Dimethyl sulphide flavour

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). 

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]. 

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-... 

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... 

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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