2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 365
Fig. 5.18. Methionine degradation to methional,
methanethiol and dimethylsulfide
Fig. 5.17. Cysteine decomposition by a Strecker degradation
mechanism: formation of H 2 S (I) or 2-mercaptoethanal
(II)
disulfide, which is cleaved again at a higher temperature
(Formula 5.11), e. g., during cooking.
If constituents which have H-atoms abstractable
by thiyl radicals, e. g., reductones, are present
in food, MFT is regenerated. This is desirable
because although the disulfide of MFT has a very
low odor threshold (Table 5.21), its meat-like
odor has a medical by-note and, unlike MFT,
its Stevens curve is much flatter (cf. 5.1.4), i. e.,
the odor is not very intensive even in a higher
concentration range.
Norfuraneol (I in Table 5.18) is under discussion
as the precursor of MFT. As proposed in
Formula 5.12, the addition of hydrogen sulfide
leads to 4-mercapto-5-methyl-3(2H)-furanone,
which yields MFT after reduction, e. g., by reductones
from the Maillard reaction, and water
elimination. MFT can also be formed in meat
by the hydrolysis of thiamine (Fig. 5.19). The
postulated intermediate is the very reactive
5-hydroxy-3-mercaptopentan-2-one.
(5.8)
(5.9)
(5.10)
(5.11)