Furan is a member of the class of compounds known as furans. These are molecules containing a heterocyclic organic group consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. Furan is aromatic because one of the lone pairs of electrons on the oxygen atom is delocalized into the ring, creating a 4n+2 aromatic system similar to benzene. Because of the aromaticity, furan is flat and lacks discrete double bonds. Furan is a colourless, flammable, highly volatile liquid with a boiling point close to room temperature (31 °C). It is soluble in common organic solvents, including alcohol, ether, and acetone, but is insoluble in water. It has a strong ethereal odour. Furan is found in heat-treated (e.g. cooked, roasted, baked, pasteurized, and sterilized) commercial foods and is produced through thermal degradation of natural food constituents (PMID:22641279). It can be found in roasted coffee, instant coffee, and processed baby foods (PMID:22641279). In particular, the highest furan levels can be detected in coffee, with mean values between 42 and 3 660 ng/g for brewed coffee and roasted coffee beans. Furan can also be detected at levels between 0.2 and 3.2 ng/g in infant formula, from 22 to 24 ng/g in baked beans, from 13 to 17 ng/g in meat products, and from 23 to 24 ng/g in soups. In soy sauce, furan is detectable at 27 ng/g (PMID:26483883). Research has indicated that coffee made in espresso makers and, above all, coffee made from capsules, contains more furan than that made in traditional drip coffee makers, although the levels are still within safe health limits. Various pathways have been reported for the formation of furan: (1) thermal degradation and/or thermal rearrangement of carbohydrates in the presence of amino acids, (2) thermal degradation of certain amino acids (aspartic acid, threonine, alpha-alanine, serine, and cysteine), (3) oxidation of ascorbic acid at higher temperatures, and (4) oxidation of polyunsaturated fatty acids and carotenoids (PMID:26483883). Several studies have reported that furan formation occurs to a large extent during the Maillard reaction. The Maillard reaction involves the thermal degradation and rearrangement of carbohydrates (i.e. non-enzymatic browning reactions during food processing and cooking). Reducing hexoses often go through the Maillard reaction in the presence of amino acids and produce reactive intermediates such as 1-deoxy- and 3-deoxyosones, aldotetrose, and 2-deoxy-3-keto-aldotetrose. 2-Deoxy-3-keto-aldotetrose typically goes through retro-aldol cleavage leading to 3-deoxyosone which undergoes alpha-dicarbonyl cleavage, followed by oxidation and decarboxylation to form 2-deoxyaldotetrose, which is a direct precursor of furan. In addition to the formation of furan via carbohydrate degradation, furan can also be formed through thermal degradation of certain amino acids. Specifically, the amino acids that can form acetaldehyde and glycolaldehyde can produce furan by aldol condensation and cyclization (PMID:26483883). Furan is toxic and may be carcinogenic. In particular, furan is a potent hepatotoxin and hepatocarcinogen in rodents, causing hepatocellular adenomas and carcinomas in rats and mice, and high incidences of cholangiocarcinomas in rats at doses ≥ 2 mg/kg (PMID:22641279).