Origin and mechanistic pathways of formation of the parent furan--a food toxicant.

Studies performed on model systems using pyrolysis-GC-MS analysis and (13)C-labeled sugars and amino acids in addition to ascorbic acid have indicated that certain amino acids such as serine and cysteine can degrade and produce acetaldehyde and glycolaldehyde that can undergo aldol condensation to produce furan after cyclization and dehydration steps. Other amino acids such as aspartic acid, threonine, and alpha-alanine can degrade and produce only acetaldehyde and thus need sugars as a source of glycolaldehyde to generate furan. On the other hand, monosaccharides are also known to undergo degradation to produce both acetaldehyde and glycolaldehyde; however, (13)C-labeling studies have revealed that hexoses in general will mainly degrade into the following aldotetrose derivatives to produce the parent furan-aldotetrose itself, incorporating the C3-C4-C5-C6 carbon chain of glucose (70%); 2-deoxy-3-ketoaldotetrose; incorporating the C1-C2-C3-C4 carbon chain of glucose (15%); and 2-deoxyaldotetrose, incorporating the C2-C3-C4-C5 carbon chain of glucose (15%). Furthermore, it was also proposed that under nonoxidative conditions of pyrolysis, ascorbic acid can generate the 2-deoxyaldotetrose moiety, a direct precursor of the parent furan. In addition, 4-hydroxy-2-butenal-a known decomposition product of lipid peroxidation-was proposed as a precursor of furan originating from polyunsaturated fatty acids. Among the model systems studied, ascorbic acid had the highest potential to produce furan, followed by glycolaldehyde/alanine > erythrose > ribose/serine > sucrose/serine > fructose/serine > glucose/cysteine.