Food Chemistry

5.3 Individual Aroma Compounds 365

Fig. 5.17.Cysteine decomposition by aStreckerdegra-
dation mechanism: formation of H 2 S (I) or 2-mer-
captoethanal (II)

Fig. 5.18. Methionine degradation to methional,

methanethiol and dimethylsulfide

disulfide, which is cleaved again at a higher tem-

perature (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,

itsStevenscurve 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 discus-

sion 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 re-

ductones from theMaillardreaction, 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)