Acetaldehyde, also known as ethanal, belongs to the class of organic compounds known as short-chain aldehydes. These are an aldehyde with a chain length containing between 2 and 5 carbon atoms. Acetaldehyde exists in all living species, ranging from bacteria to humans. Within humans, acetaldehyde participates in a number of enzymatic reactions. In particular, acetaldehyde can be biosynthesized from ethanol which is mediated by the enzyme alcohol dehydrogenase 1B. Acetaldehyde can also be converted to acetic acid by the enzyme aldehyde dehydrogenase (mitochondrial) and aldehyde dehydrogenase X (mitochondrial). The main method of production is the oxidation of ethylene by the Wacker process, which involves oxidation of ethylene using a homogeneous palladium/copper system: 2 CH2CH2 + O2 → 2 CH3CHO. In the 1970s, the world capacity of the Wacker-Hoechst direct oxidation process exceeded 2 million tonnes annually. In humans, acetaldehyde is involved in disulfiram action pathway. Acetaldehyde is an aldehydic, ethereal, and fruity tasting compound. Outside of the human body, acetaldehyde is found, on average, in the highest concentration in a few different foods, such as sweet oranges, pineapples, and mandarin orange (clementine, tangerine) and in a lower concentration in . acetaldehyde has also been detected, but not quantified in several different foods, such as malabar plums, malus (crab apple), rose hips, natal plums, and medlars. This could make acetaldehyde a potential biomarker for the consumption of these foods. In condensation reactions, acetaldehyde is prochiral. Acetaldehyde is formally rated as a possible carcinogen (by IARC 2B) and is also a potentially toxic compound. Acetaldehyde has been found to be associated with several diseases such as alcoholism, ulcerative colitis, nonalcoholic fatty liver disease, and crohn's disease; also acetaldehyde has been linked to the inborn metabolic disorders including aldehyde dehydrogenase deficiency (III) sulfate is used to reoxidize the mercury back to the mercury. Acetaldehyde was first observed by the Swedish pharmacist/chemist Carl Wilhelm Scheele (1774); it was then investigated by the French chemists Antoine François, comte de Fourcroy and Louis Nicolas Vauquelin (1800), and the German chemists Johann Wolfgang Döbereiner (1821, 1822, 1832) and Justus von Liebig (1835). At room temperature, acetaldehyde (CH3CHO) is more stable than vinyl alcohol (CH2CHOH) by 42.7 kJ/mol: Overall the keto-enol tautomerization occurs slowly but is catalyzed by acids. The level at which an average consumer could detect acetaldehyde is still considerably lower than any toxicity. Pathways of exposure include air, water, land, or groundwater, as well as drink and smoke. Acetaldehyde is also created by thermal degradation or ultraviolet photo-degradation of some thermoplastic polymers during or after manufacture. The water industry generally recognizes 20–40 ppb as the taste/odor threshold for acetaldehyde. The level at which an average consumer could detect acetaldehyde is still considerably lower than any toxicity.