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.