2- Methyl-3-furanthiol

A number of furans with thiol, sulphide or disulphide substitution have been reported as aroma volatiles, and these are particularly important in meat and coffee. In the early 1970s, it was shown that furans and thiophenes with a thiol group in the 3-position possess strong meat-like aromas and exceptionally low odour threshold values [50] however, it was over 15 years before such compounds were reported in meat itself In 1986,2-methyl-3-(methylthio)furan was identified in cooked beef and it was reported to have a low odour threshold value (0.05 pg/kg) and a meaty aroma at levels below 1 pg/kg [51]. Gasser and Grosch [52] identified 2-methyl-3-furanthiol and the corresponding disulphide, bis(2-methyl-3-furanyl) disulphide, as major contributors to the meaty aroma of cooked beef. The odour threshold value of this disulphide has been reported as 0.02 ng/kg, one of the lowest known threshold values [53]. Other thiols which may contribute to meaty aromas include mercaptoketones, such as 2-mercapto-pentan-3-one. 2-Furylmethanethiol (2-furfurylmercaptan) has also been found in meat, but is more likely to contribute to roasted rather than meaty aromas. Disulphides have also been found, either as symmetrical disulphides derived from two molecules of the same thiol or as mixed disulphides from two different thiols [54]. 

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]. 

The influence of the sensitivity of the assessors on AEDA has been studied [11], with the result that the differences in the FD factors determined by a group of six panellists amount to not more than two dilution steps (e.g. 64 and 256), implying that the key odorants in a given extract will undoubtedly be detected. However, to avoid falsification of the result by anosmia, AEDA of a sample should be independently performed by at least two assessors. As detailed in [6], odour threshold values of odorants can be determined by AEDA using a sensory internal standard, e.g. ( )-2-decenal. However, as shown in Table 16.6 these odour threshold values may vary by several orders of magnitude [8] owing to different properties of the stationary phases. Consequently, such effects will also influence the results of dilution experiments. Indeed, different FD factors were determined for 2-methyl-3-furanthiol on the stationary phases SE-54 and FFAP 2 and 2 , respectively. In contrast, 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone showed higher FD factors on FFAP than on SE-54 2 and 2, respectively. Consequently, FD factors should be determined on suitable GC capillaries [8]. However, the best method to overcome the limitations of GC-O and the dilution experiment is a sensory study of aroma models (Sect. 16.6.3). 

In addition to GC-MS, recent studies have focused on the identification and quantitative analysis of impact odorants in botrytized wines using gas chromatography-olfactometry (GC-O) analysis. Sarrazin et al. (2007a) investigated numerous botrytized and nonbotry-tized Sautemes wines. They could identify several key odorants that were responsible for the sensory differences between the wines, notably 3-mercaptohexan-l-ol, various furanons, ethyl-hexanoate, methional, phenylethanol, phenylacetaldehyde, sotolon, p-damascenone, and 2-methyl-3-furanthiol. 

The occurrence of 1 and 3 in a flavor model system has been pointed out by G.J. Hartmann et al. (j 2.. LL). Moreover, 2-methyl-3-furanthiol 1 and bis-(2-methy1-3-fury 1)-disulfide 3. have already been identified as major constituents in a model meat system that was prepared by refluxing an aqueous solution of cysteine hydrochloride, thiamin hydrochloride, and hydrolyzed vegetable protein (12,13) for four hours. 

The dipeptide camosine, /J-alanyl-i.-histidinc, is one of the most abundant N compounds present in the non-protein fraction of vertebrate skeletal muscles. It constitutes, for example, 50, 150, and 276 mg per 100 g of muscle tissue from chicken leg, bovine leg, and porcine shoulder, respectively. Chen and Ho138 examined its effects on volatile generation in a model system of ribose and cysteine (180 °C, 2 h, pH 5 and 8.5). These were complex the levels of thiophenes and some meaty compounds, such as 2-methyl-3-furanthiol, 2-furfurylthiol, and their associated dimers, were generally lowered, but those of important N compounds, such as pyrazines and thiazoles, which are known to elicit roasty and nutty flavours, were enhanced. 

Cemy and Davidek250 used [13C5]ribose to show that, when heated 3 1 with cysteine at 95 °C for 4 h in a phosphate buffer (0.5 M, pH 5), the ribose is incorporated intact not only into furylmethanethiol, but also into 2-methyl-3-furanthiol (meat-like,... 

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...

Group 3 volatiles, formed by further interactions, can reach even lower threshold values than those mentioned so far, particularly when sulfur is involved, usually derived by the break down of cysteine (bis(2-methyl-3-furyl) disulfide, 0.00002 2-methyl-3-furanthiol, 0.0004 2-furylmethanethiol, 0.005 dimethyl trisulfide, 0.01 dimethyl disulfide, 0.16 dimethyl sulfide, 0.3 hydrogen sulfide, 10). Into some volatile sulfur compounds other atoms as well as the sulfur of cysteine are incorporated (2-acetylthiazoline, 1 5-acetyl-2,3-dihydro-l,4-thiazine, 1.25 2-acetylthiazole, 10). Other volatiles are more likely to be derived from methionine (methanethiol, 0.2). 

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. 

T. Hofmann and P. Schieberle, Studies on intermediates generating the flavour compounds 2-methyl-3-furanthiol, 2-acetyl-2-thiazoline and sotolon by Maillard-type reactions, in Flavour Science Recent Developments, A. J. Taylor and D. S. Mottram (eds), Royal Society of Chemistry, Cambridge, 1996, 182-187. 

Methyl-3-buten-2-one (aliphatic ketone) 2-Methyl-3-furanthiol (furane)... 

Bouchilloux R, Damet R, Dubourdieu D. (1998b). Identification d un thiol fortement odorant, le 2-methyl-3-furanthiol, dans les vins. Vitis, 37, 177-180. 

Tominaga, T, Dubourdieu, D. (2006). A novel method for quantification of 2-methyl-3-furanthiol and 2-furanmethanethiol in wines made from Vitis Vinifera grape varieties. J. Agric. Food Chem., 54, 29-33. 

Mateo-Vivaracho, L., Ferreira, V, and Cacho, J. (2006). Automated analysis of 2-methyl-3-furanthiol and 3-mercaptohexyl acetate at ng L-1 level by headspace solid-phase... 

Figure 4. GC-MS analyses. Top 2-methyl-3-furanthiol acetate (7), sample preparation I (SP I), GC column DB-1. Middle 2-formyl-thiophene (8), SPII, GC column DB-Wax. Bottom fiirfurylmethylsulfide (9), SPII, GC column DB-Wax.

For the first time the identication of 2-methyl-3-furanthiol acetate, furfuryl-methylsulfide, 2-formyl-thiophene and 4-methyl-5-vinyl-thiazol confirms the assumption that products from thiamine degradation and yeast lees are present in wines at trace level. 

Zeiler, G. ModeUversuche zur Bildung von 2-Methyl-3-furanthiol und 2-Furfurylthiol in geko-chtem Fleisch. Doctoral thesis. Technical University of Munich, 1994. 

Boiled beef and pork differ in their aromas. Beef smells more intensely meaty, sweet-caramel-like and malty, whereas pork is stronger in sulphurous and fatty odour notes [21, 37J. According to Table 6.29, the pronounced odour notes of beef are caused by high concentration of furaneol (no. 1), 2-furfurylthiol (no. 2), 3-mercapto-2-pen-tanone (no. 3) and 2-methyl-3-furanthiol (no. 6). Omission experiments confirmed that these volatiles and in addition octanal, nonanal, (E,E)-2,4-decadienal, are the key odorants of boiled beef [21 ]. The higher concentration of the caramel-like smelling furaneol (no. 1) in beef than in pork is due to higher levels of its precursors glucose-6-phosphate and fructose-6-phosphate [37],... 

Kerscher, R., Grosch, W. (1998) Quantification of 2-methyl-3-furanthiol, 2-furfurylthiol, 3-mer-capto-2-pentanone, and 2-mercapto-3-pentanone in heated meat. J. Agric. Food Chem. 46, 1954-1958... 

Continuing our evolution of reaction flavors brings us to the reaction of materials, which are themselves, isolated from amino acids and sugars to produce unique materials which are frequently very different than the reaction of the parent amino acids and sugar. The reaction of D-Xylose and L-Cysteine is a good example of this principle. A complex meaty tasting mix is obtained from the sugar and amino acid. The mixture contains small amoimts of 2-Methyl-3-furanthiol, a potent meat flavor (equation 5). 

---