2009_Book_FoodChemistry

394 5 Aroma Compounds
Table 5.40. Types of aroma used
Aroma type
Citrus 20
Mint 15
Red fruits 11
Vanilla 10.5
Meat 10.5
Spices 8.5
Chocolate 8.5
Cheese 5.5
Nut 2.5
Others 8
a Approximate values.
Percentage (%) a
thus, designated as “natural aroma substances”.
The rest of the aroma compounds are synthetic,
but 99% of this portion is chemically identical to
their natural counterparts. Only 1% are synthetic
aroma compounds not found in nature.
5.5.1.1 Essential Oils
Essential (volatile) oils are obtained preferentially
by steam distillation of plants (whole
or parts) such as clove buds, nutmeg (mace),
lemon, caraway, fennel, and cardamon fruits
(cf. 22.1.1.1). After steam distillation, the essential
oil is separated from the water layer, clarified
and stored. The pressure and temperature used
in the process are selected to incur the least
possible loss of aroma substances by thermal
decomposition, oxidation or hydrolysis.
Many essential oils, such as those of citrus
fruits, contain terpene hydrocarbons which
contribute little to aroma but are readily autooxidized
and polymerized (“resin formation”).
These undesirable oil constituents (for instance,
limonene from orange oil) can be removed
by fractional distillation. Fractional distillation
is also used to enrich or isolate a single
aroma compound. Usually, this compound is
the dominant constituent of the essential oil.
Examples of single aroma compounds isolated
as the main constituent of an essential
oil are: 1,8-cineole from eucalyptus, 1(−)-
menthol from peppermint, anethole from anise
seed, eugenol from clove, or citral (mixture
of geranial and neral, the pleasant odorous
compounds of lemon or lime oils) from litseacuba.
5.5.1.2 Extracts, Absolues
When the content of essential oil is low in the raw
material or the aroma constituents are destroyed
by steam distillation or the aroma is lost by its
solubility in water, then the oil in the raw material
is recovered by an extraction process. Examples
are certain herbs or spices (cf. 22.1.1.1) and some
fruit powders. Hexane, triacetin, acetone, ethanol,
water and/or edible oil or fat are used as solvents.
Good yields are also obtained by using liquid
CO 2 . The volatile solvent is then fully removed
by distillation. The oil extract (resin, absolue) often
contains volatile aroma compounds in excess
of 10% in addition to lipids, waxes, plant pigments
and other substances extractable by the
chosen solvent. Extraction may be followed by
chromatographic or counter-current separation to
isolate some desired aroma fractions. If the solvent
used is not removed by distillation, the product
is called an extract. The odor intensity of
the extract, compared to the pure essential oil, is
weaker for aromatization purposes by a factor of
10 2 to 10 3 .
5.5.1.3 Distillates
The aroma constituents in fruit juice are more
volatile during the distillation concentration
process than is the bulk of the water. Hence,
the aroma volatiles are condensed and collected
(cf. 18.2.10). Such distillates yield highly
concentrated aroma fractions through further
purification steps.
5.5.1.4 Microbial Aromas
Cheese aroma concentrates offered on the market
have an aroma intensity at least 20-fold higher
than that of normal cheese. They are produced
by the combined action of lipases and Penicillium
roqueforti using whey and fats/oils of plant
origin as substrates. In addition to C 4 –C 10 fatty
acids, the aroma is determined by the presence of
2-heptanone and 2-nonanone.