Why some cooking oils get toxic when heated or left in the sun
Analysis of the Generation of Harmful Aldehydes in Edible Oils During Sunlight Exposure and Deep-Frying Using High-Field Proton Nuclear Magnetic Resonance Spectroscopy
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
Sunlight exposure created the exact same toxic aldehydes as deep-frying — just over a longer time.
People assume heat is the only culprit, but UV light alone — even at room temperature — triggers identical chemical pathways, meaning storage matters as much as cooking.
Practical Takeaways
Store all oils — especially sunflower, sesame, and rapeseed — in dark glass bottles in a cool, dark cabinet, not on the counter.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
Sunlight exposure created the exact same toxic aldehydes as deep-frying — just over a longer time.
People assume heat is the only culprit, but UV light alone — even at room temperature — triggers identical chemical pathways, meaning storage matters as much as cooking.
Practical Takeaways
Store all oils — especially sunflower, sesame, and rapeseed — in dark glass bottles in a cool, dark cabinet, not on the counter.
Publication
Journal
Foods
Year
2025
Authors
Anna Meike Freis, S. P. B. Vemulapalli
Related Content
Claims (7)
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).
Whether you leave oil in the sun or fry food in it, the same bad chemicals form — just at different speeds.
Leaving cooking oils in the sun for a long time makes them break down and form harmful chemicals, especially if they’re made from seeds with lots of unsaturated fats.
Frying food in oil at high heat for an hour creates toxic chemicals in the oil, especially in sesame oil, even after just 10 minutes.
Extra virgin olive oil isn’t used for deep frying because it breaks down too easily at high heat — even experts avoid it for this reason.