Structure-Odor Relationships of (Z)-3-Alken-1-ols, (Z)-3-Alkenals, and (Z)-3-Alkenoic Acids

Article abstract of DOI:10.1021/acs.jafc.6b04780

(Z)-3-Unsaturated volatile acids, alcohols, and aldehydes are commonly found in foods and other natural sources, playing a vital role in the attractiveness of foods but also as compounds with chemocommunicative function in entomology. However, a systematic investigation of their smell properties, especially regarding humans, has not been carried out until today. To close this gap, the odor thresholds in air and odor qualities of homologous series of (Z)-3-alken-1-ols, (Z)-3-alkenals, and (Z)-3-alkenoic acids were determined by gas chromatography-olfactometry. It was found that the odor qualities in the series of the (Z)-3-alken-1-ols and (Z)-3-alkenals changed, with increasing chain length, from grassy, green to an overall fatty and citrus-like, soapy character. On the other hand, the odor qualities of the (Z)-3-alkenoic acids changed successively from cheesy, sweaty via plastic-like, to waxy in their homologous series. With regard to their odor potencies, the lowest thresholds in air were found for (Z)-3-hexenal, (Z)-3-octenoic acid, and (Z)-3-octenal.

Full text of DOI:10.1021/acs.jafc.6b04780

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Article
Structure-Odor Relationships of (Z)-3-Alken-1-
ols, (Z)-3-Alkenals, and (Z)-3-Alkenoic acids
Katja Lorber, Gina Zeh, Johanna Regler, and Andrea Buettner
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b04780 • Publication Date (Web): 23 Dec 2016
Downloaded from http://pubs.acs.org on December 28, 2016
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Page 1 of 50
Journal of Agricultural and Food Chemistry
StructureꢀOdor Relationships of (Z)ꢀ3ꢀAlkenꢀ1ꢀols, (Z)ꢀ
3ꢀAlkenals, and (Z)ꢀ3ꢀAlkenoic acids
†‡
†
†
†‡*
Katja Lorber , Gina Zeh , Johanna Regler , Andrea Buettner
†
Professorship of Aroma Research, Department of Chemistry and Pharmacy, Emil
Fischer Center, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Henkestr. 9,
9
1054 Erlangen, Germany, andrea.buettner@fau.de
‡
Department of Sensory Analytics, Fraunhofer Institute for Process Engineering and
Packaging
(IVV),
Giggenhauser
Str.
35,
85354
Freising,
Germany,
andrea.buettner@ivv.fraunhofer.de
*
Address for correspondence
Phone +49ꢀ9131ꢀ85ꢀ22739
Eꢀmailandrea.buettner@fau.de
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Abstract
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(Z)ꢀ3ꢀUnsaturated volatile acids, alcohols, and aldehydes are commonly found in foods
and other natural sources, playing a vital role in the attractiveness of foods but also as
compounds with chemoꢀcommunicative function in entomology. However, a systematic
investigation of their smell properties, especially regarding humans, has not been
carried out until today. To close this gap, the odor thresholds in air and odor qualities of
homologous series of (Z)ꢀ3ꢀalkenꢀ1ꢀols, (Z)ꢀ3ꢀalkenals, and (Z)ꢀ3ꢀalkenoic acids were
determined by gas chromatographyꢀolfactometry. It was found that the odor qualities in
the series of the (Z)ꢀ3ꢀalkenꢀ1ꢀols and (Z)ꢀ3ꢀalkenals changed, with increasing chain
length, from grassy, green to an overall fatty and citrusꢀlike, soapy character. On the
other hand, the odor qualities of the (Z)ꢀ3ꢀalkenoic acids changed successively from
cheesy, sweaty via plasticꢀlike to waxy in their homologous series. With regard to their
odor potencies, the lowest thresholds in air were found for (Z)ꢀ3ꢀhexenal, (Z)ꢀ3ꢀoctenoic
acid, and (Z)ꢀ3ꢀoctenal.
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Keywords
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gas chromatographyꢀolfactometry; odor threshold in air; odorant; chemoꢀ
communicative; retention index; odor activity; odor intensity
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Introduction
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From the vast number of odorꢀactive compounds, a number of fatty acidꢀderived
odorants are formed as products during lipid oxidation, mostly autoxidation or enzymatic
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oxidation of linoleic or linolenic acid, but also by other biosynthetic pathways.
Corresponding to the (E)ꢀ3ꢀalkenoic acids, (E)ꢀ3ꢀalkenꢀ1ꢀols, and (E)ꢀ3ꢀalkenals that
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have been described in our previous study, the corresponding (Z)ꢀ3ꢀalkenoic acids, (Z)ꢀ
3ꢀalkenꢀ1ꢀols, and (Z)ꢀ3ꢀalkenals are commonly found as odorants or chemoꢀ
communicative substances for humans and other species in diverse foods and other
natural sources. (Z)ꢀ3ꢀHeptenoic acid, (Z)ꢀ3ꢀoctenoic acid, (Z)ꢀ3ꢀnonenoic acid, (Z)ꢀ3ꢀ
decenoic acid, (Z)ꢀ3ꢀundecenoic acid, and (Z)ꢀ3ꢀdodecenoic acid are volatile
decomposition products of various triglycerides like trilinolein and triolein, or oils (corn
oil, hydrogenated cottonseed oil) built during thermal oxidization by (simulated) deepꢀfat
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frying.
Further, (Z)ꢀ3ꢀhexenꢀ1ꢀol is a bioconversion product of (E)ꢀ3ꢀhexenoic acid,
and (Z)ꢀ3ꢀoctenꢀ1ꢀol of (E)ꢀ3ꢀoctenoic acid by the highly metabolically active fungus
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Botrytis cinerea. In previous studies, the odor of (Z)ꢀ3ꢀhexenꢀ1ꢀol has been described
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as herbal/green , green/cut grass , or green leaves-like , while (Z)ꢀ3ꢀoctenꢀ1ꢀol has
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been reported to elicit toasted nut/smoky/dusty or pungent/rancid odor qualities. A
number of the compounds investigated in this study have previously been identified in
diverse food materials, often playing a vital role in their attractiveness: (Z)ꢀ3ꢀpentenꢀ1ꢀol
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has been detected in black tea and lamb’s lettuce . (Z)ꢀ3ꢀHexenꢀ1ꢀol was, among
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others, identified in lamb’s lettuce , different melon types
, and oyster leaf . (Z)ꢀ3ꢀ
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Octenꢀ1ꢀol is, for example, part of the aroma profiles of banana and chili pepper .
With the exception of (Z)ꢀ3ꢀundecenꢀ1ꢀol and (Z)ꢀ3ꢀdodecenꢀ1ꢀol, all compounds in the
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homologous series of the (Z)ꢀ3ꢀalkenꢀ1ꢀols have been identified in food. The same
holds true for the series of (Z)ꢀ3ꢀalkenals whereas (Z)ꢀ3ꢀundecenal and (Z)ꢀ3ꢀdodecenal
have not yet been reported as food constituents. Just a few examples of the (Z)ꢀ3ꢀ
alkenals reported so far are highlighted here: (Z)ꢀ3ꢀpentenal was identified in black
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tea , (Z)ꢀ3ꢀhexenal in lamb’s lettuce , and oyster leaf , in all cases accompanied by
(Z)ꢀ3ꢀhexenꢀ1ꢀol. Moreover, (Z)ꢀ3ꢀhexenal could be identified as important odorant in
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fresh squeezed citrus juices,
but also as constituent of an offꢀodor in German
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rainbow trout. (Z)ꢀ3ꢀHeptenal, (Z)ꢀ3ꢀoctenal, (Z)ꢀ3ꢀnonenal, and (Z)ꢀ3ꢀdecenal have
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been detected in peanut butter oil. Most of the (Z)ꢀ3ꢀalkenoic acids have also been
reported before but not in such a large variety of foods as the (Z)ꢀ3ꢀalkenꢀ1ꢀols and (Z)ꢀ
3ꢀalkenals. For instance, (Z)ꢀ3ꢀhexenoic acid has shown to be part of the volatile
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spectrum of different types of tea.
These are just a few examples of the occurrence
of these structurally related compounds in food. For the interested reader, these and
several more examples together with their reported odor qualities and odor threshold
values are given in the supporting information.
Apart from that, two of the acids have been identified in nonꢀfood matrices: (Z)ꢀ3ꢀ
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pentenoic acid is a volatile in cigarette smoke , and (Z)ꢀ3ꢀhexenoic acid in the absolute
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of Gardenia jasminoides Ellis .
It is possible, maybe even likely, that compounds from these homologous series exist in
nature even if they have not yet been reported. Their current lack of detection might be
due to their potentially low concentrations, instability, or the difficulty of extraction, even
if they might be highly potent odorants.
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Besides their occurrence in food, consumables or plants, some of these compounds
also play a role in entomology, comparable to the corresponding (E)ꢀ3ꢀalkene
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compounds as we reported in our previous study. (Z)ꢀ3ꢀHexenꢀ1ꢀol and (Z)ꢀ3ꢀhexenal
are part of the defensive secretion from male Florida woods cockroaches, Eurycotis
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floridana, and the secretion of the ventral glands in Nematus sawfly larvae, as
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protection from predation. (Z)ꢀ3ꢀHexenal is a plant attractant that stimulates the
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antennae of male fall webworm moths, Hyphantria cunea. (Z)ꢀ3ꢀOctenal was also
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detected in the already mentioned ventral glands of Nematus sawfly larvae and,
likewise, in the ventral glands of the Cladius, Priophorus and Trichiocampus sawfly
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larvae. (Z)ꢀ3ꢀDecenoic acid is one compound of the anal secretion from different
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species of idolothripine thrips acting as a repellent. Further, it has been reported as
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sex pheromone in the furniture carpet beetle, Anthrenus flavipes LECONTE.
Despite the fact that various compounds from the homologous series of (Z)ꢀ3ꢀalkenꢀ1ꢀ
ols, (Z)ꢀ3ꢀalkenals, and (Z)ꢀ3ꢀalkenoic acids have been identified in foods, plants, or
insects, there has been no systematic investigation of their odor properties in terms of
their odor thresholds and smell qualities. In view of these important analytical
parameters, only a few data have been published so far (Table 1 AꢀC and Table 2 Aꢀ
C). Therefore, the aim of this work was to provide comprehensive sensory and
analytical data on these compounds to support future research on these important
substance classes. Further, odor qualities and odor properties of the corresponding
saturated compounds were determined, to provide a comprehensive understanding of
the impact of the characteristic (Z)ꢀ3ꢀmoiety on relation to the chain length and oxidation
status of the target substances, and to establish a comprehensive substance library
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comprising their analytical and sensory characteristics. This offers the possibility to
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compare the previously published data related to the (E)ꢀ3ꢀalkene compounds, with
data on the (Z)ꢀ3ꢀalkene compounds and their saturated analoga of the present
investigation.
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Materials and Methods
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Chemicals. Lindlar’s catalyst, quinoline, nꢀbutyl lithium solution (2.7 M in THF),
ethylene oxide, hexamethylphosphoramide (HMPA), nonyne, decyne, 3ꢀdodecynꢀ1ꢀol,
calcium carbonate, chromium trioxide, periodic acid, hydrochloric acid, diethyl ether,
sodium sulfate dibasic, sodium bisulfite, sodium chlorite, magnesium sulfate, Dessꢀ
Martin periodinane, sodium bicarbonate, acetonitrile, toluene, tetrahydrofurane
anhydrous, and sodium thiosulfate were purchased from SigmaꢀAldrich (Steinheim,
Germany). Silica gel (Normasil 60, 40 – 63 ꢁm), sodium chloride, dichloromethane, nꢀ
hexane, and ethyl acetate were purchased from VWR International GmbH (Darmstadt,
Germany). (Z)ꢀ3ꢀPentenꢀ1ꢀol was purchased from abcr (Karlsruhe, Germany), (Z)ꢀ3ꢀ
hexenꢀ1ꢀol, (Z)ꢀ3ꢀheptenꢀ1ꢀol and (Z)ꢀ3ꢀnonenꢀ1ꢀol were from SigmaꢀAldrich (Steinheim,
Germany), and (Z)ꢀ3ꢀoctenꢀ1ꢀol from TCI Europe (Zwijndrecht, Belgium). All chemicals
were used without further purification.
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Nuclear Magnetic Resonance (NMR) Spectra. H and C NMR spectra were
recorded in CDCl on an Avance 360 spectrometer, 360 MHz, and an Avance 600
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spectrometer, 600 MHz (Bruker Biospin, Rheinstetten, Germany), at room temperature
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operated at 360 or 600 MHz ( H) and 90 or 150 MHz ( C), with tetramethylsilane (TMS)
as internal standard.
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Preparative HighꢀPerformance Liquid Chromatography (PrepꢀHPLC). The
preparative HPLC separation (AS 2057plus autosampler, two PU 2087plus gradient
pumps, a mixing chamber, DG 98050 degasser, and UV 2077plus UV/vis detector, all
by JASCO, GrossꢀUmstadt, Germany) was performed using a C18ꢀmaterial preparative
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column (Nucleodur 100ꢀ5 C18ec, 10 × 250 mm, 5 ꢂm particle size, with inlineꢀfilter,
MachereyꢀNagel, Düren, Germany) with gradient elution program (A: acetonitrile, B:
water; flow rate 3 mL/min, gradient A/B: 20:80 to 100:0 in 60 min) and detection by UV
at 254, 280, and 300 nm. Fractions from several runs containing the target compound
were collected manually, combined, dried over MgSO , and evaporated.
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GCꢀFID, GCꢀOlfactrometry (GCꢀO) and GCꢀElectron ImpactꢀMass Spectrometry
(GCꢀEIꢀMS). GCꢀFID and GCꢀO analyses were performed with a Trace GC Ultra
(Thermo Fisher Scientific GmbH, Dreieich, Germany) by using the following capillaries:
FFAP (30 m x 0.32 mm i.d. fused silica capillary, free fatty acid phase FFAP, 0.25 ꢁm;
Chrompack, Mühlheim, Germany) and DB5 (30 m x 0.32 mm i.d. fused silica capillary
DBꢀ5, 0.25 ꢁm; J & W Scientific, Fisons Instruments). The samples were applied by the
coolꢀonꢀcolumn injection technique at 40 °C. After 2 minutes, the temperature of the
oven was raised at 10 °C/min to 240 °C, then raised at 40 °C/min to 280 °C (DB5), or at
10 °C/min to 240 °C (FFAP), respectively, and held for 5 minutes. The flow rate of the
carrier gas helium was 2.5 mL/min. At the end of the capillary, the effluent was split in a
ratio 1:1 (by volume) into an FID and a sniffing port using two deactivated but uncoated
fused silica capillaries (50 cm x 0.32 mm). The FID and the sniffing port were held at
250 °C, respectively. GCꢀEIꢀMS analyses were performed with an Agilent MSD 5975C
(Agilent Technologies, Waldbronn, Germany) and a Thermo ITQ 900 (Thermo Fisher
Scientific, Dreieich, Germany) with the capillaries described above, using the same
temperature programs as for the GCꢀFID and GCꢀO measurements, with a flow rate of
the carrier gas helium of 1.0 mL/min. Mass spectra in the electron impact mode (EIꢀMS)
were generated at 70 eV.
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Retention indices (RI). Retention indices were determined according to the method
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previously described by Van den Dool and Kratz (1963).
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Panelists. Panelists were trained volunteers from the University of ErlangenꢀNürnberg
(Erlangen, Germany), exhibiting no known illness at the time of examination and with
audited olfactory function. In preceding weekly training sessions, the assessors were
trained for at least half a year in recognizing orthonasally about 150 selected known
odorants at different concentrations according to their odor qualities, and in naming
these according to an inꢀhouse developed flavor language. Furthermore, the panel was
trained every two weeks on specific attributes with the help of specifically developed
sniffing sticks; in the course of this training, all panelists also had to fill the same
questionnaire (hedonic, intensity) to obtain insights into their specific sensitivities or
insensitivities which were systematically recorded. Based on these tests, panelists were
regularly tested if they complied with the established flavor language.
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Odor threshold values. Thresholds in air were determined by GCꢀO with (E)ꢀ2ꢀdecenal
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as internal standard.
Of every dilution, 2 ꢁL were applied for injection into the GC
system. The thresholds were determined by five panelists (one male, four female), with
each experiment being conducted once. GC analyses were performed on capillary
FFAP as described above. The purity of all commercially available and synthesized
compounds was taken into account in the GCꢀO experiments. All synthesized
compounds were further checked for potential olfactorily active impurities by sniffing
each single substance on both capillaries of different polarity, to exclude any
interferences.
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Odor quality determination. The odor qualities, determined during GCꢀO evaluation,
were related in comparison to odor qualities of commercially available reference
compounds. Thereby, panelists were asked to freely choose the respective odor quality
descriptors based on the inꢀhouse developed flavor language that is related to these
references (cf. panelists). No additional descriptors were supplied to the panelists. The
panelists determined the qualities during sniffing of the solution corresponding to FD 1
(injection of 2 ꢁL). The panelists were instructed to record any changes in odor qualities
in all following dilutions.
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67 Syntheses, general procedures:
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3ꢀAlkynꢀ1ꢀols (Figure 1 A). nꢀButhyllithium (2.7 M in hexane, 1 eq) was injected into a
cooled solution (≤ 0 °C) of the corresponding 1ꢀalkyne (1 eq) in dry THF (approx. 10
mL/12 mmol alkyne) under nitrogen atmosphere. Ethylene oxide (1.2 eq) was injected
from a precooled syringe, and then HMPA (ca. 2 eq) was injected. The resulting mixture
was stirred over night at room temperature and then worked up by dilution with water
and extraction with hexane. After evaporation of the solvent, the crude product was
purified by column chromatography (silica gel, eluent: hexane/EtOAc = 4/1) to give the
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pure 3ꢀalkynꢀ1ꢀol as a colorless to pale yellow oily liquid.
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(Z)ꢀ3ꢀAlkenꢀ1ꢀols (6 to 8, Figure 1 B). 3ꢀAlkynꢀ1ꢀol was added to a slurry of Lindlar’s
catalyst (5% Pd on CaCO poisoned with Pb) in pentane and quinoline at room
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temperature. The reaction flask was swept with H gas, evacuated three times and
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stirred under H atmosphere overnight. The reaction mixture was filtered through a plug
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of CaCO and rinsed with Et O twice. The filtrate was partitioned against 1.0 M HCl
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saturated with NaCl, washed with saturated brine and dried over MgSO . After filtration
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and evaporation of the solvent the crude product was purified by column
chromatography (silica gel, eluent: hexane/EtOAc = 4/1) to give the pure (Z)ꢀ3ꢀalkenꢀ1ꢀ
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ol as a colorless oily liquid.
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(Z)ꢀ3ꢀAlkenals (9 to 16, Figure 1 C). A solution of (Z)ꢀ3ꢀalkenꢀ1ꢀol (1 eq) in CH Cl (1
2 2
mL/mmol alcohol) was added dropwise to a suspension of DessꢀMartin periodinane (1.1
eq) in CH Cl (2 mL/mmol DessꢀMartin periodinane). After a few minutes, in some cases
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the reaction mixture started to boil and was allowed to do so for about five minutes. The
obtained suspension was stirred for three hours at room temperature. It was then
filtered through a glass frit and the filtrate was washed with saturated aqueous NaHCO3
solution containing Na S O (25%) (3.5 mL/mmol alcohol). The resulting clear solution
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was dried over MgSO , filtered and the solvent was removed under reduced pressure to
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give the corresponding (Z)ꢀ3ꢀalkenal as a colorless to pale yellow oily liquid. (Z)ꢀ3ꢀ
Decenal and (Z)ꢀ3 dodecenal were purified by preparative HPLC.
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(Z)ꢀ3ꢀAlkenoic acids (17, 22 to 24, Figure 1 D). By dissolving H IO (11.4 g, 50 mmol),
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and CrO (23 mg, 1.2 mol %) in wet MeCN (0.75 v % water) to a volume of 114 mL a
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stock solution of H IO /CrO was prepared (complete dissolution took 1 – 2 hours). The
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H IO /CrO solution (11.4 mL) was added to a solution of the (Z)ꢀ3ꢀalkenꢀ1ꢀol 1, 6, 7 or 8
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(2.0 mmol) in wet MeCN (10 mL, 0.75 v % water) over 30 to 60 minutes while
maintaining the reaction temperature at 0ꢀ5 °C. The mixture was aged at 0 °C for 30
minutes and quenched by adding an aqueous solution of Na HPO (0.6 g in 10 mL
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H O). Toluene (15 mL) was added, the organic layer was separated, washed with 10
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mL of a 1/1 brine/water mixture twice, then with 5 mL of an aqueous NaHSO (0.22 mg
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in 5 mL water) once, and finally with 5 mL of brine once. The solvent was removed
under reduced pressure to give the corresponding (Z)ꢀ3ꢀalkenoic acid 17, 22, 23 or 24
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as a colorless oily liquid or solid.
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(Z)ꢀ3ꢀAlkenoic acids (18 to 21, Figure 1 D). To a solution of (Z)ꢀ3ꢀalkenal (1 eq) in
DMSO (5 eq) an aqueous solution of NaClO (1.1 to 1.2. eq) with a pH of 4.3 (set with
2
NaH PO ) was added in portions and stirred over night at room temperature. The two
2
4
layers were extracted with EtOAc thrice, and the combined organic layers were washed
with brine. The resulting clear solution was dried over anhydrous MgSO . After
4
evaporation of the solvent the residue was purified by column chromatography (silica
gel, eluent: hexane/EtOAc = 4/1) to give the pure (Z)ꢀ3ꢀalkenoic acid as a colorless oily
38ꢀ40
liquid.
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15 Results and Discussion
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The 3ꢀalkynꢀ1ꢀols that were required as intermediates for the subsequent synthetic
steps were successfully obtained following to the method reported by Heath et al. 1988
34, 35
and Argenti et al. 2006.
The (Z)ꢀ3ꢀalkenꢀ1ꢀols were generated therefrom by
selective hydrogenation of the corresponding 3ꢀalkynꢀ1ꢀol with poisoned Lindlar’s
catalyst, and the corresponding (Z)ꢀ3ꢀalkenals were obtained via mild oxidation of the
35, 36
corresponding (Z)ꢀ3ꢀalkenꢀ1ꢀols with DessꢀMartin periodinane as oxidizing agent.
The respective (Z)ꢀ3ꢀalkenoic acids were synthesized following two different methods.
Compounds 17 and 22 to 24 were obtained by oxidation with H IO /CrO in wet MeCN
5
6
3
37
according to Zhao et al. 1998, whereas compounds 18 to 21 were synthesized
38ꢀ40
applying a Pinnick oxidation reaction (Figure 1 AꢀC).
The synthesized compounds were then used to determine their retention indices, and
their mass spectrometric and NMR data, thereby unambiguously confirming their
chemical identity. Following the analytical characterization, the individual odor threshold
values in air were determined by a panel using GCꢀO, thereby yielding the median odor
threshold of the panel as well as the odor threshold distribution amongst the group, and
the main odor qualities of the homologous (Z)ꢀ3ꢀalkenꢀ1ꢀols, (Z)ꢀ3ꢀalkenals, and (Z)ꢀ3ꢀ
alkenoic acids; all data are compiled in Table 1 and Figure 2.
As in our previously published study on the structure–odor relationships of the related
(E)ꢀ3ꢀalkenoic acids, (E)ꢀ3ꢀalkenꢀ1ꢀols, and (E)ꢀ3ꢀalkenals, the geometric mean values
3
are given in addition to the median odor threshold values reported in Table 3. In the
following, however, only the median values will be discussed. In view of this it is
important to note that strong deviations between the single values would influence the
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geometric mean more significantly than the median. However, when comparing the
median and geometric mean values, it becomes evident that both values showed the
same tendencies in the present study, and that the differences between both values
were generally low, if any.
When evaluating the odor qualities of the investigated compounds it was found that the
main odor quality in the series of (Z)ꢀ3ꢀalkenꢀ1ꢀols was grassy-green for (Z)ꢀ3ꢀpentenꢀ1ꢀ
ol, (Z)ꢀ3ꢀhexenꢀ1ꢀol and (Z)ꢀ3ꢀheptenꢀ1ꢀol, then changing to fatty for (Z)ꢀ3ꢀoctenꢀ1ꢀol,
(Z)ꢀ3ꢀnonenꢀ1ꢀol and (Z)ꢀ3ꢀdecenꢀ1ꢀol, and finally to soapy, citrus-like for (Z)ꢀ3ꢀ
undecenꢀ1ꢀol and (Z)ꢀ3ꢀdodecenꢀ1ꢀol (Table 1 AꢀC).
The (Z)ꢀ3ꢀalkenals revealed very similar main odor qualities and the smell changes
followed a comparable pattern with increasing chain length: starting with grassy, green
for (Z)ꢀ3ꢀpentenal and (Z)ꢀ3ꢀhexenal, changing to fatty for (Z)ꢀ3ꢀheptenal, (Z)ꢀ3ꢀoctenal,
(Z)ꢀ3ꢀnonenal and (Z)ꢀ3ꢀdecenal, and finally reaching citrus-like, soapy impressions for
(Z)ꢀ3ꢀundecenal and (Z)ꢀ3ꢀdodecenal.
In contrast to this, the (Z)ꢀ3ꢀalkenoic acids were associated with odor qualities that are
typical for fatty acids: (Z)ꢀ3ꢀpentenoic acid, (Z)ꢀ3ꢀhexenoic acid and (Z)ꢀ3ꢀheptenoic acid
were found to mainly elicit a sweaty odor impression. With increasing chain length, a
plastic-like note developed for (Z)ꢀ3ꢀoctenoic acid and (Z)ꢀ3ꢀnonenoic acid, whereas (Z)ꢀ
3ꢀdecenoic acid, (Z)ꢀ3ꢀundecenoic acid and (Z)ꢀ3ꢀdodecenoic acid were primarily
described as smelling waxy and acidic.
Nevertheless, some variations were observed in the individual naming of odor attributes
between panelists (Table 2 AꢀC). (Z)ꢀ3ꢀdodecenꢀ1ꢀol, for example, was described as
citrus-like and soapy, but also musty and earthy, metallic, or pungent, tangy. This
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variance in selection of descriptors was especially pronounced in the group of (Z)ꢀ3ꢀ
alcohols with smell attributes ranging from grassy, citrus-like to waxy, metallic, or
pungent.
In contrast to the alcohols, the selected odor qualities of the (Z)ꢀ3ꢀalkenals were much
more consistent as most panelists reported the terms grassy, green for (Z)ꢀ3ꢀpentenal
and (Z)ꢀ3ꢀhexenal, fatty for (Z)ꢀ3ꢀheptenal and (Z)ꢀ3ꢀoctenal, and fatty, citrus-like for (Z)ꢀ
3ꢀnonenal. However, the attributes named for (Z)ꢀ3ꢀdecenal, (Z)ꢀ3ꢀundecenal and (Z)ꢀ3ꢀ
dodecenal differed to some extent. These compounds were reported as smelling citrus-
like, soapy, fresh but also metallic-waxy.
The (Z)ꢀ3ꢀalkenoic acids were perceived as acidic, sweaty or cheesy in case of the
shortꢀchain (Z)ꢀ3ꢀalkenoic acids changing to sweaty, plastic-like or waxy with increasing
chain length. Interestingly, panelist 3 consistently reported fatty impressions for all
acids, accompanied by soapy for (Z)ꢀ3ꢀpentenoic, (Z)ꢀ3ꢀoctenoic, (Z)ꢀ3ꢀdecenoic and
(Z)ꢀ3ꢀundecenoic acid, or green for (Z)ꢀ3ꢀheptenoic, (Z)ꢀ3ꢀoctenoic and (Z)ꢀ3ꢀ
dodecenoic acid.
When regarding the odor thresholds, the lowest median value was determined for (Z)ꢀ3ꢀ
hexenal with 0.27 ng/L , followed by (Z)ꢀ3ꢀoctenoic acid with 0.42 ng/L , (Z)ꢀ3ꢀoctenal
air
air
with 0.93 ng/L , and (Z)ꢀ3ꢀheptenoic acid with 0.94 ng/L . Overall, comparison of all
air
air
three homologous series shows, that the (Z)ꢀ3ꢀalkenals were the compounds with the
lowest median odor threshold values, ranging between 0.27 and 2.32 ng/L . However,
air
for the alcohol and acid derivatives, ranging between 9.5 and 191 ng/L , or 0.42 and 77
air
ng/L , respectively, no clear trend is observable: for the compounds containing five
air
carbon atoms, (Z)ꢀ3ꢀpentenoic acid (median OT 21 ng/L ) showed a lower value by a
air
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factor of about 3 than (Z)ꢀ3ꢀpentenꢀ1ꢀol (median OT 57 ng/L ), whereas it was the other
air
way around for the C6ꢀderivatives. On the other hand, (Z)ꢀ3ꢀheptenoic acid (median OT
0.94 ng/L ) and (Z)ꢀ3ꢀoctenoic acid (median OT 0.42 ng/L ) showed significantly lower
air
air
median values than the corresponding (Z)ꢀ3ꢀalkenꢀ1ꢀols, and somewhat lower median
OTs than the related aldehydes (Z)ꢀ3ꢀheptenal (1.08 ng/L ) and (Z)ꢀ3ꢀoctenal (0.93
air
ng/Lair).
Regarding the individual odor threshold values, the extent of the spreading of the
individual OT values was inconsistent for substances of the same substance class: as
an example, the OT ranges of (Z)ꢀ3ꢀpentenꢀ1ꢀol and (Z)ꢀ3ꢀhexenꢀ1ꢀol were notably
narrower with a factor of 16 between their minimum and maximum threshold values of
both substances than those of the related (Z)ꢀ3ꢀheptenꢀ1ꢀol or (Z)ꢀ3ꢀoctenꢀ1ꢀol
comprising a factor of 64 (Table 1 A, Table 3 A and Figure 2 A). For (Z)ꢀ3ꢀhexenꢀ1ꢀol,
it is especially interesting to note that the individual odor threshold values were
distributed in a relatively narrow range in comparison to all other compounds of the
homologous series of (Z)ꢀ3ꢀalkenꢀ1ꢀols (lowest individual odor threshold value for four of
five panelists), potentially indicating a somewhat exceptional biological meaning of this
substance. This observation might be related to the fact that this compound is
commonly produced in the course of lipid oxidation processes, for example when plant
tissue is disrupted. Likewise, a second narrow distribution of the individual threshold
values was observed for (Z)ꢀ3ꢀnonenꢀ1ꢀol, which is potentially also linked to its
preferential formation in nature. Further, regarding the quotient of highest and lowest
value, (Z)ꢀ3ꢀheptenꢀ1ꢀol and (Z)ꢀ3ꢀoctenꢀ1ꢀol showed the highest factor of 64 (cf. Table
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3 A), thus the broadest variance in individual threshold levels, followed by (Z)ꢀ3ꢀ
undecenꢀ1ꢀol and (Z)ꢀ3ꢀdecenꢀ1ꢀol with factors of 33 and 32, respectively.
In contrast to that, much more consistent data were obtained for the individual panelists
when regarding the (Z)ꢀ3ꢀalkenals with a variance that was commonly only in the range
of approximately a factor 8 to 16 between extremes (Table 1 B and Figure 2 B).
Nevertheless, the spreading of the values followed a comparable pattern, albeit in a
narrower range than for the corresponding alcohols. Highly reproducible values could
be observed for the individual OT values of (Z)ꢀ3ꢀhexenal, with a factor of only 4
between minimum and maximum value. Further, (Z)ꢀ3ꢀdodecenal showed a threshold
range from 0.03 ng/L to 1.06 ng/L which corresponds to a factor of 35 (Table 3 B),
air
air
spanning two orders of magnitude.
While the single values of the (Z)ꢀ3ꢀalkenals from all panelists were quite reproducible,
single values of some of the (Z)ꢀ3ꢀalkenoic acids varied to a higher extent between the
lowest and the highest individual threshold value (Table 1 C, Table 3 C and Figure 2
C). In case of (Z)ꢀ3ꢀheptenoic acid and (Z)ꢀ3ꢀoctenoic acid, the minimum and maximum
values even spanned 6 and 5 orders of magnitude, respectively. Neither the (Z)ꢀ3ꢀ
alkenꢀ1ꢀols, nor the (Z)ꢀ3ꢀalkenals showed such high variation in threshold levels.
Nevertheless, despite these differences, the ranking of the OTs of the individual
panelists of a specific homologous series was comparable (cf. Table 3 AꢀC). For
instance, the lowest individual odor threshold of the (Z)ꢀ3ꢀalkenꢀ1ꢀols was that of (Z)ꢀ3ꢀ
hexenꢀ1ꢀol for four out of five panelists as discussed above.
Further, we did not observe any correlation between the individual odor descriptors and
the individual odor threshold values.
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The variations in odor thresholds between panelists appear, at first sight, as being huge,
especially when being compared to other studies: in the odor threshold determination
33
study of Czerny et al. , for example, a maximum factor of 8 has been reported.
However, the number of panelists was commonly comparatively lower in other studies
with e.g. only two panelists in the study of Czerny et al. and only aromatic compounds
having been investigated but no openꢀchained substances. Accordingly, data are only
comparable to a limited extent.
There may be several reasons for interꢀindividual variance in odor thresholds and odor
qualities. First, the receptor repertoire might be different between individuals as has
41, 42
been reported in several studies on receptor expression in humans.
Second, it is
known that a large number of odorants activate not only one but often a number of
different receptors whereas one receptor can be activated by a range of different
43, 44
compounds, obviously resulting in a complex coding of smell.
The potential
variation in this code between individuals is not yet resolved. Third, odorants may be
bioꢀtransformed during their passage through the nasal cavity, for example by
cytochrome P450 metabolization; such processes are also termed peri-receptor
45ꢀ50
events.
This can modify the structure and quantity of a substance, and might,
accordingly, lead to differences in odor threshold and quality. In view of this it is
especially interesting to note that the (Z)ꢀ3ꢀcompounds investigated in this study might
be prone to e.g. oxidative attack potentially leading to epoxides and a series of other
45, 49, 50
derivatives as described previously.
To translate the data of the present study into a comparison with other structurally
related odorants, we listed the median values of the OTs of the (Z)ꢀ3ꢀalkene
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compounds, (E)ꢀ3ꢀalkene compounds, and the corresponding saturated compounds in
Table 4 AꢀC for direct comparison. Overall, the investigated (Z)ꢀ3ꢀcompounds showed
lower odor thresholds in air than the corresponding saturated compounds, followed by
the (E)ꢀ3ꢀanaloga. Accordingly, the (Z)ꢀ3ꢀconfiguration obviously represents an
important stereoꢀchemical feature in these substance groups.
Our study demonstrates that the investigated compounds of the series of the (Z)ꢀ3ꢀ
alkenꢀ1ꢀols, (Z)ꢀ3ꢀalkenals and (Z)ꢀ3ꢀalkenoic acids show a broad variance of odor
qualities, and partially very low odor thresholds corresponding to high odor potencies.
This study is important insofar as it not only provides data on the smell properties of
these substances, but as it also highlights the at times relatively high variance in
olfactory perception between individuals; this latter aspect is, in our opinion, not
sufficiently regarded in odorant research. More detailed studies would be required but
the laborious and time consuming analyses as applied in this study are commonly just
executed by one or two panelists only in other investigations in this field.
Another relevant aspect of our study is the fact that several of the 24 investigated
substances have already been identified in foods or generally in nature; accordingly,
some of the remaining substances may be also promising candidates to be discovered
as natural compounds in future studies. The analytical data compiled in this study, such
as retention indices, mass spectra, odor threshold data in air and odor qualities are
intended to aid their future discovery. Moreover, this study aims at raising attention to
this substance class not only in terms of some of the compounds being potentially
important odorants in food but also with regard to other biological meaning, for example
as compounds with chemoꢀcommunicatory function, like pheromones, attractants or
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repellents in the research field of entomology. Substance libraries as generated in the
present study will support their future discovery and systematic investigation.
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77 Abbreviations
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GCꢀO
Gas Chromatography ꢀ Olfactometry
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79 Acknowledgments
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We thank all members of our working group for their participation in the sensory
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82 Associated content
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Supporting Information
Spectroscopic data (MSꢀEI, NMR), yield and purity of all synthesized compounds as
well as a table including the entry numbers and structures of all investigated (Z)ꢀ3ꢀ
compounds, a table of foods / consumables the substances have been identified in,
besides tables of literature data on odor qualities and odor threshold values, and a table
comprising the concentrations of the FD1 solutions of the respective odorants and
retention indices of the investigated saturated compounds are given in the
supplementary material, together with figures visualizing the individual odor thresholds.
This material is available free of charge via the Internet at http://pubs.acs.org.
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92 References
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