5.1.1 Concept
When
food is consumed, the interaction of taste, odor and textural feeling provides
an overall sensation which is best defined by the English word “flavor”. German
and some other languages do not have an adequate expression for such a broad
and comprehensive term. Flavor results from compounds that are divided into two
broad classes: Those responsible for taste and those responsible for
odors, the latter often designated as aroma substances. However, there are
compounds which provide both sensations.
Compounds
responsible for taste are generally nonvolatile at room temperature.
Therefore, they interact only with taste receptors located in the taste buds of
the tongue. The four important basic taste perceptions are provided by: sour,
sweet, bitter and salty compounds. They are covered in separate sections (cf.,
for example, 8.10, 22.3, 1.2.6, 1.3.3, 4.2.3 and 8.8). Glutamate stimulates the
fifth basic taste (cf. 8.6.1).
Aroma
substances are volatile compounds which are
perceived by the odor receptor sites of the smell organ, i. e. the olfactory
tissue of the nasal cavity. They reach the receptors when drawn in through the
nose (orthonasal detection) and via the throat after being released by chewing
(retronasal detection). The concept of aroma substances, like the concept of
taste substances, should be used loosely, since a compound might contribute to
the typical odor or taste of one food, while in another food it might cause a
faulty odor or taste, or both, resulting in an off-flavor.
5.1.2
Impact Compounds of Natural Aromas
The
amount of volatile substances present in food is extremely low (ca. 10–15 mg/kg). In general, however, they
comprise a large number of components.
Especially foods made by thermal processes, alone (e. g., coffee) or in
combination with a fermentation process (e. g., bread, beer, cocoa, or tea),
contain more than 800 volatile compounds. A great variety of compounds is often
present in fruits and vegetables as well.
All
the known volatile compounds are classified according to the food and the class
of compounds and published in a tabular compilation (Nijssen, L. M.
et al., 1999). A total of 7100 compounds in more than 450 foods are
listed in the 1999 edi-tion, which is also available as a database on the
internet.
Of
all the volatile compounds, only a limited number are important for aroma.
Compounds that are considered as aroma substances are primarily
those which are present in food in concen-trations higher than the odor and/or
taste thresh-olds (cf. “Aroma Value”, 5.1.4). Compounds with concentrations
lower than the odor and/or taste thresholds also contribute to aroma when
mix-tures of them exceed these thresholds (for ex-amples of additive effects,
see 3.2.1.1, 20.1.7.8, 21.1.3.4).
Among
the aroma substances, special attention is paid to those compounds that provide
the charac-teristic aroma of the food and are, consequently, called key
odorants (character impact aroma com-pounds). Examples are given in Table 5.1.
In
the case of important foods, the differentiation between odorants and the
remaining volatile com-pounds has greatly progressed. Important find-ings are
presented in the section on “Aroma” in the corresponding chapters.
Table
5.1. Examples of key odorants
Compound
|
Aroma
|
Occurrence
|
(R)-Limonene
|
Citrus-like
|
Orange
juice
|
(R)-1-p-Menthene-
|
Grapefruit-
|
Grapefruit
juice
|
8-thiol
|
like
|
|
Benzaldehyde
|
Bitter
|
Almonds,
|
almond-like
|
cherries,
plums
|
|
Neral/geranial
|
Lemon-like
|
Lemons
|
1-(p-Hydroxy-
|
Raspberry-
|
Raspberries
|
phenyl)-3-butanone
|
like
|
|
(raspberry ketone)
|
||
(R)-()-1-Octen-3-ol
|
Mushroom-
|
Champignons,
|
like
|
Camembert
|
|
cheese
|
||
(E,Z)-2,6-
|
Cucumber-
|
Cucumbers
|
Nonadienal
|
like
|
|
Geosmin
|
Earthy
|
Beetroot
|
trans-5-Methyl-2-
|
Nut-like
|
Hazelnuts
|
hepten-4-one
|
||
(filbertone)
|
||
2-Furfurylthiol
|
Roasted
|
Coffee
|
4-Hydroxy-2,5-
|
Caramel-
|
Biscuits,
|
dimethyl-3(2H)-
|
like
|
dark beer,
|
furanone
|
coffee
|
|
2-Acetyl-1-pyrroline
|
Roasted
|
White-bread
|
crust
|
||
Threshold Value
The
lowest concentration of a compound that is just enough for the recognition of
its odor is called the odor threshold (recognition threshold). The detection
threshold is lower, i. e., the concen-tration at which the compound is
detectable but the aroma quality still cannot be unambiguously established. The
threshold values are frequently determined by smelling (orthonasal value) and
by tasting the sample (retronasal value). With a few exceptions, only the
orthonasal values are given in this chapter. Indeed, the example of the
carbonyl compounds shows how large the difference between the ortho- and
retronasal thresholds can be (cf. 3.7.2.1.9).
Threshold
concentration data allow comparison of the intensity or potency of odorous
substances. The examples in Table 5.2 illustrate that great differences exist
between individual aroma com-pounds, with an odor potency range of several
or-ders of magnitude.
In
an example provided by nootkatone, an es-sential aroma compound of grapefruit
peel oil (cf. 18.1.2.6.3), it is obvious that the two enan-tiomers (optical
isomers) differ significantly in their aroma intensity (cf. 5.2.5 and 5.3.2.4)
and, occasionally, in aroma quality or character.
The
threshold concentrations (values) for aroma compounds are dependent on their vapor
pres-sure, which is affected by both temperature and medium. Interactions with other
odor-producing substances can result in a strong increase in the odor
thresholds. The magnitude of this effect is demonstrated in a model experiment
in which the odor thresholds of compounds in water were determined in the presence
and absence of 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HD3F). The results in
Table 5.3 show that HD3F does not influence the threshold value of
4-vinylguaiacol. However, the threshold values of the other odorants increase in the presence of
HD3F. This effect is the greatest in the case of β-damascenone, the threshold value being increased by a factor of
90. Other examples in this book which show that the odor threshold of a
compound increases when it is influenced by other odor-producing substances are
a comparison of the threshold values in water and beer (cf. Table 5.4) as well
as in water and in aqueous ethanol.
Compound
|
Threshold
value
|
(mg/l)
|
|
Ethanol
|
100
|
Maltol
|
9
|
Furfural
|
3.0
|
Hexanol
|
2.5
|
Benzaldehyde
|
0.35
|
Vanillin
|
0.02
|
Raspberry
ketone
|
0.01
|
Limonene
|
0.01
|
Linalool
|
0.006
|
Hexanal
|
0.0045
|
2-Phenylethanal
|
0.004
|
Methylpropanal
|
0.001
|
Ethylbutyrate
|
0.001
|
(+)-Nootkatone
|
0.001
|
(-)-Nootkatone
|
1.0
|
Filbertone
|
0.00005
|
Methylthiol
|
0.00002
|
2-Isobutyl-3-methoxypyrazine
|
0.000002
|
1-p-Menthene-8-thiol
|
0.00000002
|
Table 5.3. Influence
of 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HD3F) on the odor threshold of aroma
sub-stances in water
Compound
|
Threshold
|
value (µg/1)
|
Ratio
|
|
Ia
|
IIb
|
II to I
|
||
4-Vinylguaiacol
|
100
|
90
|
≈1
|
|
2,3-Butanedione
|
15
|
105
|
7
|
|
2,3-Pentanedione
|
30
|
150
|
5
|
|
2-Furfurylthiol
|
0.012
|
0.25
|
20
|
|
β-Damascenone
|
2
|
×10−3
|
0.18
|
90
|
a I, odor threshold of
the compound in water.
I, odor threshold of
the compound in water.
bII, odor threshold of the
compound in an aqueous HD3F solution having a concentration (6.75 mg/1, aroma value A = 115) as high as in a coffee drink.
Table 5.4. Comparison
of threshold values a in water and beer
Compound
|
Threshold (mg/kg) in
|
||
Water
|
Beer
|
||
n-Butanol
|
0.5
|
200
|
|
3-Methylbutanol
|
0.25
|
70
|
|
Dimethylsulfide
|
0.00033
|
0.05
|
|
(E)-2-Nonenal
|
0.00008
|
0.00011
|
|
5.1.4 Aroma Value
As already indicated, compounds with
high “aroma values” may contribute to the aroma of foods. The “aroma value” Ax
of a compound is calculated according to the definition:
The
examples presented in Fig. 5.1 show that the exponent n and, therefore, the
dependency of the odor intensity on the concentration can vary substantially.
Within a class of compounds, the range of variations is not very large, e. g., n
= 0.50−0.63
for the alkanals C4–C9.
In
addition, additive effects that are difficult to assess must also be
considered. Examinations of mixtures have provided preliminary information.
They show that although the intensities of com-pounds with a similar aroma note
add up, the in-tensity of the mixture is usually lower than the sum of the
individual intensities (cf. 3.2.1.1). For substances which clearly differ in
their aroma note, however, the odor profile of a mixture is composed of the
odor profiles of the components added together, only when the odor intensities
are approximately equal. If the concentration ratio is such that the odor
intensity of one component pre-dominates, this component then largely or
com-pletely determines the odor profile.
Examples
are (E)-2-hexenal and (E)-2-decenal which have clearly different odor profiles
(cf. Fig. 5.2 a and 5.2 f). If the ratio of the odor intensities is
approximately one, the odor notes of both aldehydes can be recognized in the
odor profile of the mixture (Fig. 5.2 d). But if the dominating odor intensity
is that of the decenal (Fig. 5.2 b), or of the hexenal (Fig. 5.2 e), that
particular note determines the odor profile of the mixture.
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