Olives grow on trees, right?

So how come you've never seen a fresh, tree-ripened olive in the produce section at the grocery store?

Why are they always swimming in salty brine?

Oh, and did you know that black olives are really green?

Everything you know about these salty, oily stone fruits just might be a lie.

Legend has it that the Greek goddess Athena won naming rights to the city of Athens thanks to her gift of olives.

But while the Olea europaea tree is valued for its wood and oil, the fresh fruit tastes absolutely disgusting.

The main culprit is a bitter compound called oleuropein.

And fresh olives contain a ton of it: up to 14% of its dry weight.

And it tastes absolutely terrible.

Professional olive processors and intrepid home cooks have three main options to get the oleuropein out and make olives taste not terrible.

Soaking them in water or fermenting them in salt brine can slowly leach away the bitter compound.

But that takes weeks.

Option 3 is a chemical shortcut.

It involves soaking the olives in sodium hydroxide.

Good old NaOH is commonly known as lye, and is used in soap making.

Instead of soaking out the bitter oleuropein over time, sodium hydroxide speeds up its chemical breakdown to less bitter compounds, like hydroxytyrosol,.

The lye takes a week or so to do its job, depending on the process.

Caustic lye doesn't end up in your food, since the olives are thoroughly rinsed.

Then, they're packed in a salt brine that helps preserve them before they're shipped.

But the lye does have one other effect on your olives, at least the familiar canned ones -- the kind you put on pizza, the ones you might have put on your fingers as a kid.

Those don't start out black.

Fresh olives do cover a color spectrum -- but the ones you buy probably didn't come off the tree looking like leather boots.

Back in the late 1800s, olives were mostly grown for oil.

But a California farmer named Freda Ehmann couldn't use her crop for oil and needed a way to sell the bitter fruit instead.

So she started experimenting with a process that not only preserved them and made them taste better, it also turned her green olives black.

She did it by mixing air into the olives during the lye treatment, which exposes them to oxygen.

When oxygen is present, hydroxytyrosol and other compounds in the olives undergo further chemical reactions that create dark brown or black pigments.

Those chemicals aren't totally stable, and can degrade over time, giving the olives an unappetizing mottled look -- like this cat, but less adorable.

WHO'S A GOOD GIRL!

So the processors also add a fixative in the form of ferrous gluconate.

The iron forms a complex and stops the olives from turning catty, so they come out of the can a shiny uniform black.

This processing is known as the Ripe Olive style, or California-Style.

Not all lye-treated olives turn black.

If no oxygen is present, those color-changing reactions don't happen and the olives stay green.

What does this mean to you?

Not a lot.

Oxygen is reactive stuff.

Not only can it turn green olives black, but it can also change a lot of the flavor compounds that we love in the more gently cured olives.

But the black ones are still pretty tasty on pizza.

If you're feeling salty about the lies you've been told about olives, don't be.

Just thank chemistry you're able to enjoy them.