Propenal

Acrolein, the intensely pungent three-carbon aldehyde, a colorless liquid whose vapors sting eyes and airways at trace levels. It arises when fats and oils are overheated, lending a harsh acrid note to abused frying media, and microbes can generate it during cider and spirit fermentations, where it spoils flavor. Its toxicity and reactivity make it an industrial hazard as well as a food-quality marker: processors avoid the time-temperature abuse that creates it, and analysts treat it as a sentinel compound for oil degradation.