When you fry food over and over again, some cooking oils break down faster than others—oils with more saturated fats (like coconut or palm oil) hold up better and make fewer harmful gunk particles than oils with lots of unsaturated fats (like sunflower or olive oil).

The more you fry, the hotter it gets, and the more times you reuse the oil, the faster the oil breaks down and goes bad.

Oils like peanut oil don’t break down as quickly when you fry food compared to butter or vegetable oils, making them a bit more stable at high heat.

When you reuse cooking oil over and over again, it starts to break down and create bad-smelling chemicals that can make food taste funny and might be bad for your health.

Oils with more unsaturated fats (like sunflower and sesame) break down faster and make more harmful chemicals when heated or left in the sun than oils with more saturated fats (like peanut or olive).

Oils with polyunsaturated fatty acid content exceeding 10% are susceptible to thermal oxidation during cooking, generating harmful aldehydes and polar compounds even below smoke point.