Acrolein

An aldehyde, synonym propenal, classed among the aroma compounds of foods but notorious chiefly as a source of off-odour and off-flavour. It forms when fats overheat — glycerol dehydrates into acrolein — giving abused frying oil its harsh, pungent, eye-watering character, and it can taint spirits distilled under poor control. Its presence is therefore a chemical signal of thermal abuse: fryers and distillers who push temperatures too far generate it, and sensory panels pick it up long before instruments are needed. Beyond food, it is a potent industrial irritant, which is why occupational exposure around overheated fats is taken seriously.