Frying oil as dark as coffee: what happens when you reuse it again and again?

Every batch ages the oil a little

In the frying pan, oil faces three "enemies" at once:

1. Heat: deep-frying usually happens at about 170 to 190°C.
2. Oxygen: the surface of the oil is constantly touching air.
3. Water from food: drop in a piece of fish or potato and it sizzles as water steams up through the oil.

Together, these cause three kinds of change:

• Oxidation: oxygen attacks the fat, creating compounds that smell rancid or burnt.
• Hydrolysis: water splits fat into free fatty acids. The oil smokes more easily.
• Polymerization: broken pieces stick together into big molecules. The oil gradually turns thick and syrupy.

On top of that, crumbs of batter and meat sink to the bottom, burn black and make the oil break down even faster.

Six signs it's time to toss the oil

You don't need a lab. Your eyes, nose and hands are enough:

1. Much darker color, like strong tea or coffee.
2. Thick and syrupy, pours slowly, clings to the bottle.
3. Lots of foam that doesn't settle when you add food.
4. Smokes early, at a heat level that used to be fine.
5. Odd smell: foul, burnt, rancid like candle wax, or fishy from last time.
6. Fried food darkens fast without getting crisp, and soaks up more oil.

Spot two or three of these signs, and it's time to pour the oil out.

So how many times can you reuse it?

We'd love to give you a number. But honestly: there's no fixed number. How fast oil breaks down depends on lots of things:

In restaurants, people measure something called polar compounds, the total of all compounds formed as oil breaks down. Some European countries require frying oil to be changed once this passes about 24 to 27%. At home, just rely on the six signs above.

The health side: what we know and don't know

What's fairly certain:

• The longer oil is used for frying, the more oxidation products it builds up, including aldehydes.
• Old oil soaks into food more, so fried food absorbs more oil.
• Frying at high heat for a long time can create a small amount of extra trans fat.

What's still being studied: how these compounds affect people over the long term, at normal amounts in a diet. Animal experiments show plenty of unfavorable signs, but human data is still limited.

So the sensible advice sits in the middle: reusing oil a few times while it's still in good shape is fine; don't keep a pot of oil until it's black and thick. And swap a few fried meals for other cooking methods.

If you reuse oil, do it right

1. Let the oil cool completely before handling it. Hot oil burns very easily.
2. Strain it through a fine sieve lined with paper towel or cheesecloth to catch every crumb.
3. Store it in a tightly sealed glass jar in a dark, cool place. Light and air age oil quickly.
4. Keep oils by dish. Oil from frying fish stays separate, for fish dishes. Oil from frying potatoes goes to vegetables.
5. Salt food away from the pan, not directly over the oil. Salt is thought to make oil break down faster.
6. Topping up old oil with new oil only slows things down; it doesn't "refresh" the pot.

When throwing oil away: don't pour it down the sink. Cooled oil solidifies and clogs the pipes. Let it cool, pour it into an old plastic bottle, cap it and put it in the trash.

Fry less, stay crispy

A few ways to get crispy, fragrant food without a big pot of oil:

• Air fryer: blows hot air around the food. A thin brush of oil is enough for crunch.
• Oven: set food on a rack so hot air circulates, and flip once halfway.
• Pan-sear with little oil: use a nonstick pan, one spoon of oil for the whole pan.
• Foil-bake or steam: great for fish, keeping all its natural sweetness.

Frequently asked questions