Mercaptans flavour

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). 

Cysteine can be obtained by hydrolysis from cysteine-rich proteins in hair or feathers or from petrochemical sources. Cysteine is an important raw material in Maillard reactions for the preparation of process flavours, but it can also serve as a source of ammonia and hydrogen sulfide for the preparation of flavour chemicals, such as the terpene sulfur compounds mentioned in Sect. 13.2.4 and furfuryl mercaptan mentioned in Sect. 13.4.2.4. 

Since sulphur compounds are important in the off-flavour of UHT milk, attempts to improve its flavour have focused on reducing the concentration of these, e.g. by adding thiosulphonates, thiosulphates or cystine (which react with mercaptans) or sulphydryl oxidase, an indigenous milk enzyme (which oxidizes sulphydryls to disulphides Chapter 8). 

Sulphur compounds furfuryl mercaptane with its very strong roasted coffee, sulphurous burnt character. This unstable component varies its flavour profile with the concentration. Methanethiol has a strong rotten, disgusting sulphurous odour, but at low concentrations it enhances an interesting, freshly brewed character to the coffee flavour. 

Clinton W.P. and Pettit F.E. (1969) Enhancing coffee flavour with methyl mercaptan. French Pat. 2000347. 

Parliment T.H. and Stahl H.D. (1995b) Formation of furfuryl mercaptan in coffee model systems. Dev. Food Sci., Food Flavours, Morton and Macleod Eds, Elsevier, Amsterdam 37A, 805-13. 

Pascual E.C., Blank I., Goodman B.A. and Yerantzian C. (2000) The detection and characterization of free radicals generated during the decomposition of solutions of the coffee flavour compound furfuryl mercaptan. 18th Int. Colloq Chem Coffee (Helsinki, 2-6.8.1999) (ASIC, 2000), 50-7. 

Phenylethyl- mercaptan 1665 6263-65-6 No Europe 0.01 USA ND Japan ND Yes. The NOEL of 0.43 mg/kg bw perday for the related substance 2,6-dimethylthiophenol (No. 530) (Peano et al., 1981) is >2 million times the estimated daily intake of ( )-1-phenylethylmercaptan when used as a flavouring agent. See note 8 No safety concern... 

Another type of commercial hop extract is made by borohydride reduction of an isomerized extract of a-acids and is claimed to be less sensitive to light than a normal isomerized extract [124]. When beer, particularly lager beer, is exposed to sunlight in clear bottles it develops an unpleasant sun-struck flavour due to the formation of isopentenyl mercaptan (98). It is envisaged that photolysis of isohumulone cleaves the isohexenoyl side-chain to form a 3-methylbut-2-enyl radical which reacts with hydrogen sulphide, or any available thiol, in the beer to produce (98) [125]. 

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. 

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... 

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... 

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