Why olive oil goes rancid and how to tell

HostMost of us have a bottle of olive oil sitting right next to the stove. It's the one thing we reach for every time we cook, but it's easy to forget that it's actually a fresh food. Sometimes you pour it out and it just smells a bit off, like something from a dusty garage. How does something that's basically pure fat actually go bad?

GuestIt helps to stop thinking of olive oil as a shelf staple like salt or sugar and start thinking of it like a fruit juice. Because that's really what it is. It's the juice squeezed out of a small green or black fruit. Most fats are pretty stable, but the stuff that makes olive oil so good for you also makes it very fragile. When we say it goes rancid, we really mean the oil is rusting. Just like a piece of metal rusts when it sits out in the rain, the fats in the oil start to fall apart when they hit the air. This happens through a process where oxygen molecules latch onto the fat chains and break them into smaller, smelly pieces. Once those chains break, the oil loses its flavor and all those healthy bits we want.

HostSo it's literally breaking apart at a tiny level. But if it's just sitting in a bottle with the cap on, how's it getting enough air to rust?

GuestWell, every time you open that cap to drizzle some on a salad, new air rushes in. But air isn't the only thing at work here. There are three big enemies: light, heat, and air. Light is a huge one. You know how some olive oils come in those pretty clear glass bottles? That's actually the worst way to store it. The green stuff in the oil that gives it that color actually helps light kill the flavor even faster. It acts like a little magnifying glass for the sun. Heat does the same thing. If you keep your oil right next to the stove where it gets warm every time you make dinner, you're basically speeding up the clock. The heat gives those molecules more energy to bounce around and break those fat chains even faster.

HostI always thought the clear bottles were just to show off the color, but you're saying that's actually hurting it. What about the fridge? If heat is the enemy, shouldn't I just keep it in there with the milk?

GuestYou can, but it's not great for the oil. When olive oil gets cold, it turns into a thick, cloudy sludge. It can even get hard like butter. To use it, you have to let it warm up again, and that constant shift from cold to warm can mess with the flavor. Plus, if you leave it in the fridge, you might get water building up inside the bottle from the air, and water is another thing that makes oil go bad. The best spot is actually a dark, cool cupboard away from the oven. Think of it like a wine cellar for your fat. But unlike wine, olive oil doesn't get better with age. It's at its best the very second it comes out of the press. From that moment on, the clock is ticking. Most oils only stay truly fresh for about two to three months once you crack the seal.

HostOnly two or three months? I have definitely had bottles in my pantry for a year or more. But here is the thing, sometimes I taste a fresh oil and it has this really sharp, peppery burn in the back of my throat. I always thought that meant it was turning bad, but now I'm not sure.

GuestThat peppery kick is actually exactly what you want. Those are the healthy parts of the oil, the bits that protect your heart and fight off swelling in the body. If your oil stings your throat a little, that means it's fresh and full of life. When the oil goes rancid, that pepperiness is the first thing to die. It becomes flat and greasy. To really tell if your oil is bad, you have to use your nose. Put a little bit in a glass, cover it with your hand to warm it up, and then take a deep breath. Fresh oil should smell like grass, or green tomatoes, or maybe even lemons. If it smells like old peanuts, or wax crayons, or a bottle of Elmer’s glue, it's gone.

HostGlue and crayons? That's a very specific smell.

GuestIt's unmistakable once you know what to look for. Think about the smell of a box of old crayons you found in the back of a closet. That heavy, waxy, dull scent is the smell of fats that have died. Another big sign is how it feels in your mouth. Fresh olive oil should feel clean. It might be thick, but it should wash away easily. Rancid oil feels like it's coating your tongue in plastic. It feels heavy and sticky, and that greasy feeling lingers long after you swallow.

HostIs it dangerous to eat, or does it just taste like a school supply closet?

GuestIt probably won't make you sick to your stomach right away like bad meat or old milk would. But it's not good for you. When those fats break down, they create things that can cause stress in your body. All those health benefits people talk about with the Mediterranean diet? You lose all of that when the oil goes bad. You're basically just eating empty, smelly grease at that point. If you see a big jug of oil on a bottom shelf that looks like it has been there since the store opened, it's better to skip it and buy a smaller bottle you know you can finish quickly.

GuestFresh oil should smell like a field of green grass, not a box of old wax sticks.

HostThat bottle by the stove is more than just a kitchen buddy, and keeping it cool and dark is the only way to save that morning toast from the taste of old socks.