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Perhaps you’ve found yourself on the oil aisle in the grocery store, wondering what the difference between those different kinds of olive oils were, and why one was so much more expensive than the other.

I, too, have wondered. And it’s a pretty simple answer.

Madrid, Spain is home to the International Olive Council, which determines the different grades of olive oil. There are five kinds of oil that have been deemed suitable for human consumption. (Makes me wonder what they do with the unsuitable oils. Euch.)

The different grades of olive oil explained.

Extra-Virgin Olive Oil

Extra-virgin olive oil is highest possible grade oil and is noted for its strong, rich taste. It’s got a lower acidity, no more than 0.8% acididty, a very highly desirable quality in olive oils because it allows the flavors of the olive to come through more cleanly and fully.

Producers obtain extra-virgin olive oil through a mechanical extraction, as opposed to a chemical one. What this means is the olives arrive at the facility as whole olives, not in any kind of altered form, and they must be the ripest, freshest olives: no green, under-ripe olives that might produce a bitter oil (low acidity, remember?) or overripe or old olives that might produce a rancid oil. They’re ground into a paste, a process that takes between 20 to 40 minutes, then spread onto fibrous disks and stacked atop each other in the press. The pressure from the press begins to push the oil out, but there’s still quite a bit of vegetal liquid. Back in the day, people would just wait for the oil to rise to the surface (oil is lighter than water) but now centrifuges are used to separate the two.

The leftover paste is called pomace. We’ll get back to it in a second.

And voila, extra virgin olive oil.

Virgin Olive Oil

Virgin Olive Oil has a slightly higher acidity at 1.5%, but is produced in the same method. The main difference is the kinds of olives used: maybe some green ones got thrown in the mix by mistake, but it’s still a good-tasting oil.

Olive Pomace Oil

I’m skipping down the list of grades (which are usually structured around acidity content) because pomace oil is used as a mixing agent with the middle two tiers of oil quality. When the olives have been all ground up into pomace, they still retain about 5-10% of their oil quantity, but it won’t come out through more pressing, so producers turn to chemical methods to extract that remaining oil. They use a combination of heat and chemical solvents to squeeze that extra oil out, and this takes place in a processing plant, not an oil mill.

Because heat has a negative effect on fats, pomace oil doesn’t have the same kind of texture or flavor that you’d find in a cold-pressed oil. Even so, pomace oil is chemically very similar to higher-quality oils, and so retains the health benefits of virgin oils.

But this oil also has a higher smoke point than virgin oils, meaning it starts to denature, or break down, at a higher temperature than its more “pure” counterparts. This makes it a better cooking oil, and plenty of restaurants and chefs employ it in their kitchens.

Pure Olive Oil

I have no idea why it’s called “pure” olive oil, since it’s a mixture of pomace oil and virgin oils. The acidity usually hovers between 1.5% and 2%.

Olive Oil

This olive oil is more of the same–a mixture of pomace and virgin oils with an acidity at 2%.

So now you can confidently stride down the oil aisle, armed with the knowledge of where those extra three bucks are going.

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