3-methyl-2-butene-l-thiol

The product was previously identified in beer by Andrews (1987) and in human sweat by Polak et al. (1988), and characterized as sunstruck or lightstruck off-flavor. The cause has been attributed to the degradation of bitter isohumulones from hops under the influence of ultraviolet light (Bondeel et al., 1987). By photofragmentation trans-isohumulone liberates a prenyl radical which can trap a thiol radical, producing an undesired flavor for beer. Holscher et al. (1992) have also shown that 3-methyl-2-buten-l-thiol and 3-mercapto-3-methylbutanol are principally formed under pyrolytic conditions in roast model reactions of prenyl alcohol and sulfur-containing amino acids. 

The resulting compound is ofl 3-methyl-2-butene-l-thiol. Lee WJanson, Brew Chem 101 The Basics of Homebrewing Chemistry (Pownal, Vermont Storey Communications, Inc. 1996) ... 

Some reaction systems, which have been described in the patent literature for the production of meat aromas, regard thiamine as precursor. 3-Methyl-2-butene-l-thiol is one of the roast odorants of coffee (cf. 21.1.3.3.7) and can cause on off-flavor in beer (cf. Table 5.5). In general, only very small amounts are formed which are still aroma active on account of the very low odor threshold (Table 5.21). The formation of the thiol is explained by the fact that the 3-methyl-2-butene radical is formed from terpenes by photolysis (beer) or under the drastic conditions of the roasting process (coffee). This radical then meets a SH -radical formed from cysteine under these conditions. In the case of beer, humulons (cf. 20.1.2.3.2) are under discussion as the source of the alkyl radical. In coffee 3-methyl-2-butene-l-ol (prenyl alcohol) is also a possible precursor, which yields the thiol after water elimination and hydrogen sulfide addition. 

Figure 2 Principle of the method used to enrich 3-methyl-2-butene-l-thiol from distillates containing beer volatiles, a. Binding and transfer into the aqueous phase b, release and transfer to solvent.

Figure 3 Proposed formation pathway for 3-methyl-2-butene-l-thiol from isohumulone and cysteine when exposed to light in the presence of riboflavin. (Data from Ref. 5.)...

Table 3 Influence of Riboflavin on the Formation of 3-Methyl-2-Butene-l-Thiol in Beer and a Model Solution ...

In summary, the results indicate that 3-methyl-2-butene-l-thiol may be a key odorant in certain types of beer that contributes to their typical aroma. The data suggest that the tolerable amount for a positive aroma contribution is about 0.2pg/L, but this has to be proved by further sensory experiments. 

AJ Irwin, L Bordeleau, RL Barker. Model studies and flavor threshold determination of 3-methyl-2-butene-l-thiol in beer. J Am Soc Brew Chem 51 1-3, 1993. 

Figure 8 Chemical structures of some aroma impact compounds (FD > 12.5) found in the headspace of roast and ground Arabica and Robusta coffee acetaldehyde (no. 1), methanethiol (no. 2), diacetyl (no. 5), 3-methylbutanal (no. 6), 2-methylbutanal (no. 7), 2,3-pentanedione (no. 8), and 3-methyl-2-butene-l-thiol (no. 9). The numbers correspond to those in Table 1.

Methyl-2-butene-l -thiol Roasted coffee Prenyl alcohol/H2S 34... 

Furfuryl ethyl ether 3- Methyl-2-butene-l -thiol S-Methyl hexanethioate Methional [6270-56-0 [5287-45-6 [2432-77-1 [3268-49-3 ... 

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