2009_Book_FoodChemistry

346 5 Aroma Compounds
Table 5.6. Possible changes in aromas during the isolation of volatile compounds
Reaction
Enzymatic
1. Hydrolysis of esters (cf. 3.7.1)
2. Oxidative cleavage of unsaturated fatty acids (cf. 3.7.2.3)
3. Hydrogenation of aldehydes (cf. 5.3.2.1)
Non-enzymatic
4. Hydrolysis of glycosides (cf. 5.3.2.4 and 3.8.4.4)
5. Lactones from hydroxy acids
6. Cyclization of di-, tri-, and polyols (cf. 5.3.2.4)
7. Dehydration and rearrangement of tert-allyl alcohols
8. Reactions of thiols, amines, and aldehydes (cf. 5.3.1.4) in the
aroma concentrate
9. Reduction of disulfides by reductones from the Maillard reaction
10. Fragmentation of hydroperoxides
At the low pH values prevalent in fruit, nonenzymatic
reactions, especially reactions 4–7
shown in Table 5.6, can interfere with the
isolation of aroma substances by the formation
of artifacts. In the concentration of isolates from
heated foods, particularly meat, it cannot be
excluded that reactive substances, e. g., thiols,
amines and aldehydes, get concentrated to such
an extent that they condense to form heterocyclic
aroma substances, among other compounds
(Reaction 8, Table 5.6).
In the isolation of aroma substances, foods which
owe their aroma to the Maillard reaction should
not be exposed to temperatures of more than
50 ◦ C. At higher temperatures, odorants are
additionally formed, i. e., thiols in the reduction
of disulfides by reductones. Fats and oils contain
volatile and non-volatile hydroperoxides which
fragment even at temperatures around 40 ◦ C.
An additional aspect of aroma isolation not to be
neglected is the ability of the aroma substances to
bind to the solid food matrix. Such binding ability
differs for many aroma constituents (cf. 5.4).
The aroma substances present in the vapor space
above the food can be very gently detected by
headspace analysis (cf. 5.2.1.3). However, the
amounts of substance isolated in this process
are so small that important aroma substances,
which are present in food in very low concentrations,
give no detector signal after gas
chromatographic separation of the sample. These
substances can be determined only by sniffing
the carrier gas stream. The difference in the
detector sensitivity is clearly shown in Fig. 5.4,
taking the aroma substances of the crust of white
bread as an example. The gas chromatogram
does not show, e. g., 2-acetyl-1-pyrroline and
2-ethyl-3,5-dimethylpyrazine, which are of great
importance for aroma due to high FD factors
in the FD chromatogram (definition in 5.2.2.1).
These aroma substances can be identified only
after concentration from a relatively large amount
of the food.
5.2.1.1 Distillation, Extraction
The volatile aroma compounds, together with
some water, are removed by vacuum distillation
from an aqueous food suspension. The
highly volatile compounds are condensed in
an efficiently cooled trap. The organic compounds
contained in the distillate are separated
from the water by extraction or by adsorption
to a hydrophobic matrix and reversed phase
chromatography and then prefractionated.
The apparatus shown in Fig. 5.5 is recommended
for the gentle isolation of aroma substances from
aqueous foods by means of distillation. In fact,
a condensate can be very quickly obtained because
of the short distances. As in all distillation
processes, the yield of aroma substances decreases
if the food or an extract is fatty (Table
5.7).
After application of high vacuum (≈5mPa) the
distillation procedure is started by dropping the