2009_Book_FoodChemistry

374 5 Aroma Compounds
Table 5.26. Precursors and sensory properties of amines
Amine Amino Odor
acid Quality Threshold (mg/l)
precursor
Water a Oil
2-Methylpropyl Val Fishy, amine-like, malty 8.0 48.3
2-Methylbutyl Ile Fishy, amine-like, malty 4.9 69.7
3-Methylbutyl Leu Fishy, amine-like, malty 3.2 13.7
2-Phenylethyl Phe Fishy, amine-like, honey-like 55.6 89.7
3-(Methylthio)propyl Met Fishy, amine-like, boiled potato 0.4 0.3
a pH 7.5.
reaction predominates. An especially odor
intensive amine, trimethylamine, is formed in the
degradation of choline (cf. 11.2.4.4.4).
5.3.1.9 Phenols
Phenolic acids and lignin are degraded thermally
or decomposed by microorganisms into phenols,
which are then detected in food. Some of these
compounds are listed in Table 5.27.
Smoke generated by burning wood (lignin pyrolysis)
is used for cold or hot smoking of meat and
fish products. This is a phenol enrichment process
since phenol vapors penetrate the meat or
fish muscle tissue. Also, some alcoholic beverages,
such as Scotch whiskey, and also butter have
low amounts of some phenols, the presence of
which is needed to roundoff their typical aromas.
Ferulic acid was identified as an important precursor
in model experiments. 4-Vinylguaiacol is
formed as the main product in pyrolysis, the secondary
products being 4-ethylguaiacol, vanillin
and guaiacol. To explain such a reaction which,
for example, accompanies the process of roasting
coffee or the kiln drying of malt, it has to be assumed
that thermally formed free radicals regulate
the decomposition pattern of phenolic acids
(cf., for example, heat decomposition of ferulic
acid, Fig. 5.26). In the pasteurization of orange
juice, p-vinyl-guaiacol can also be formed from
ferulic acid, producing a stale taste at concentrations
of 1 mg/kg.
5.3.2 Enzymatic Reactions
Aroma compounds are formed by numerous reactions
which occur as part of the normal meta-
Fig. 5.26. Thermal degradation of ferulic acid. 4-Vinylguaiacol
(I), vanillin (II), and guaiacol (III) (according
to Tressl et al., 1976)
bolism of animals, plants and microorganisms.
The enzymatic reactions triggered by tissue disruption,
as experienced during disintegration or
slicing of fruits and vegetables, are of particular
importance. Enzymes can also be involved indirectly
in aroma formation by providing the preliminary
stage of the process, e. g. by releasing