Sulphidic flavours

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. 

Many cheeses contain the same or similar compounds but at different concentrations and proportions chromatograms of some cheese varieties are shown in Figure 10.25. The principal classes of components present are aldehydes, ketones, acids, amines, lactones, esters, hydrocarbons and sulphur compounds the latter, e.g. H2S, methanethiol (CH3SH), dimethyl sulphide (H3C-S-CH3) and dimethyl disulphide (H3C-S-S-CH3), are considered to be particularly important in Cheddar cheese. The biogenesis of flavour compounds has been reviewed by Fox et al. (1993, 1996a) and Fox, Singh and McSweeney (1995). 

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... 

Other materials Herbs, spices and extracts Flavouring substances identified in food Water Thiamine and thiamine hydrochloride Ascorbic, citric, lactic, fumaric, succinic and tartaric acid and their salts (Na, K, Ca, Mg, NH ) Guanylic acid, inosinic acid and their salts (Na, K, Ca) Inositol Sodium, potassium and ammonium sulphides, hydrosulphides and polysulphides Led thine pH regulators acetic, hydrochloric, phosphoric and sulphuric acid salts thereof sodium, potassium, calcium and ammonium Polymethylsiloxane... 

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). 

CIC the typical sulphurous flavour is represented by the high concentration of dimethyl sulphide, combined with traces of 1,2-dithia-cyclo-pentene. The vegetable-green note results from 2-isopropyl-3-methoxy pyrazine, resembling raw potatoes, and 2-sec-butyl-3-methoxy pyrazine, a green bell pepper note. 

CIC the earthy odour of fresh potatoes is represented by 2-isopropyl-3-methoxy pyrazine. This earthy note is supported by the mushroom character of l-octen-3-ol. The key component of boiled potatoes is 3-(methylthio)-propanal, balanced with dimethyl sulphide. The high reaction temperatures in baked and fried potatoes start the Maillard reaction to form mainly heterocyclic components 2-ethyl-3,5-dimethyl pyrazine, 2-ethyl-6-vinyl pyrazine, 5-methyl-6,7-dihydro-(5H)cyclopenta-pyrazine, 2-acetyl-l,4,5,6-tetrahydro-pyridine are responsible for the roasted, nutty cracker-like flavour. The heat-induced degradation of the potato lipids and the frying oil imparts a fatty, tallowy character to the french fried potatoes. (E,E)-2,4-Decadienal, 2-octenal, octanoic acid and decanoic acid are main contributors to this fatty note. 

The way in which an oil is distributed physically in an aqueous phase can affect the headspace concentration of flavouring substances. In the case of dimethyl sulphide, a higher concentration of flavouring substance is needed in the oil-in-water emulsion to achieve the concentration in the gas phase which is measured above the non-emulsi-fied system (Fig. 5.22A). This points to an adsorption of the dimethyl sulphide on the boundary surfaces or to some other type of interaction. In the case of allyl mustard oil, a higher concentration is measured in the headspace of the emulsion than in that of non-emulsified system (Fig. 5.22B). Apparently the mustard oil has little or no affinity with the boundary surfaces. 

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]. 

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]. 

Dimethyl sulphide (DMS, CHa-S-CHj) is recognized as making a significant contribution to beer flavour, particularly in lagers [87]. DMS is derived from a heat-labile precursor in malt it is also produced during fermentation. The heat-labile precursor of the DMS derived from malt is S-methyl meth-... 

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]. 

The formation of off-flavours in beer has been reviewed [40], Autoxidation of the lipids present in beer produces carbonyl compounds with very low taste thresholds. In particular, linoleic acid is oxidized to trihydroxyoctadecenoic acids (Table 22.7) which break down into 2-/mAz.y-nonenal. This aldehyde and related compounds impart a cardboard flavour to beer at very low concentrations. Other carbonyl are formed from the lipids in beer by irradiation with light including the C9, Cjo, and Cu-alka-2,4-dienals (thresholds 0 5, 0 3 and 0 01 ppb respectively) [40]. The level of diacetyl and pentane-2,3-dione in a range of commercial beers is given in Table 22.11. Quantities in excess of 0 15 ppm impart a buttery flavour more noticeable in lagers than in ales. Bacterial contamination and petite mutants of yeast result in high levels of diacetyl. The sulphur compounds characterized in beer are listed in Table 22.19 with some threshold data. Dimethyl sulphide is the major volatile... 

The flavour components in butter and butterfat are complex and include y- and d-lactones (Boldingh and Taylor, 1962), methyl ketones (Forss, 1972 Langeer and Day, 1964), aldehydes (Badings 1970), short-chain fatty acids (Forss, 1972 Langeer and Day, 1964), sulphides (Patton etal., 1956 Day etal., 1957 Badings etal., 1975), alcohols (Forss, 1972 Langeer and Day, 1964), proteins, sugars and secondary reaction products (Teranishi et al., 1981). 

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)... 

Numerous odour-active VSCs have been detected in beers. Examples of beer VSCs encompass methanethiol, ethanethiol, H2S, dimethyl sulphide, dimethyl disulphide, methional, methionol, 3-(methylthio)propyl aceate and 2-mercapto-3-methyl-l-buta-nol (Angelino, 1991 HiU Smith, 2000). Most VSCs cause off-odours such as rotten egg-like, cabbage-like, onion-like and garlic-like. However, some VSCs have a positive impact on beer flavour by accentuating fruitiness (e.g. 3-mercaptohexanol and 3-mercaptohexyl acetate). 

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