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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)

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