but it does, further reinforcing the idea that there are at least two receptors that detect saltiness, not just the one.

Salt receptor two.

(air thumping) So, chemically, (pipe clinking) how does salt change other flavors?

We don't know.

Chemically, how do we taste salt to begin with?

We also mostly don't know.

And that brings me to an important lesson in science.

Sometimes, when you set out to find something, you end up knowing far, far less than you did when you started.

(beer bottle clinking) And then, sometimes a year later, sometimes 50 years later, science finally answers the simple question that you started with, which results in 27 more questions.

And so, the cycle continues.

And I still don't like IPAs.