Food Chemistry

5.3 Individual Aroma Compounds 363

(5.6)

Fig. 5.16.Formation of 5-ethyl-3-hydroxy-4-methyl-
2(5H)-furanone from threonine by heating

Table 5.19.Odor thresholds of 4-hydroxy-5-methyl-
(I) and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (II) as
a function of the pH value of the aqueous solution

pH Threshold (μg/l)

III

7. 023 , 000 60


8. 5 2100 31


9. 0 2500 21

5.3.1.4 Thiols,Thioethers,Di-andTrisulfides......................

An abundance of sulfurous compounds is ob-
tained from cysteine, cystine, monosaccharides,
thiamine and methionine by heating food. Some

Table 5.20.Occurrence of 4-hydroxy-2,5-dimethyl-

3(2H)-furanone

Food mg/kg

Beer, light 0. 35

Beer, dark 1. 3

White bread, crust 1. 96

Coffee drinka 1 .5–7

Emmental cheese 1. 2

Beef, boiled 9

Strawberry 1–30

Pineapple 1 .6–35

aCoffee, medium roasted, 54 g/l water.

are very powerful aroma compounds (Table 5.21)

and are involved in the generation of some

delightful but also some irritating, unpleasant

odor notes.

Thiols are important constituents of food aroma

because of their intensive odor and their oc-

currence as intermediary products which can

react with other volatiles by addition to carbonyl

groups or to double bonds.

Hydrogen sulfide and 2-mercaptoacetaldehyde

are obtained during the course of theStrecker

degradation of cysteine (Fig. 5.17). In a simi-

lar way, methionine gives rise to methional,

which releases methanethiol byβ-elimination

(Fig. 5.18). Dimethylsulfide is obtained by

methylation during heating of methionine in the

presence of pectin:

(5.7)

Methanethiol oxidizes easily to dimethyldisul-

fide, which can disproportionate to dimethylsul-

fide and dimethyltrisulfide (Formula 5.8).