2009_Book_FoodChemistry

366 5 Aroma Compounds
(5.12)
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-1-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-
1-ol (prenyl alcohol) is also a possible precursor,
which yields the thiol after water elimination and
hydrogen sulfide addition.
It is unclear whether sulfides I–III in Fig. 5.20
and trithioacetone, analogous to trithioacetaldehyde
(I), are really formed during the cooking
of meat or whether these compounds are artifacts
that are produced on concentration of the volatile
fraction in the course of analysis (cf. 5.2.1).
Fig. 5.19. Formation of 2-methyl-3-furanthiol and
bis(2-methyl-3-furyl)disulfide from thiamine
Fig. 5.20. Formation of 2,4,6-trimethyl-s-trithiane (I),
3,5-dimethyl-1,2,4-trithiolane (II) and 2,4,6-trimethyl-
5,6-dihydro-1,3,5-dithiazine (III)