Methyl-3-furanthiol, 2-, formation

The routes involved in the formation of the various furan sulphides and disulphides involve the interaction of hydrogen sulphide with dicarbonyls, furanones and furfurals. Possible pathways are shown in Scheme 12.8. Furanthiols have been found in heated model systems containing hydrogen sulphide or cysteine with pentoses [56-58]. 2-Methyl-3-furanthiol has also been found as a major product in the reaction of 4-hydroxy-5-methyl-3(2H)-furanone with hydrogen sulphide or cysteine [21, 59]. This furanone is formed in the Maillard reaction of pentoses alternatively it has been suggested that it may be produced by the dephosphorylation and dehydration of ribose phosphate, and that this may be a route to its formation in cooked meat [21, 60]. 

To explain the formation of 3-mercapto-2-pentanone without its 2,3-isomer, a new pathway was proposed (Scheme 5.15), in which 1,4-dideoxyosone and 5-hydroxy-3-mercapto-3-penten-2-one are intermediates. Both 2-methyl-3-furanthiol and 2-methyl-3-hydroxyfuran are readily derived from this pathway without involving norfuraneol. 

Scheme 5.15 Proposed formation of 3-mercapto-2-pentanone and 2-methyl-3-furanthiol from ribose and cysteine via l,4-dideoxy-2,3-diketose2X...

As far as 2-methyl-3-furanthiol is concerned, it can be formed from cysteine, but thiamine constitutes its more important precursor by far.256 When the reaction is carried out in the presence of thiamine in an aqueous medium at 120 °C for 1 h, only about 8% of the thiol is derived from cysteine, and, in the absence of thiamine from the mix, no thiol was detected. The probable mechanism of formation from thiamine is shown in Scheme 5.16.257... 

Scheme 5.16 Proposed formation of 2-methyl-3-furanthiol from thiamine251...

Comparative aroma dilution analyses of the headspace of aqueous solutions, containing either the total volatiles isolated from a fresh coffee brew or these volatiles mixed with the melanoidins isolated from coffee brew, revealed drastic losses of odorous thiols, 2-furfurylthiol, 3-methyl-2-butenethiol, 3-mercapto-3-methylbutyl formate, 2-methyl-3-furanthiol, and methanethiol, in the presence of melanoidins.509 The first compound was affected most, the reduction being 16-fold, and was accompanied by an overall reduction in roasty-sulfury aroma. The rapid loss of thiols was confirmed by stable-isotope dilution analysis. [2H]-NMR and LC-MS gave strong evidence that the thiols become covalently bound via Maillard-derived pyrazinium compounds. 

T. A. Bolton, G. A. Reineccius, R. Liardon, and T. Huynh Ba, Role of cysteine in the formation of 2-methyl-3-furanthiol in a thiamine-cysteine model system, in Thermally Generated Flavors Maillard, Microwave, and Extrusion Processes, T. H. Parliment, M. J. Morello, and R. J. McGorrin (eds), American Chemical Society, Washington, DC, 1994, 270-278. 

The work also demonstrates that IMP in meat is a precursor for 2-methyl-3-furanthiol and mercaptoketones, although it does not seem to be as important in the formation of 2-furylmethanethiol. The roles of IMP and ribose as sources of these thiols have been discussed previously (12,19). The mechanism involves the Maillard reaction and could require the intermediate formation of 4-hydroxy-5-methyl-3(2H)-furanone and dicarbonyls, such as butanedione and pentanedione, which is then followed by their reaction with hydrogen sulfide or cysteine. The concentrations of IMP in meat vary considerably between different animals and different muscles, and are affected by production conditions both pre- and post-slaughter. The present results indicate that the amount of IMP in the meat at the time of cooking may be an important factor in determining the amount of meaty flavor. 

Fig. 5.19. Formation of 2-methyl-3-furanthiol and bis(2-methyl-3-furyl)disulfide from thiamine...

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