2009_Book_FoodChemistry

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-hydroxyacetophenone
(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-hydroxyacetophenone
(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 carbohydrate
degradation is the source of ethanal only.
Due to its aroma activity at higher concentrations
ethanal is of great importance for the fresh
note, e. g., in orange and grapefruit juice.
Linoleic and linolenic acids in fruits and vegetables
are subjected to oxidative degradation by
lipoxygenase alone or in combination with a hydroperoxide
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 oxygen.
A part of the aldehydes is enzymatically reduced
to the corresponding alcohols (see below).
In comparison, lipoxygenases and hydroperoxide
lyases from mushrooms exhibit a different reaction
specificity. Linoleic acid, which predominates
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 allyl
alcohol is oxidized to a small extent by atmospheric
oxygen to the corresponding ketone.
Owing to an odor threshold that is about hundred
times lower (cf. Table 3.32), this ketone together
with the alcohol accounts for the mushroom
odor of fresh champignons and of Camembert.
Aldehydes formed by the Strecker degradation
(cf. 5.3.1.1; Table 5.16) can also be obtained as
metabolic by-products of the enzymatic transamination
or oxidative deamination of amino acids.
First, the amino acids are converted enzymatically
to α-keto acids and then to aldehydes by decarboxylation
in a side reaction:
(5.22)
Unlike other amino acids, threonine can eliminate
a water molecule and, by subsequent decarboxylation,
yield propanal:
(5.23)
Many aldehydes derived from amino acids occur
in plants and fermented food.