2009_Book_FoodChemistry

5.2 Aroma Analysis 345
Fig. 5.3. The cause of aroma defects in food
5.2 Aroma Analysis
The aroma substances consist of highly diversified
classes of compounds, some of them
being highly reactive and are present in food
in extremely low concentrations. The difficulties
usually encountered in qualitative and
quantitative analysis of aroma compounds are
based on these features. Other difficulties are
associated with identification of aroma compounds,
elucidation of their chemical structure
and characterization of sensory properties.
The results of an aroma analysis can serve as
an objective guide in food processing for assessing
the suitability of individual processing steps,
and for assessing the quality of raw material,
intermediate- and endproducts. In addition, investigation
of food aroma broadens the possibility of
food flavoring with substances that are prepared
synthetically, but are chemically identical to those
found in nature, i. e. the so-called “nature identical
flavors” (cf. 5.5).
The elucidation of the aroma of any food is carried
out stepwise; the following instrumental and
sensory analyses are conducted:
• Isolation of the volatile compounds
• Differentiation of the aroma substances from
the remaining components of the volatile fraction
by dilution analyses
• Concentration and identification
• Quantification and calculation of aroma values
• Simulation of the aroma on the basis of the
analytical results
• Omission experiments
5.2.1 Aroma Isolation
The amount of starting material must be selected
to detect even those aroma substances which are
present in very low concentrations (ppb range),
but contribute considerably to the aroma because
of still lower odor thresholds. The volatile compounds
should be isolated from food using gentle
methods because otherwise artifacts can easily be
produced by the reactions listed in Table 5.6.
Additional difficulties are encountered in foods
which retain fully-active enzymes, which can
alter the aroma. For example, during the homogenization
of fruits and vegetables, hydrolases split
the aroma ester constituents, while lipoxygenase,
together with hydroperoxide lyase, enrich the
aroma with newly-formed volatile compounds.
To avoid such interferences, tissue disintegration
is done in the presence of enzyme inhibitors,
e. g., CaCl 2 or, when possible, by rapid sample
preparation. It is useful in some cases to inhibit
enzyme-catalyzed reactions by the addition of
methanol or ethanol. However, this can result
in a change in aroma due to the formation of
esters and acetals from acids and aldehydes
respectively.