2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 363
(5.6)
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 drink a 1.5–7
Emmental cheese 1.2
Beef, boiled 9
Strawberry 1–30
Pineapple 1.6–35
a Coffee, medium roasted, 54 g/l water.
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)
I
II
7.0 23,000 60
4.5 2100 31
3.0 2500 21
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 occurrence
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 the Strecker
degradation of cysteine (Fig. 5.17). In a similar
way, methionine gives rise to methional,
which releases methanethiol by β-elimination
(Fig. 5.18). Dimethylsulfide is obtained by
methylation during heating of methionine in the
presence of pectin:
5.3.1.4 Thiols, Thioethers, Di- and Trisulfides
An abundance of sulfurous compounds is obtained
from cysteine, cystine, monosaccharides,
thiamine and methionine by heating food. Some
(5.7)
Methanethiol oxidizes easily to dimethyldisulfide,
which can disproportionate to dimethylsulfide
and dimethyltrisulfide (Formula 5.8).