398 5 Aroma Compounds
bly to the off-flavor formed on storage of
lemon juice. Citral is also the precursor of
p-cresol.
In citrus oils, limonene andγ-terpinene are also
attacked in the presence of light and oxygen. Car-
vone and a series of limonene hydroperoxides are
formed as the main aroma substances.
5.5.5 EncapsulationofAromas.................................
Aromas can be protected against the chemical
changes described in 5.5.4 by encapsulation.
Materials suitable for inclusion are polysaccha-
rides, e. g., gum arabic, maltodextrins, modified
starches, and cyclodextrins. The encapsulation
proceeds via spray drying, extrusion or formation
of inclusion complexes. For spray drying, the
aroma substances are emulsified in a solution
or suspension of the polysaccharide, which
contains solutizer in addition to the emulsifying
agent.
In preparation for extrusion, a melt of wall ma-
terial, aroma substances, and emulsifiers is pro-
duced. The extrusion is conducted in a cooled
bath, e. g., isopropanol.
β-Cyclodextrins, among other compounds,
can be used for the formation of inclusion
complexes (cf. 4.3.2). Together with the aroma
substances, they are dissolved in a water/ethanol
mixture by heating. The complexes precipitate
out of the cooled solution and are removed by
filtration and dried. Criteria for the evaluation
of encapsulated aromas are: stability of the
aroma, concentration of aroma substance, av-
erage diameter of the capsules and, amount of
aroma substance adhering to the surface of the
capsule.
(5.49)5.6 RelationshipsBetweenStructureandOdor
5.6.1 GeneralAspects
The effect of stimulants on the peripheral
receptors of an organism results in responses
that are characterized by theirqualityand their
intensity. The intensity is quantifiable, e. g., by
determining odor threshold values (cf. 5.1.3).
The quality can be described only by comparison.
Odor stimulants can be grouped into those of
the same or similar qualities, e. g., compounds
with a caramel-like odor (cf. 5.3.1.2 and 5.3.1.3)
or roasted smelling N-heterocyclic compounds
(cf. 5.3.1.6). The dependence of the odor thresh-
old on the structure is of great interest since the
specificity of the odor detection is the reason
why aroma substances are only a fraction of the
volatile compounds occurring in foods (cf. 5.3).
The specificity of the sense of smell will be
elucidated by using two classes of compounds
as an example. Studies have shown how the
odor thresholds in these classes change when the
structures are systematically varied. Only odor
thresholds in air are used because the influence
of a solvent or a solid carrier does not have to be
considered.5.6.2 Carbonyl Compounds
In the series of saturated aldehydes C 5 –C 10 ,the
odor threshold reaches a minimum with octanal
(Table 5.44). An E-configurated double bond in
the 2-position raises the odor threshold in the
case of the alkenals 5:1 to 8:1 compared with the
corresponding alkanals. (E)-2-Nonenal is the ex-