Mercaptans in coffee

Parliment, T. H., Stahl, H. D., Formation of furfuryl mercaptan in coffee model systems, Dev. Food Sci., 37A, 805, 1995. (CA123 196907b)... 

T. Parliment and H. Stahl, Formation of Furfuryl Mercaptan in Coffee Model Systems, in Developments in Food Science V37A Food Flavors Generation, Analysis and Process Influence (G. Charalambous, ed.), Elsevier, New York, 1995, p. 805. G. B. Nickerson and S. T. Likens, Gas chromatographic evidence for the occurrence of hop oil components in beer, J. Chromatog. 21 1-3 (1966). 

Some of these reaction flavors occur in nature. The oxidation of mercaptans to form disulfides and other products is in part responsible for the loss of flavor from fresh roasted coffee only a short time after brewing. Furfiiryl Mercaptan is one of the most potent flavor components in coffee and is responsible for a fresh brewed coffee flavor. While it is present at only 1.55 ppm versus Furfuryl Alcohol (515 ppm) and Furfural (157 ppm), its flavor contribution is 310,014 times its flavor threshold (0.005 ppb). If there were a 100% conversion of Furfuryl Mercaptan to Diflirfuryl Disulfide, the coffee would appear to be 10 times weaker due to Difurfliryl Disulfide s flavor threshold being 0.05 ppb, or ten times weaker than Furfuryl Mercaptan (equation 10). The overall effect of this oxidation is that the coffee has lost its flavor. 

The authors observed that an exhaustive list of all the chemicals present in coffee flavor had not yet been compiled, but they believed they had identified the components that are present at the higher ratio of weight, and those which principally control the odor note. Most of the substances identified were well-known compounds present in other roasted products as well, for instance in caramel sugar, cocoa, baked bread and—partially—even in wood tar. However, some of the chemicals detected were new and, obviously, characteristic of roasted coffee. Traces of methyl mercaptan, which was already known at that time and which smells even worse, were also detected in coffee aroma. Commenting on this observation, Reichstein and Staudinger note that it is generally known that many popular raw materials and synthetic perfume compounds owe their characteristic note, which is extremely pleasant to the olfactory sense, to their content of small quantities in additives which carry a rather unpleasant odor in themselves but prove very attractive in thinned solutions and in admixture with other oils. The authors tried to reconstitute coffee aroma, and only by combining over 40 of the substances extracted from coffee... 

Parliment T.H. and Stahl H.D. (1994) Generation of furfuryl mercaptan in cysteine-pentose model systems in relation to roasted coffee. Am. Chem. Soc. Symp. Ser. 564, 160-70. 

Generation of Furfuryl Mercaptan in Cysteine—Pentose Model Systems in Relation to Roasted Coffee... 

More than 60 years ago Reichstein and Staudinger (1) reported the presence of furfuryl mercaptan (Fur-SH) or 2-fiirfurylthiol in coffee. Tressl reported that the level of Fur-SH in Robusta coffee is about double that of Arabica coffee, ranging from about 1-2 ppm in Arabicas to about 2-3.8 ppm in Robustas (2). While not present in many other foods, it is also a flavor constituent of chicken, beef and pork. 

Coffee Precursors. The primary precursors of furfiiryl mercaptan in green coffee are postulated to be the free or polymeric forms of pentose sugars and sulfiir amino acids. Hexose sugars may also be a source upon fragmentation. 

Some furans containing a sulfur atom in the aUphatic side chain, and also some thienofurans, contribute significantly to the aroma of roasted coffee. Examples of compounds of the first group are 2-furanmethanethiol (furfuryl mercaptan), its 5-methyl derivative (8-165) and 2-methyl-3-furanthiol which are found in coffee. The presence of 2-furanmethanethiol and 2-methyl-... 

T. H. Parliment and H. D. Stahl, Generation of Furfuryl Mercaptan in Cysteine-Pentose Model Systems in Relation to Roasted Coffee, in Sulfur Compounds in Foods (C. Mussinan and M. Keelan, eds.), American Chemical Society, Washington,... 

Sulfur compounds such as furfuryl mercaptans have a rotten odor but in small amounts are coffee-like.15 Furfuryl mercaptan itself has an odor threshold of 0.005 ppb in water but at 10 ppb in water it imparts a distinctly stale odor.19 The particular precursors of furfuryl mercaptan seem to be the coffee cell wall material which contains both arabinogalactan as a pentose sugar source and protein such as glutathione.84 Other sulfur compounds such as kahweofuran and methyldithiofurans impart a meaty odor if their concentrations are high enough.19... 

Furfural comes from pentose sugars in cereal straws and brans. Furfural is the precursor of furfuryl mercaptan and its disulfide, difurfuryl disulfide, which are both important chemicals for coffee, meat and roasted flavours. They are prepared by the reaction of furfural and hydrogen sulfide (Scheme 13.15). 

SAFETY PROFILE Poison by intraperitoneal route. Experimental reproductive effects. Used as a flavoring in chocolate, fruit, nuts, and coffee. WTien heated to decomposition it emits toxic fumes of SOx. See also MERCAPTANS. 

Group 14 lists among other compounds various furyl disulphides and in particular the coffee compound furfuryl mercaptan (Flavis 13.026, FEMA 2493). 

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). 

The odor of rotten cabbage is very diffusive and objectionable (Arctander, 1967). On the other hand, despite these characteristics and certainly thanks to its originality, this mercaptan gives one of the most intense noted to roasted coffee. The threshold concentration in water varies between 0.02 ppb (Guadagni et al., 1963a), and 2.1 ppb (quoted by Persson and von Sydow, 1973). 

Sulfur analogs of alcohols and ethers are thiols or mercaptanes and sulfides, respectively. According to the nomenclature rules, thiols are named following the same rules as for alcohols, the only difference being that the suffix -ol is replaced with -thiol. Most of these substances can be recognized by their unpleasant odor. Ethanethiol is used as an additive to natural gas since its odor serves as an indicator of a gas leak in home installations. Pure natural gas is odorless. Some animals such as skunks release an unpleasant odor which originates from thiols shown in the next scheme. However, some thiols have a pleasant odor, for instance the components of the aroma of coffee. 

It has been known for more than 50 years that furfiiryl mercaptan is one of the very few critical coffee aroma compounds. Aqueous solutions of ribose and cysteine were reacted in a high temperature/short time reactor and the aromatics generated were separated and identified by GC/MS. This study investigates the effect of pH, time and temperature on fiirfuryl mercaptan formation. At higher temperatures furfuryl mercaptan is the major compound generated. The kinetics describing its formation are covered. 

Thiols arising in the MaUlard reaction, mostly derived from heterocyclic compounds, such as furan, thiophene, thiazole and other heterocycles, are important flavour-active components of meat, coffee and many other foods. For example, 2-furanmethanethiol (furan-2-ylmethanethiol, furfuryl mercaptan) has an odour resembling roasted coffee, N- (2-mercaptoethyl) -1,3-thiazolidine (8-127), which is formed in the reaction of fructose with cysteamine, and has a very strong odour resembling popcorn. Its odour threshold is 0.005 ng/1 in air. 

Particularly important substances for the basic flavour of baked and cooked meat are aliphatic thiols (such as methanethiol), sulfides (such as dimethyldisulfide, dimethytrisulfide and dimethyltetra-sulfide), aldehydes (such as acetaldehyde, 2-methylpropanal and 3-methylbutanal), furans, pyridines and thiophenes with a mer-capto group in position C-3 and their corresponding disulfides and some other aliphatic and heterocyclic sulfur compounds. Examples of important aliphatic thiols are 3-mercaptobutan-2-one (8-179) and 3-mercaptopentan-2-one found in cooked beef. A mixture of 3-mercapto-2-methylpentane-l-ol diastereoisomers (Figure 8.86) has a broth-like, sweaty and leek-hke flavour. Very low odour threshold concentrations and an odour reminiscent of roasted meat are found in 2-methylfuran-3-thiol (8-180), which also occurs in beef broth, roasted coffee and other foods, 2,5-dimethylfuran-3-thiol, their corresponding disulfides and 2-furanmethanethiol (furfuryl mercaptan). The typical aroma of roast beef is found 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). 

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