What actually happens when cooking oil is reused and why it matters

When oil is heated again and again, it doesn’t just “get a little darker.” What’s really happening is a slow, invisible chain reaction. Heat, oxygen, moisture, and food particles conspire to rewrite the oil’s chemistry. Triglycerides, the molecules that make up most edible oils, begin to split. The double bonds in unsaturated fats oxidize, and the oil forms polar compounds, peroxides, and aldehydes. What was once a neutral medium for frying starts turning unstable and reactive. These changes aren’t harmless or cosmetic. Repeated heating alters the oil’s structure, taste, and smoke point and what’s more important, how it behaves inside the human body. That faint burnt smell, the darker hue, the slightly sticky texture? Those are clues that the oil is now chemically different from what cam