Methyl mercaptan thresholds

Humans are extremely sensitive to hydrogen sulfide, most likely as a danger signal. The recognition threshold lies at 0.00047 ppm (vol) (cited in Cain, 1978). Only 40 molecules of methyl mercaptan, distributed over several receptors, sufficed for detection (cited in Harborne, 1993). 

The oxidation of mercaptans can be useful to prepare other flavor molecules. Furfuryl Mercaptan and Methyl Mercaptan can be oxidized together to give Methyl Furfuryl Disulfide, a potent material useful for bread, pork and other meat products. It is also the third most active flavor material in fresh brewed coffee, being present at 0.38 ppm or 9,623 times its flavor threshold (0.04 ppb) (equation 11). 

Some volatile sulfur compounds were obtained from methionine and riboflavin after light irradiation under acidic conditions (Table I). Methyl mercaptan seems to be one of the important off-odor components. At this time, because of using the solvent extraction with dichloromethane, the amount of highly volatile methyl mercaptan seemed to be low. Therefore, we focused the other off-odor components, that is, methional, dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) and methyl methanethiosulfonate (MMTS). In particular, DMTS is considered to be significant because of its extremely low threshold value. Figure 1 shows that the amount of DMDS and DMTS increased with decreasing pH value. 

The impact of a compound cannot be predicted from its OAV value, but is the result of the interaction of the odorant with the other odorants present in the mixture. In general, experience has shown that impact compounds, whenever they exist, are odorants present at concentrations higher than the threshold and showing, usually, an odor quite different from the rest of odors present in the wine and demonstrating a personal character. For instance, out of all fruity esters of wine, only isoamyl acetate and phenylethyl acetate, both of which have clear specific and differentiable aromas, have been found to be able to act as impact compounds (Van Wyk et al. 1979 Tat et al. 2007). Similarly, the three mercaptans derived from cysteine, 4-methyl-4-mercaptopentan-2-one, 3-mercaptohexanol and 3-mercaptohexyl acetate, all of which have specific aromas, have been found to be key compounds in some wines (Darriet et al. 1995 Murat et al. 2001 Ferreira et al. 2002a Campo et al. 2005). 

Some sulfur-based compounds (especially those with thiol functions) make a positive contribution to the varietal aromas of certain grape varieties. The role of 4-methyl-4-mercaptopentanone in Sauvignon Blanc aroma is well known (Section 7.4.1). However, most sulfur derivatives have a very bad smell and have detection thresholds as low as 1 p,g/l. These compounds are known as mercaptans, referring to their capacity to be precipitated by mercury salts. 

An important sulfide is methional (8-37). Methional in beer and wine is formed by the activity of microorganisms. It is partly reduced to the corresponding alcohol methionol (8-13) and reaction with acetyl-CoA yields 3-methylthiopropyl acetate (8-129), which is an important component of various fermented foods. Another ester of acetic acid 3-(methylthio)hexyl acetate is a component that posseses attractive tropical fruity notes on dilution. The less odoriferous (-)-(J )-enantiomer (8-130) is reminiscent of passion fruit, while the (-l-)-(S)-form has a more herbaceous odour. The odour thresholds of these thiols in air are 0.10 ng/1 and 0.03 ng/1, respectively. Both isomers have been found in passion fruit (Passiflora edulis, Passifloraceae), guava Psidium guajava, Myrtaceae) and aromatic white wines. Methyl-3-(methylthio)propionate, or pineapple mercaptan (8-131), has a flavour reminiscent of pineapple. S-Methylthiohexanoate (8-132) is a component of the durian fruit smell. Condensation of methional with ethanol yields (Z)-2-(methylthio)methylbut-2-enal also known as 2-ethylidenemethional (8-133), which is an important component of potato chips aroma. It also occurs in... 

Furfurylthiol is the primary character impact compound for the aroma of roasted Arabica coffee (45). It has a threshold of 5 ppt and smells like freshly brewed coffee at concentrations between 0.01 and 0.5 ppb (46). At higher concentrations it exhibits a stale coffee, sulfury note. Other potent odorants in roasted coffee include 5-methylfurfurylthiol (0.05 ppb threshold), which smells meaty at 0.5-1 ppb, and changes character to a sulfury mercaptan note at higher levels (46). Furfuryl methyl disulfide has a sweet mocha coffee aroma (15). A key aromatic that markedly contributes to coffee aroma is 3-mercapto-3-methylbutyl formate. The pure compound has a blackcurrant-like, catty note, however, in the context of brewed coffee, it contributes roast coffee likeness (47,48). 

---