376 5 Aroma Compounds
Table 5.28.Pyrolysis products of some phenolic acids
(T: 200◦C; air)
Phenolic acid Product Distribution
(%)
Ferulic 4-Vinylguaiacol 79. 9
acid Vanillin 6. 4
4-Ethylguaiacol 5. 5
Guaiacol 3. 1
3-Methoxy-4-hydroxy-
acetophenone
(Acetovanillone) 2. 6
Isoeugenol 2. 5
Sinapic 2,6-Dimethoxy-4-
acid vinylphenol 78. 5
Syringaldehyde 13. 4
2,6-Dimethoxyphenol 4. 5
2,6-Dimethoxy- 1. 8
4-ethylphenol
3,5-Dimethoxy-
4-hydroxy-
acetophenone
(Acetosyringone) 1. 1
amino acids from available proteins, sugars from
polysaccharides, or ortho-quinones from phenolic
compounds. These are then converted into aroma
compounds by further nonenzymatic reactions.
In this way, the enzymes enhance the aroma of
bread, meat, beer, tea and cacao.
5.3.2.1 Carbonyl Compounds, Alcohols...........................
Fatty acids and amino acids are precursors of
a great number of volatile aldehydes, while carbo-
hydrate degradation is the source of ethanal only.
Due to its aroma activity at higher concentra-
tions ethanal is of great importance for the fresh
note, e. g., in orange and grapefruit juice.
Linoleic and linolenic acids in fruits and vegeta-
bles are subjected to oxidative degradation by
lipoxygenase alone or in combination with a hy-
droperoxide lyase, as outlined in sections 3.7.2.2
and 3.7.2.3. The oxidative cleavage yields oxo
acids, aldehydes and allyl alcohols. Among the
aldehydes formed, hexanal, (E)-2-hexenal, (Z)-3-
hexenal and/or (E)-2-nonenal, (Z)-3-nonenal,
(E,Z)-2,6-nonadienal and (Z,Z)-3,6-nonadienal
are important for aroma.
Frequently, these aldehydes appear soon after the
disintegration of the tissue in the presence of oxy-
gen. A part of the aldehydes is enzymatically re-
duced to the corresponding alcohols (see below).
In comparison, lipoxygenases and hydroperox-
ide lyases from mushrooms exhibit a different re-
action specificity. Linoleic acid, which predom-
inates in the lipids of champignon mushrooms,
is oxidatively cleaved to R(–)-1-octen-3-ol and
10-oxo-(E)-8-decenoic acid (cf. 3.7.2.3). The al-
lyl alcohol is oxidized to a small extent by at-
mospheric oxygen to the corresponding ketone.
Owing to an odor threshold that is about hun-
dred times lower (cf. Table 3.32), this ketone to-
gether with the alcohol accounts for the mush-
room odor of fresh champignons and of Camem-
bert.
Aldehydes formed by theStreckerdegradation
(cf. 5.3.1.1; Table 5.16) can also be obtained as
metabolic by-products of the enzymatic transam-
ination or oxidative deamination of amino acids.
First, the amino acids are converted enzymati-
cally toα-keto acids and then to aldehydes by de-
carboxylation in a side reaction:(5.22)Unlike other amino acids, threonine can eliminate
a water molecule and, by subsequent decarboxy-
lation, yield propanal:(5.23)Many aldehydes derived from amino acids occur
in plants and fermented food.