Oxidation Unsaturated fatty acids form hydroperoxides that can decompose into aldehydes, ketones, acids, and other secondary products.

Hydrolysis Water released from food can split triacylglycerols, increasing free fatty acids and partial glycerides.

Polymerization Sustained frying conditions can form larger molecules, increasing viscosity and total polar material.

Is reheated oil “toxic”? The chemistry of degradation is well established. What is less certain is how a specific household reuse pattern translates into a precise long-term disease risk in humans. Much toxicology comes from isolated compounds, animal models, or exposure levels that do not map directly onto normal cooking.

Everyday Lipid takeaway Avoid prolonged overheating, keep frying temperatures appropriate, filter food particles when reusing oil, and discard oil that shows substantial degradation.

References - Chemical alterations taken place during deep-fat frying... Chem Phys Lipids. 2012. PMID: 22800882. - Mechanism, indexes, methods, challenges, and perspectives of edible oil oxidation analysis. PMID: 34845958.