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Analysis of strawberry flavour - discrimination of aroma types by
quantification of volatile compounds
Article in Zeitschrift für Lebensmittel-Untersuchung und -Forschung · September 1997
DOI: 10.1007/s002170050154
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Z Lebensm Unters Forsch A (1997) 205: 218 – 223
 Springer-Verlag 1997
O R I G I N A L PA P E R
Detlef Ulrich ? Edelgard Hoberg ? Adolf Rapp
Steffen Kecke
Analysis of strawberry flavour ± discrimination of aroma types by
quantification of volatile compounds
Received: 13 November 1996 / Revised version: 4 February 1997
AbstractmOver a period of 3 years, the essential volatile
compounds of several strawberry varieties were analysed
by gas chromatography, gas chromatography-olfactometry
and mass spectrometry. In general, a strong variability in
the dependence of the amount of these compounds on the
ripening stage, climate and location was found, nevertheless, the key compounds of the aroma showed typical,
genetically determined basic patterns. The quantification
of the key aroma compounds in cultivated and wild strawberries resulted in a definition of aroma types which
corresponded with the sensory evaluation. These aroma
types can be used to establish a criterion for the selection
of quality in strawberry breeding.
Key wordsmStrawberry ? Flavour ? Aroma ? Volatiles ? Gas
chromatography-olfactometry ? Sensory evaluation ? Plant
breeding
Introduction
Strawberries are cultivated in nearly all countries of the
world and are one of the most popular fruits that are
consumed as fresh, conserved or manufactured products.
The actual number of strawberry varieties is immense and
every year new varieties come into trade and cultivation.
The German Federal Office of Plant Varieties tests a
collection of about 200 strawberry varieties every year.
Because of its typical aroma, the strawberry has always
been a favoured object in aroma analysis. In addition, the
chemical basis of the strawberry aroma has been examined
D. Ulrich ( ) ? E. Hoberg ? S. Kecke
Federal Center for Breeding Research on Cultivated Plants, Institute
for Quality Analysis, Neuer Weg 22/23, D-06484 Quedlinburg,
Germany
A. Rapp
Institute for Grapevine Breeding Geilweilerhof,
D-76833 Siebeldingen, Germany
in depth [1]. The flavour industry is able to supply synthetic
aromas for the flavouring of many types of foods, as well as
pharmaceutical and other products. One of the key compounds of the strawberry aroma, 2,5-dimethyl-4-hydroxy3(2H)-furanone (Furaneol, DHF) [2], represents one of the
most important synthetic flavour in terms of mass and
value.
On the other hand, the sensory quality of traded strawberries is often criticized by consumers. Throughout this
century the predominant aim of plant breeding has been to
increase not the sensory quality, but rather parameters such
as yield, habitat and the ability to withstand storage,
transport and processing. As is the case for other fruit and
vegetable species, only the old cultivated varieties have
excellent sensory characteristics [3, 4].
Plant breeding and quality control demand measurement
of objective parameters for the assessment of sensory
quality. The aim of the work published here was to
characterize the chemical profile of the strawberry aroma,
with special regard being given to the application of aroma
analysis to selection in plant breeding.
The flavour substances of strawberries, especially the
volatile compounds, have been studied extensively. In the
literature, more than 360 volatile compounds have been
reported [1]. As well as sugars and acids about 15–20
volatile compounds are important for sensory perception
[5–8]. In Table 1 the aroma compounds are summarized
according to their gas chromatographic retention indices on
a wax column. The contribution of these compounds to the
total aroma impression is the aroma value, which is defined
as the quotient of concentration and odour thresholds. It
varies within relatively large ranges depending on the
individual variety [8–13]. Staudt et al. [14] attempted to
characterize wild species by quantification of volatile
compounds, especially the esters. In the paper of Larsen
et al. [6] two types of cultivated strawberries are distinguished by their high or low ester content.
Another important direction in strawberry breeding is
concerned with the introduction of the typical “wood
strawberry flavour” into cultivated varieties by crossing
genotypes of Fragaria ananassa Duch. and Fr. vesca L.,
219
Fig. 1mSensory profiles of four strawberry varieties with different
aroma characters and popularities. The values are given as the average
of over 3 years. Wild strawberry: 1 - Fragaria vesca L.; cultivated
strawberries (Fr. ananassa Duch.): 2 - Polka, 3 - Elsanta, 4 - Senga
Gourmella. Description of the sensory parameters – Smell: 1, green
(grassy like cis-3-hexenal); 2, aromatic (typical cultivated or wild
strawberry); 3, fruity (ester-like); 4, flowery (like β-ionon); Taste: 5,
sweet; 6, sour; 7, flavour (intensity of overall flavour); 8, stale; 9, ripe;
10, popularity
which are normally unable to cross-pollinate. By using
these parents Fr. vescana was recombinated after extensive
breeding experiments [15, 16]. A simple sensory comparison of the aromas of Fr. ananassa Duch. and Fr. vesca L.
types indicates significant differences in the chemical
composition which, to date, have not been clearly interpreted.
A precondition of using instrumental-analytical methods
for breeding and quality control is the determination of the
differences between strawberry species in terms of their
chemical composition.
Materials and methods
Material
Four European cultivated strawberry varieties and one wild species
were harvested from 1993 to 1995 in the experimental garden of the
Federal Center of Breeding Research on Cultivated Plants. Only fully
ripe fruits typical of the respective varieties were sampled. The
characteristics of the varieties discussed correspond to those in [17].
Wild strawberry (“wood strawberry”): A – Fr. vesca L., local clone.
Cultivated strawberries (Fr. ananassa Duch.; “pineapple strawberry”):
B – “Mieze Schindler”; breeder, Professor Schindler, Dresden; origin,
crossing from “Lucida perfecta” and “Johannes Müller”; registration in
the 1930s; C – “Polka”; breeder, IVT Wageningen, The Netherlands;
origin, crossing from “Induka” and “Sivetta”; registration in 1988; D –
“Elsanta”; breeder, IVT Wageningen, The Netherlands; origin, crossing
from “Gorella” and “Holiday”; registration in 1982; E – “Senga
Gourmella”; breeder, von Sengbusch; registration in 1971.
Fig. 2mGas chromatography-olfactometry: comparison of flavour
dilution (FD) chromatograms. Sensory impressions with FD-factors
below 2 and the “green” notes are excluded from the chromatogram.
Compounds numbered 9a and 18a are unknown
and mixed. Normally it took 3 h from harvest to testing. The sensory
evaluation of strawberries was carried out by a trained panel consisting
of 15 members with five samples per session. After collecting a choice
of altogether 11 sensory features the quantification was made. The
characteristics were defined both verbally and in comparison with
reference substances. The panellists quantified the features on a nongraduated 10-cm-long linear scale. Every variety was tested six times
in the experimental period.
Instrumental analysis
Sample preparation. Fruits were washed and sepals dissected. Then,
200 g fruits and 200 ml of a 20 mass% NaCl solution were homogenized for 2 min in a Waring blender. The centrifugation at 3500 g and
4 °C (30 min) was followed by addition of the internal standard, 2,6dimethyl-5-hepten-2-ol (0.1 ppm v/v to the supernatant of the centrifuged material). Following this, 200 ml of the supernatant was
extracted for 20 h at a water bath temperature of 27 °C with trichlorofluoro methane (Merck No. 818 749, boiling point 23 °C, 1x distilled).
Gas chromatography-olfactometry. Hewlett-Packard 5890 A; injector
temperature 180 °C; split 10 ml. Column: HP-INNO wax
30 m × 0.32 mm ID/0.5 µm; 2 ml hydrogen/min; temperature programme: 50 °C to 180 °C at 3 °C/min, FID detector 250 °C. Split of
carrier gas: 33% to FID and 66% to sniffing port. For the aroma extract
dilution analysis the extracts were stepwise diluted with solvent (freon)
in a geometrical line [18]. The gas chromatography-olfactometry
analysis was repeated for the five varieties every year. The panellists
who carried out the olfactory analysis were specially trained members
of the above-mentioned sensory panel.
Sensory evaluation
The same sample was used for instrumental and sensory analyses. The
berries were washed and, after dissection of sepals, divided into four
Gas chromatography. Hewlett-Packard 5890 A; injector temperature
180 °C; split 40 ml. Column: HP-INNO wax 60 m × 0.32 mm ID/
0.5 µm; 2 ml hydrogen/min, temperature programme: 50 °C to 180 °C
220
Table 1mAroma compounds in the strawberry. Out of the 120 identified compounds the 23 listed are quantified by the method given in the
text. The substances 2, 3, 6, 8, 10 – 14, 16, 17 and 19 were selected for
Peak no.
the aroma patterns in Fig. 3. References for threshold values: [1, 5, 6,
23, 24]. [DMF 2,5-Dimethyl-4-methoxy-3(2H)-furanone, DHF 2,5dimethyl-4-hydroxy-3(2H)-furanone]
Retention
index
Compound
Threshold in water (ppb)
Sensory description
1
2
3
4
5
940
945
977
1006
1120
Pentanone-2
Methyl butanoate
Ethyl butanoate
Butyl acetate
Heptanone-2
50
59
0.13
5000
50
Thinner, acetone
Fruity, ester-like, green
Fruity, ester-like, sweet
Thinner
Meaty
6
6a
7
8
9
10
1125
1147
1164
1183
1398
1573
Methyl hexanoate
(Z)-3-Hexenal
(E)-2-Hexenal
Ethyl hexanoate
(Z)-3-Hexen-1-ol
Linalool
70
0.25
17
3
70
6
11
12
13
14
15
15 a
1625
1669
1711
1882
1943
1957
DMF
Butanoic acid
2-Methyl butanoic acid
Hexanoic acid
2-Phenyl ethanol
β-Ionone
0.03
240
180
3000
1100
0.007
Fruity, caramel, green
Buttery
Fruity, buttery
Unpleasant
Flowery, rose
Floral, crude, violet
16
16 a
17
18
18 a
19
2042
2063
2123
2143
2163
2216
DHF
Octanoic acid
γ-Decalactone
Eugenol
δ-Decalactone
Methyl anthranilate
0.04
8800
50
11
160
3
Caramel, sweet
Unpleasant, fatty
Fruity, lactone-like, cocos
Spicy, nutmeg
Lactone-like
Aromatic, fruity, sweet, soap
Table 2mKey compounds of strawberry aroma. Values are relative
concentrations of aroma key compounds. The areas of the volatile
compound peaks were normalized by the area of internal standard
Peak no.
Compound
Methyl butanoate
Ethyl butanoate
Methyl hexanoate
Ethyl hexanoate
Linalool
11
12
13
14
16
DMF
Butanoic acid
2-Methyl butanoic acid
Hexanoic acid
DHF
17
19
γ-Decalactone
Methyl anthranilate
peak. The data are given as average of a 3-year trial, with 38
observations. Varieties: A – Fragaria vesca L., B – Mieze Schindler,
C – Polka, D – Elsanta, E – Senga Gourmella (n. d. Not detectable)
Relative concentration of key aroma
compounds in variety:
A
2
3
6
8
10
Fruity, ester-like
Green, grassy
Green, fatty
Fruity, ester-like, green apple
Grassy, green
Flowery, sweet
B
C
D
E
0.4
2.1
0.1
0.5
0.2
5.3
2.0
1.2
1.4
0.1
7.9
3.5
0.8
0.6
0.3
0.5
0.1
0.4
0.1
0.8
0.3
50.1
0.1
50.1
1.1
19.3
0.2
0.8
2.1
2.9
0.1
2.2
7.1
45.6
8.2
6.0
0.4
3.9
8.4
2.4
4.3
6.7
8.6
31.2
2.5
34.4
5.5
7.2
94.3
5.4
2.0
5.5
0.1
1.3
0.2
n.d.
8.0
n.d.
77.5
n.d.
at 1.5 °C, FID detector 250 °C. The quantification was carried out by
comparison of areas of the peaks to those of the internal standard (2,6dimethyl-5-hepten-2-ol, ROTH No. 9237.1).
Results and discussion
GC/MS. Hewlett-Packard MSD 5972 with HP 5890 Series 2 plus, data
base NBS75 K and Wiley138. Column: INNO-wax 60 m × 0.32 mm
ID/0.5 µm; 2 ml helium/min; temperature programme: 50 °C to 180 °C
at 1.5 °C/min.
Between 1993 and 1996 the authors examined altogether 27
European cultivated strawberry varieties and 20 wild species. For discussion in this paper five species from the years
1993–1995 were selected, which represent the typical set of
sensory and chemical features.
Statistical calculation. Programme SAS 6.08; Proc RANK, Proc
CLUSTER. Procedure: the metric data were standardized and processed by cluster analysis (method “average linkage”).
Sensory profiles
Figure 1 shows the sensory profiles of four strawberry
species. The data represent the average of 3 years. The
221
Fig. 3mAroma pattern of four different strawberry varieties. Aroma
value versus number of volatile compounds listed in Table 1. The
compounds with “green” characters (Z)-3-hexenal (6a), (E)-2-hexenal
(7) and (Z)-3-hexenol (9) are excluded. The data shown are the average
of 38 observation from 3 years
wild species Fr. vesca L. differs notably from the cultivated
varieties because it has an aromatic and flowery odour
(feature 2 and 4). After long-term trials at the German
Federal Office of Plant Varieties [17] and the Federal
Center of Breeding Research, the variety “Polka” has
been shown to be a strawberry with a good or very good
taste, which is reflected in its sensory profile by high values
for flavour and popularity. The varieties “Elsanta” and
“Gourmella” compare badly in all of these features.
Although sensory sensations are also influenced by
parameters other than aroma compounds, the strawberry
odour and taste impressions recorded in several growing
periods were relatively stable and clearly different. Because
of the limited number of samples and the changing weather
conditions the sensory results are not statistically significant but show trends according to year-long trials of other
investigators [17].
Gas chromatography-olfactometry
The olfactometric analysis of strawberries led to the characterization of five basic sensory impressions of strawberries: fruity, green, caramel, lactone-like and buttery [5, 8].
Strawberry is a fruit with a very intensive aroma which is
based on its high content of chemical substances. Concentrated extracts therefore lead to a high number of odour
impressions in gas chromatography-olfactometry analysis.
Figure 2 summarizes the flavour-dilution chromatograms
(FD-chromatogram) of four varieties. In order to get a rapid
survey, weakly intensive odour impressions with FD-factors
below 2 are excluded.
Green odour notes based on hexenals and hexenols have
an exceptional quality. In particular, (Z)-3-hexenal, with its
very low threshold value of 0.25 ppb, contributes essentially to the fresh aroma impression and is necessary for the
positive strawberry aroma [5]. Thermally processed or
frozen berries lack such “green” components and differ
significantly from fresh fruits [5, 7, 11]. Precursors of green
components are multiple-unsaturated fatty acids [19, 20]
which are decomposed by enzymes. Investigations of the
bell pepper demonstrate that the formation of these substances only starts after destruction of cell tissue in the
receptacles during homogenization [21]. Observations by
Luning et al. show that, in contrast to the situation for other
volatile compounds, the quantity of “green” components is
highly varied within one variety and that an obvious
relationship exists between the ripening stage of the fruits
and the method of sample preparation.This is also the
reason why the “green” odour impressions are not represented in the FD-chromatograms.
The varieties evaluated contain all the compounds that
give rise to the basic sensory impressions, but in different
quantities. The wood strawberry Fr. vesca L. differs from
the cultivated types in that it contains a very aromatic,
plainly recognizable woodstrawberry-like semivolatile
compound. It was identified as methylanthranilate, reported
previously by Hirvi and Honkanen [9]. This component was
detected first in the nearly 50-year-old “Mieze Schindler”
(see Table 2). “Polka” is distinguished from all other
varieties by virtue of the fact that it smells strongly of
ester-like compounds. In “Elsanta” this group is weakened
and is completely absent from “Senga Gourmella”, which,
in addition, contains an off-flavour (hexanoic acid).
Gas chromatography
The aroma value was calculated by dividing the relative
concentrations of volatile compounds (in area %) by the
corresponding odour threshold value in water [18], as
indicated in the literature (see Table 1). Figure 3 shows
the aroma patterns of the four typical varieties mentioned
above.
The aroma patterns of the four varieties correlate sufficiently with the sensory impressions made in the olfactometry experiments. In all tested strawberries the substances 2,5-dimethyl-4-methoxy-3(2H)-furanone (Mesifuran, DMF) and 2,5-dimethyl-4-hydroxy-3(2H)-furanone
(Furaneol, DHF) have the highest aroma values. Wood
strawberry is characterized by its medium to high aroma
values for methylanthranilate. “Polka” is notable because of
its high quantities of butanoic and hexanoic acids esters.
The sensorially less preferable varieties contain less of the
esters and more γ-decalactone and hexanoic acid. The
absence of esters in conjunction with high concentrations
222
Fig. 4mCluster analysis of a 3-year trial with five strawberry varieties
and 38 observations. Analysis of 23 volatile compounds by GC after
liquid-liquid extraction. Wild strawberry: 1 – Fragaria vesca L.
Cultivated strawberries (Fr. ananassa Duch.): 2 – Mieze Schindler,
3 – Polka, 4 – Elsanta and Senga Gourmella. (OBS Number of
observation, D euclidean distance)
of DMF, DHF and hexanoic acid seems to result in an
aroma impression which is perceived as unpleasant.
Statistical analysis
The relative concentrations of aroma substances for the five
varieties harvested from 1993 to 1995 were calculated by
cluster analysis. The results are shown in Fig. 4. The
marked clusters of volatile compounds composition conform with the sensorially distinguishable genotypes.
Fig. 5mAroma types of strawberries. [MA Methylanthranilate, DHF
2,5-dimethyl-4-hydroxy-3(2H)-furanone]
other positive characters such as a good tolerance to storage
and shipping.
The results discussed allow further conclusions to be
drawn that are relevant to plant breeding. The instrumental
analysis can support an effective selection for sensory
quality. From the generally high number of descendants,
seedlings with positive aroma patterns can be selected, e.g.
ester-rich or methylanthranilate-rich types. Therefore, applied breeding should use effective analytical methods
(rapid methods). One of the opportunities already used for
analysis of strawberry aroma is gas chromatography in
combination with solid-phase microextraction [22].
AcknowledgementsmThe authors wish to thank M. Dießner, H. Elstner, S. Eunert and H. Hastrich for their technical assistance.
Strawberry aroma types
The results gained from the instrumental analysis of the five
typical strawberry varieties discussed in this paper allow
the conclusion that distinct aroma types can be defined
(Fig. 5). This is supported by the sensory analysis trends
mentioned above.
A very important role is played by methylanthranilate,
which is responsible for the typical character of the wood
strawberry (Fr. vesca L.) aroma, which is characterized by
an intensive spicy-aromatic and flowery note. All genotypes investigated can be subdivided into a methylanthranilate-containing and methylanthranilate-free types. As
well as some wild species “Mieze Schindler” is a methylanthranilate-containing type. For decades this old cultivar
has been known as one of the most favoured strawberries in
Germany. Depending on climatic conditions “Mieze
Schindler” contains low to medium concentrations of
methylanthranilate.
In contrast, the methylanthranilate-free group is divided
in a sensorially pleasant group (ester-type) and a less
pleasant group (DHF-type). Fr. virginiana L. – one of the
parental genotype of Fr. ananassa Duch., with a fresh and
fruity aroma – can be assigned to the ester-type. The third
group, the DHF-type, contains varieties with medium to
poor flavour; however, often this flavour is combined with
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