Hydrogen sulphide flavour

The first group contains compounds produced in the early stages of the reaction by the breakdown of the Amadori or Heynes intermediates, and includes similar compounds to those found in the caramelisation of sugars. Many of these compounds possess aromas that could contribute to food flavour, but they are also important intermediates for other compounds. The second group comprises simple aldehydes, hydrogen sulphide or amino compounds that result from the Strecker degradation occurring between amino acids and dicarbonyl compounds. 

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

The reaction of pulegone or isopulegone with hydrogen sulphide in triethyl-amine leads to the interesting p-menthane-8-thiol-3-one (163), a constituent of Buchu leaf oil Barosina betulina Bartl., Rutaceae), which has a black-currant flavour. Reductive dimerization of pulegone affords the alcohol (164), which... 

A powerful sulphury aroma compound, 3-mercapto-2-methylpentan-l-ol (51), has recently been identified [13] in a complex process flavouring. It has a low odour threshold of 0.15 mg/L of water. The formation could be traced back to the onions present in the process flavouring and its formation is explained from propanal present in onions via aldol condensation, addition of hydrogen sulphide and enzymatic reduction (Fig. 3.31). 

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]. 

Off-flavours may arise from the activities of all the wild yeasts encountered. Pichia and Candida species will oxidize ethanol in a vessel exposed to air to produce acetic acid, while Saccharomyces diastaticus will attack malto-tetraose and often dextrins to produce ethanol. There are also differences between culture yeast and wild yeast with respect to the production of esters, fusel alcohols, organo-sulphur compounds, hydrogen sulphide, vicinal diketones and other products important in beer flavour. Phenolic flavours are known to be produced by certain wild yeasts. [95]. 

Beer quality Altered flavour Altered volatile spectrum Reduced hydrogen sulphide Reduced dimethyl sulphide Alcohol acetyltransferase Manipulation of BAP2 to modulate higher alcohols Increased copy number of MET25 gene Removal of dimethyl sulphide oxidase by deletion of MXRl Fujii et al. (1994) Kodama et al. (2(X)1) Qmura, Shibano, Fukui, and Nakatani (1995) Hansen, Braun, Bech, and Gjermansen (2002)... 

The basic reaction of amino acids under irradiation conditions is the formation of a radical intermediate on the a-carbon atom. From this, ketoacids, iminoacids and degradation products are formed (Scheme 7.3). Cysteine is decomposed to cystine, with hydrogen sulphide formation causing the off flavour of irradiated protein-rich products. 

For most volatile flavouring substances the type of binding to lipids can be explained by the distribution laws. A well-known exception is the affinity of dimethyl sulphide to emulsion boundary surfaces, mentioned above. In the case of phenolic compounds, hydrogen bridges could also be involved [6]. 

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