SEARCH

SEARCH BY CITATION

Keywords:

  • headspace solid microextraction;
  • aroma compound;
  • cherry;
  • gas chromatography–olfactometry

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References

This study was conducted to determine the aroma-active compounds of five sweet cherry cultivars grown in Yantai region, China, viz. ‘Lapins’, ‘Rainier’, ‘Stella’, ‘Hongdeng’ and ‘Zhifuhong’. The samples were extracted by headspace solid phase microextraction (HS–SPME) and analysed by gas chromatography–mass spectrometry (GC–MS) on DB-wax and DB-5 columns. A total of 52 volatiles were identified. Among these, hexanal, (E)-2-hexenal, 1-hexanol, (E)-2-hexen-1-ol, benzaldehyde and benzyl alcohol were the main volatile compounds in the five cherries. Furthermore, the aroma compounds of five cherry samples were evaluated using a combination of HS–SPME and GC–olfactometry (GC–O) dilution analysis, and a total of 40 aroma-active compounds were identified. The results suggested that hexanal, (E)-2-hexenal, (Z)-3-hexenal, nonanal, benzaldehyde and geranylacetone (FD ≥ 16), responsible for the green, orange, almond and floral characters of the cherries, were the potentially important common odorants in these cherry cultivars. Benzyl alcohol and linalool were significant aroma compounds in most cherries, with the exception of ‘Stella’ and ‘Rainier’. In addition, (E,Z)-2,6-nonadienal (cucumber-like odour) could be important to ‘Hongdeng’ and ‘Zhifuhong’, and (E,E)-2,4-nonadienal (fatty odour) probably made great contributions to the aromas in ‘Lapins’ and ‘Stella’. From the present result, it was concluded that the aroma profiles were similar in the five cherry cultivars, but significant variation was found in the contributions of these compounds to each cherry. Copyright © 2010 John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References

The sweet cherry (Prunus avium L.) is important commercially as a table fruit and as an ingredient for fruit cocktails and maraschino cherries. The fresh consumption of sweet cherries is important in China, especially in Yantai, the main region of production. The sweet cherry crop occupies more than 12 000 ha in this region, and the current annual production level is > 50 000 tons (50% of China's production), creating a sales revenue of more than 2 billion Chinese yuan.[1] Nowadays, Yantai sweet cherry has become a geographical trademark in China. As Yantai sweet cherries became more popular in China over the past decade, their characteristic and distinct flavour began to receive more scrutiny from consumers.

The aroma compounds in sweet cherry are made up of a great number of organic components, including aldehydes, alcohols, esters, acids and terpenes. As early as 1986, Schmid and Grosch[2] reported that benzaldehyde, (E)-2-hexenal, and hexanal contributed to fruit flavour in cherries. Mattheis et al.[3] detected 28 aroma compounds from Prunus avium ‘Bing’ by means of headspace sampling, and benzaldehyde and hexanal, with high aroma values, were identified. Girard and Kopp[4] studied the volatiles of 12 sweet cherries, using a dynamic headspace GC method; a total of 50 volatile compounds were identified. Of these, (E)-2-hexenol, benzaldehyde, hexanal and (E)-2-hexenal were predominant flavour volatiles and could be used to segregate commercial and new cherry selections into various subgroups. Zhang and co-workers[5] characterized the volatile compounds in sweet cherry ‘Hongdeng’ during riping via HS–SPME, followed by GC–MS, and a total of 37 compounds were identified; hexanal, (E)-2-hexenal, benzaldehyde, (E)-2-hexen-1-o1, ethyl acetate and ethyl hexanoate were considered to be the characteristic aroma compounds of ‘Hongdeng’.

GC–O has become a standard technique to evaluate the odour-active compounds in natural products and complex mixtures.[6] The application of SPME to GC–O dilution analysis has been developed by varying the thickness of the fibre phase, the length of exposure and the split ratios of injected samples in the gas chromatograph's injection port.[6,7] A combination of HS–SPME and GC–O techniques has been applied to detect the aroma compounds in orange juice,[8] apple ciders,[9] Chinese liquor[10] and Elsholtzia splendens.[11] Although Yantai sweet cherry is a major table fruit in China, its aroma has not yet been fully understood. Hence, the objective of this study was to employ HS–SPME GC–O dilution analysis technique to identify and compare the aroma compounds in five cherry fruits from Yantai region.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References

Chemicals

Acetaldehyde, 2-methylpropanal, 3-methylbutanal, hexanal, (E)-2-hexenal, octanal, nonanal, decanal, benzaldehyde, (E,Z)-2,6-nonadienal, β-phenylacetaldehyde, ethyl acetate, ethyl butanoate, ethyl hexanoate, methyl salicylate, ethyl hexadecanoate, acetic acid, 3-methylbutanoic acid, pentanoic acid, hexanoic acid, octanoic acid, decanoic acid, limonene, linalool, menthol, geranylacetone, 6-methyl-5-hepten-2-one, 1-butanol, 1-pentanol, 1-hexanol, (Z)-3-hexen-1-ol, 3-methylbutanol, 1-octen-3-ol, 1-octanol, α-phenethyl alcohol, 1-nonanol, benzyl alcohol, β-phenethyl alcohol, 3-octanol (internal standard) and C7–C30 alkane standards were obtained from Aldrich–Sigma (Shanghai, China). Sodium chloride was purchased from China National Pharmaceutical Ground Corporation (Shanghai, China).

Cherry Samples

Fruits of Prunus avium L. cultivars, viz. ‘Lapins’, ‘Rainier’, ‘Stella’, ‘Hongdeng’ and ‘Zhifuhong’, were obtained from the Menlou cherry orchard (Yantai, China). Healthy cherries (1 kg) were randomly picked at commercial maturity during July 2008. The fruits were immediately placed on ice and transported to the laboratory, where they were were quickly frozen (QF) and stored at –23oC for 2 days until analysis.

Extraction of Volatile Components

A total of 100 g cherry fruits were taken out of the ice box. The fruits were destemmed and defrosted at 5°C under nitrogen until they were soft and ready to be crushed (always icy and cold). Fruit disruption was performed under nitrogen in a glass blender jar (Jiangyin Scientific Research Devices Factory, Jiangyin, China) for 1 min. Calcium chloride (1 g) was added to inhibit enzyme activity before crushing.[12] The crushed cherry was clarified by centrifugation at 4500 × g for 10 min at 0°C, and a portion of 10 ml was transferred to a 20 ml glass vial spiked with 5 µl of 3-octanol (internal standard, 10 mg/l standard solution in ethanol). After the addition of 3.0 g NaCl and a stirring bar for solid-phase microextraction, the vial was capped with a Teflon septum and an aluminium cap (Chromacol, Herts, UK).

Solid-phase Microextraction

The SPME holder for manual sampling and fibres used in the analyses were purchased from Supelco (Aldrich, Bellefonte, PA, USA). Three SPME fibre coatings were tested and used: 50/30 µm DVB/CAR/PDMS, 75 µm CAR/PDMS and 100 µm PDMS. Before extraction, the fibres was conditioned by being inserted into the GC injector at 250°C for 2 h to prevent contamination. Then the fibres were exposed to the headspace of the above prepared glass vial, which was placed in a thermostated bath adjusted to 50°C to promote the transference of the compounds from the sample to the headspace for 40 min with continuous stirring. Desorption of the fibre was taken at 250°C for 5 min. All analyses were performed in triplicate. Operating conditions were optimized by realizing SPME extractions of cherry samples at different adsorption temperatures (30°C, 40°C, 50°C and 60°C) and time (10, 20, 30, 40, and 50 min).

Linearity of HS–SPME

To check the linearity of HS–SPME for cherry fruit analysis, ‘Hongdeng’ juice was sequentially diluted with distilled water at a 1:1 ratio. The volatiles were extracted using a manual SPME holder with a 50/30 µm DVB/CAR/PDMS fibre. The samples were analysed on a DB-wax column (30 m × 0.25 mm i.d., 0.25 µm film thickness; J&W Scientific). The samples were equilibrated at 50oC for 10 min and extracted for 40 min at the same temperature with stirring. After extraction, the fibre was inserted into the injection port of the GC apparatus (at 250°C) to desorb the extracts for 5 min. Each dilution was analysed twice, and the first dilution was assigned a concentration number of ten.[10]

Direct GC–O

Before GC–O dilution analysis, a similarity test was performed on the SPME odours that issued from the same fresh cherry juice. According to Rega's method,[13] an Agilent 6890 gas chromatograph equipped with an olfactometer and a short capillary of untreated silica (1 m × 0.32 mm i.d.) was used. The flow rate of the carrier gas (nitrogen) was 25 ml/min, and the oven temperature was kept at 50°C. The SPME extracts were injected into the GC port at a splitless mode (250°C). Since no chromatographic separation was generated by the silica capillary, aroma compounds arrived simultaneously at the sniffing port. For each SPME extract, two assessors who had been familiarized with cherry juice perceived and evaluated the similarity of global odour. The fibres were kept in the GC inlet until the end of the sensorial stimulus.

GC–O Analysis

GC–O analysis was performed on a Hewlett-Packard 5890 gas chromatograph equipped with a flame ionization detector (FID) and an olfactometer. The column carrier gas was nitrogen at a constant flow rate of 2 ml/min. Half of the column flow was directed to the FID and the other half to the olfactometer. The samples were analysed on a DB-wax column (30 m × 0.25 mm i.d., 0.25 µm film thickness; J&W Scientific, Folsom, CA, USA). Each extract was injected in splitless mode. The oven temperature was kept at 40°C for 2 min, then raised to 230°C at a rate of 6°C/min and held at 230°C for 15 min. The injector and detector temperatures were both 250°C. The samples were stepwise diluted with distilled water, using a 1:1 dilution before being extracted with HS–SPME and analysed by the GC–O dilution analysis technique. The flavour dilution (FD) factors were determined for the odour-active compounds in each sample.[10]

Two experienced olfactory assessors were employed. Aroma descriptions and retention time were recorded for each sample. A peak was considered aroma-active if at least one assessor found it at the same retention time in at least two evaluation tests with a similar description.

GC–MS Analysis

Identification was carried out using a Trace MS (Finingan, USA). The sample was analysed on a DB-Wax column (30 m × 0.25 mm i.d., 0.25 µm film thickness; J&W Scientific) and on a DB-5 column (60 m × 0.32 mm i.d., 1 µm film thickness; J&W Scientific). Each extraction was injected in splitless mode. The oven temperature was kept at 40°C for 2 min, then raised to 230°C at a rate of 6°C/min and held at 230°C for 15 min on the DB-wax column, while the final temperature was 250°C for 15 min on the DB-5 column. The column carrier gas was helium at a constant flow rate of 2 ml/min. The mass spectrometer was operated in the electron impact mode (EI) at 70 eV, scanning the range 34–348 m/z, and the ion source temperature was set at 230°C.

Compound Identification

Mass spectra of unknown compounds were compared with those in the Wiley 275.L Database (Agilent Technologies). Retention indices (RIs) of unknown compounds were calculated in accordance with a modified Kováts method.[14] Identification of unknown compounds was achieved by comparing the mass spectra and RIs of the standards or from the literature (RIL).

Quantitative Analysis

Relative peak area (relative to internal standard) was used for quantitative analysis of volatiles in cherry. The internal standard solution (3-octanol) were individually prepared in ethanol and stored at 4°C. Dilutions were made with ethanol at the same temperature. The integral for all chromatogram peaks used the selective ion method (SIM).

Statistical Analysis

Statistical procedures were carried out using the SPSS version 13.0 statistical package for Windows (SPSS Inc., Chicago, IL, USA). ANOVA and Duncan's multiple range test were applied to the data to determine significant differences between the analysed volatile compounds; the model was statistically significant, with a value of p ≥ 0.01.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References

Parameters of HS–SPME

SPME offers a rapid, solvent-free method for the extraction of organic compounds from aqueous samples. It has been pointed out that some SPME applications demonstrate a bias for extracting higher boiling-point compounds over lower boiling-point compounds, and the extracts will not be representative of the gas phase at equilibrium as it is perceived by the nose.[15] However, by the optimization of SPME variables (choice of fibre, thickness of the fibre phase, length of exposure, salt addition, extraction temperature and time, etc.), it was possible to obtain representative and reproducible HS–SPME extracts of the typical odour of flavour foods.[7,10,11]

The presence of salts has been shown to improve the adsorption of analytes for SPME analyses[16,17] because the dissociated ions disrupt the sample matrix by decreasing the solubility of the aroma molecules, which then are more readily absorbed by the headspace fibre[16]. The same phenomenon has also been found in the absorption of volatiles in cherry by SPME phases. For this study, all samples were prepared with saturated sodium chloride, and it was observed that the addition of salt significantly improved the extraction efficiency of most volatiles, especially the acids (acetic acid, 3-methylbutanoic acid, etc.) and some highly volatile components (acetaldehyde and ethyl acetate). In contrast, the presence of sodium chloride slightly decreased the extraction efficiency for limonene and geranylacetone (data not shown). This phenomenon is probably due to competitive absorption of the SPME fibre.[18]

In the current investigation, several SPME variables were studied, including SPME fibre, temperature and time of extraction. Three fibres, coated with 50/30 µm DVB/CAR/PDMS, 75 µm CAR/PDMS and 100 µm PDMS, were evaluated for the extraction of aroma-active compounds in cherry. PDMS fibres exhibited greater extraction of non-polar compounds (aldehydes and esters) than mixed fibres. PDMS/DVB and CAR/DVB/PDMS fibres extracted a similar proportion of volatile compounds; however, DVB/CAR/PDMS fibre extracted more polar and middle polar compounds in cherry, such as acetic acid, 3-methyl butanoic acid and pentanoic acid, and it has been used for the analysis of aroma compounds in apple[19] and longan[20].

To select the optimum extraction temperature and time, a 50/30 µm CAR/DVB/PDMS fibre was used. Extraction was carried out at 30°C, 40°C, 50°C and 60°C for 10, 20, 30, 40, and 50 min. Figure 1 shows the result in graphic form, expressed as the sum of areas of all the volatile compounds obtained from cherry by use of each set of conditions. The best result was obtained at 50°C for 40 min.

thumbnail image

Figure 1. Effect of sampling conditions (adsorption temperature and time) on the extraction of volatiles in cherry

Download figure to PowerPoint

Identification of Compounds

GC–MS was used for the determination of volatile compounds present in the extracts. A majority of compounds were identified by comparison of their spectra with those recorded in the Wiley 275.L database. Meanwhile, the RIs of unknown compounds were calculated using GC retention index standards (hydrocarbons from straight-chain, C7–C30) compared with the RIs of the standards or those reported in published work.[21,22] The data in Table 1 show that a total of 52 compounds were identified in five cherries on the DB-Wax and DB-5 columns. Most compounds have been reported previously in cherry[3,5,23] and consisted largely of 18 alcohols, 16 aldehydes, six acids, six esters, four terpenes, one ketone and one furan. ‘Stella’ showed the richest composition, with 48 compounds. ‘Lapins’, ‘Hongdeng’, and ‘Zhifuhong’ had a moderate composition, with 46, 41 and 44 compounds, respectively; ‘Rainier’ gave the simplest mixture, with only 36 identified compounds.

Table 1. The volatile compounds in five cherry fruits identified by GC–MS analysis on DB-Wax and DB-5 columns
RI (DB-Wax)RI (DB-5)CompoundsIdentification*Relative peak area **
LapinsRainierHongdengStellaZhifuhong
  • *

    MS, compounds were identified by MS spectra; RI, compounds were identified by comparison to pure standard; RIL, compounds were identified by comparison with RIs from the literature.

  • **

    The numbering refers to elution order; values (area %) represent averages of three determinations and standard deviation (± SD) is given in parentheses (t, traces, <0.05%); a different superscript letter in the same row represents significant difference at p ≤ 0.01 by Duncan's multiple range test. ND, not detected.

Aldehydes   50.55 (1.63)d18.20 (1.56)b25.19 (1.73)c46.45 (0.95)d9.11 (0.44)a
<900<700AcetaldehydeRI, MS0.34 (0.14)bc0.23 (0.14)ab0.17 (0.03)a0.11 (0.11)a0.43 (0.13)c
<900<7002-MethylpropanalRI, MS0.11 (0.01)b0.03 (0.01)aND0.06 (0.03)ab0.24 (0.05)c
9717033-MethylbutanalRI, MS0.65 (0.08)bNDND0.81 (0.04)b0.25 (0.01)a
1073804HexanalRI, MS19.71 (1.54)b1.55 (0.76)a1.79 (0.22)a21.00 (1.00)b3.59 (0.27)a
1136817(Z)-3-HexenalRIL[24], MS0.24 (0.05)a0.28 (0.04)ab0.38 (0.03)ab0.43 (0.04)c0.29 (0.04)bc
1192843(Z)-2-HexenalRIL[24], MS0.17 (0.02)c0.10 (0.01)ab0.12 (0.01)b0.08 (0.01)a0.10 (0.01)ab
1211861(E)-2-HexenalRI, MS2.92 (0.16)bc1.07 (0.06)a3.35 (0.25)c6.81 (0.77)d1.68 (0.08)ab
12821006OctanalRI, MS0.71 (0.13)bND0.23 (0.05)a0.60 (0.04)b0.16 (0.02)a
1333962(E)-2-HeptenalRIL[29,30], MS0.43 (0.12)bNDND0.38 (0.00)aND
13831110NonanalRI, MS2.25 (0.11)d0.18 (0.02)a0.53 (0.06)b1.91 (0.06)c0.43 (0.02)ab
14171071(E)-2-OctenalRIL[29,30], MS2.77 (0.04)cND0.03 (0.01)a1.76 (0.04)bND
14881210DecanalRI, MS0.07 (0.01)cND0.10 (0.02)c0.03 (0.01)a0.04 (0.01)ab
1517965BenzaldehydeRI, MS17.74 (0.62)c13.50 (0.80)bc16.77 (1.56)bc11.03 (0.84)b2.18 (0.20)a
15691121(E,Z)-2,6-NonadienalRI, MSNDND0.37 (0.03)bND0.22 (0.03)a
16201047β-PhenylacetaldehydeRI, MS0.57 (0.00)dND0.22 (0.01)b0.30 (0.04)c0.09 (0.01)a
16971305(E,E)-2,4-NonadienalRIL[27], MS0.27 (0.01)cND0.13 (0.00)a0.22 (0.01)bND
Esters   0.98 (0.14)a2.32 (0.21)c1.52 (0.04)b1.24 (0.09)ab1.11 (0.04)ab
<900<700Ethyl acetateRI, MSND0.06 (0.02)a0.04 (0.00)a0.07 (0.01)a0.45 (0.04)b
1023808Ethyl butanoateRI, MSND0.18 (0.03)b0.05 (0.02)a0.34 (0.02)cND
12321001Ethyl hexanoateRI, MS0.08 (0.01)a0.10 (0.01)b0.03 (0.01)ab0.07 (0.01)ab0.09 (0.03)b
13201013Hex-2-enyl acetateRIL[24], MS0.18 (0.01)bND0.23 (0.04)b0.19 (0.03)b0.08 (0.00)a
17711192Methyl salicylateRI, MS0.48 (0.06)b0.52 (0.03)b0.58 (0.03)b0.22 (0.03)a0.27 (0.02)a
22531999Ethyl hexadecanoateRI, MS0.24 (0.06)a1.47 (0.16)b0.58 (0.14)a0.34 (0.04)a0.22 (0.01)a
Alcohols   59.09 (0.44)b110.83 (3.97)e97.41 (1.34)d72.06 (2.44)c49.84 (1.07)a
11167012-PentanolRIL[29,31], MS0.06 (0.01)aNDND0.36 (0.04)c0.26 (0.02)b
11187153-PentanolRIL[29,32], MS0.15 (0.01)aNDND0.29 (0.03)b0.35 (0.04)b
11566701-ButanolRI, MS0.54 (0.01)bND0.05 (0.01)a2.01 (0.13)c2.43 (0.11)d
12087383-MethylbutanolRI, MSND1.17 (0.21)bNDND0.04 (0.01)a
12457523-Methyl-3-buten-1-olRIL[32], MS0.40 (0.09)a1.37 (0.14)c0.88 (0.14)b0.68 (0.08)ab0.44 (0.07)ab
12497651-PentanolRI, MS0.39 (0.01)c0.09 (0.00)a0.13 (0.01)a0.26 (0.02)b0.14 (0.02)a
13167703-Methyl-2-buten-1-olRIL[31,32], MS0.81 (0.03)b1.62 (0.08)d1.39 (0.09)c1.81 (0.04)d0.49 (0.02)a
13588701-HexanolRI, MS2.75 (0.21)ab2.22 (0.05)a3.36 (0.27)b2.55 (0.11)a2.86 (0.09)ab
1373862(Z)-3-Hexen-1-olRI, MS0.28 (0.02)a0.35 (0.04)a0.31 (0.04)a0.48 (0.08)a0.68 (0.03)b
1405882(E)-2-Hexen-1-olRIL[24], MS21.15 (1.39)b16.29 (0.53)a25.05 (1.38)b22.75 (0.65)b16.25 (0.49)a
14419751-Octen-3-olRI, MS0.52 (0.05)d0.36 (0.06)c0.13 (0.01)a0.23 (0.02)bcND
14459601-HeptanolRI, MS0.33 (0.03)c0.15 (0.02)ab0.16 (0.02)ab0.18 (0.01)b0.10 (0.01)a
155810761-OctanolRI, MS0.76 (0.09)c0.13 (0.02)aND0.32 (0.04)b2.15 (0.06)d
165311651-NonanolRI, MS0.54 (0.05)b0.28 (0.00)a0.47 (0.06)b0.26 (0.03)a0.25 (0.02)a
17941069α-Phenethyl alcoholRI, MSNDNDNDND0.15 (0.03)
18601041Benzyl alcoholRI, MS28.86 (1.89)ab82.84 (3.98)d63.35 (2.79)c37.88 (2.01)b23.62 (0.78)a
18961116β-Phenylethyl alcoholRI, MS0.71 (0.06)a2.25 (0.11)c1.37 (0.03)b0.73 (0.12)a0.81 (0.06)a
195714631-DodecanolRI, MS0.85 (0.04)a1.71 (0.15)c0.75 (0.07)a1.29 (0.11)b0.97 (0.08)ab
Terpenes   1.48 (0.07)a0.86 (0.06)a2.31 (0.31)b0.79 (0.11)a1.36 (0.14)a
11691036LimoneneRI, MSND0.03 (0.01)a0.05 (0.01)b0.03 (0.00)a0.10 (0.01)c
15511106LinaloolRI, MS0.48 (0.11)bcND0.51 (0.06)c0.22 (0.04)a0.24 (0.04)ab
16331157MentholRI, MS0.20 (0.02)aND0.48 (0.08)bND0.27 (0.01)a
18561447GeranylacetoneRI, MS0.79 (0.03)a0.83 (0.05)ab1.27 (0.15)b0.54 (0.15)a0.75 (0.10)a
Furans   0.74 (0.02)bNDND0.30 (0.07)aND
12759952-PentylfuranRIL[32], MS0.74 (0.02)bNDND0.30 (0.07)aND
Ketones   1.56 (0.06)c0.46 (0.08)a1.47 (0.16)c1.37 (0.06)c0.92 (0.06)b
13249886-Methyl-5-hepten-2-oneRI, MS1.56 (0.06)c0.46 (0.08)a1.47 (0.16)c1.37 (0.06)c0.92 (0.06)b
Acids   6.42 (0.53)d5.84 (0.19)cd3.30 (0.20)b4.89 (0.26)c1.90 (0.19)a
1435<700Acetic acidRI, MS0.02 (0.00)a0.04 (0.01)a0.02 (0.01)a0.03 (0.01)a0.05 (0.00)b
16608393-Methylbutanoic acidRI, MS0.41 (0.02)a1.88 (0.14)d1.11 (0.10)c0.81 (0.06)bc0.59 (0.08)ab
1728922Pentanoic acidRI, MS0.13 (0.04)b0.08 (0.00)aND0.1 (0.02)bND
1840984Hexanoic acidRI, MS4.22 (0.31)b1.22 (0.07)c0.21 (0.08)a2.90 (0.29)d0.31 (0.04)a
20511176Octanoic acidRI, MS0.74 (0.01)b1.05 (0.10)c0.76 (0.03)b0.42 (0.04)a0.46 (0.04)a
22641374Decanoic acidRI, MS0.90 (0.15)ab1.57 (0.07)c1.20 (0.15)b0.58 (0.09)a0.49 (0.03)a

According to the results of the quantitative analyses, aldedydes and alcohols had higher peak areas in five cherry fruits (over 80% of total volatiles), then followed the acids, esters and terpenes. The most represented class of compounds in all five cherries was C6 compounds and aromatic compounds, according to their relative peak areas, i.e. hexanal (1.55–21.00), (E)-2-hexenal (1.07–6.81), 1-hexanol (2.22–3.65), (E)-2-hexen-1-ol (16.25–25.05), benzaldehyde (2.18–17.74) and benzyl alcohol (23.62–82.84). The highest content of hexanal and (E)-2-hexenal was present in ‘Stella’, whereas the lowest content was found in ‘Rainier’. 1-Hexanol and (E)-2-hexen-1-ol had the highest content in ‘Hongdeng’. The result was similar to the previous report about sweet cherry ‘Hongdeng’ by Zhang et al.[5]. C6 compounds are known to have a characteristic ‘green leaf’ odour, as released from green plant tissue following mechanical damage.[24] As for benzaldehyde (almond-like odour) and benzyl alcohol (floral odour), they had the largest relative peak areas, in ‘Lapins’ and ‘Rainier’, respectively.

In addition, two ketones (geranylacetone and 6-methyl-5-hepten-2-one) were found with higher relative peak areas (0.54–1.27 and 0.46–1.56, respectively) in the five cherries. 6-Methyl-5-hepten-2-one has been found previously in some sweet cherry cultivars,[4,23] such as ‘Lapins’, ‘Stella’, ‘Ambrunes’ and ‘Pico Colorado’, and geranylacetone has been reported in Monastrell and Cabernet Gernischt grapes.[25,26] It is worth noticing that geranylacetone identified in the present investigation has not been reported in other studies about cherries.

Statistically significant differences were found between the average content of most compounds in the five cultivars by ANOVA and Duncan's multiple range test (Table 1). The most important compounds in differentiating samples were the total alcohols and β-phenylacetaldehyde.

GC–O Dilution Analysis

The GC–O dilution analysis was performed by successively diluting the cherry samples with distilled water. The concentrations of compounds extracted by SPME fibre had a better linear relationship with the dilutions at lower concentration, which had been evaluated by Deibler[6,7], Fan[10] and Choi[11].

To ensure that the results of GC–O dilution analysis were reliable, a linear relationship was drawn between the concentration of extracted compounds by SPME fibre and the dilutions. ‘Hongdeng’ juice was sequentially diluted at a 1:1 ratio with distilled water, and the responses (total ion abundance) for several alcohols, aldehydes, acids and esters were evaluated. The results showed that 1-hexanol, (E)-2-hexen-1-ol, benzyl aclcohol, hexanal, (E)-2-hexenal, benzaldelyde, ethyl hexanoate, linalool and hexanoic acid had adequate linearity for GC–O dilution analysis (Figure 2), since their linear correlation coefficients (R2) were all > 0.92. Among these, 1-hexanol generated the best correlation (R2 = 0.9918).

thumbnail image

Figure 2. Linearity of alcohols, aldehydes, acids and esters from ‘Hongdeng’ cherry juice extracted by HS–SPME (DVB/CAR/PDMS fibre) and detected on a DB-wax column using a series of 1:1 dilutions; the first dilution was assigned a number of 10. (A) 1-Hexanol, (E)-2-hexen-1-ol and benzyl aclcohol; (B) hexanal, (E)-2-hexenal and benzaldelyde; (C) ethyl hexanoate, linalool and hexanoic acid

Download figure to PowerPoint

Before GC–O dilution analysis, a direct GC–O technique was performed to make a comparison of SPME extracted odour with fresh cherry juice. Judging by the evaluation results of two assessors, the desorbed odour by GC from SPME was quite similar to that of the reference juice. In the meantime, ‘cooked’ odour was perceived in the juice at the first time of dilution, but this unpleasant odour gradually decreased along with sequential dilutions, and was hardly perceived by the fourth dilution.

On the basis of the FD values on a DB-Wax column (Table 2), a total of 40 aroma-active compounds were detected in the five cherries. The potentially important compounds were hexanal, (Z)-3-hexenal, (E)-2-hexenal, nonanal and geranylacetone (FD ≥ 16) in all five cherries. These compounds impart green, orange and floral odours, respectively. Hexanal and (E)-2-hexenal, as the important aroma compounds in sweet cherry fruits, have been reported previously.[4,5,23]

Table 2. Potent aroma compounds detected by HS–SPME and GC–O dilution analysis on a DB-Wax column
RI (DB-Wax)CompoundsDescriptorFD factors*
LapinsRainierHongdengStellaZhifuhong
  • *

    ND, not detected by GC–O.

Aldehydes
<900AcetaldehydeFruity1688816
<9002-MethylpropanalMalty882816
9713-MethylbutanalFruity16NDND168
1073HexanalGreen≥643232≥6432
1136(Z)-3-HexenalGreen1616163216
1192(Z)-2-HexenalGreen84444
1211(E)-2-HexenalGreen323232≥6432
1282OctanalFruity1648168
1305(Z)-2-HeptenalAlmond4ND24ND
1383NonanalOrange321632≥64≥64
1417(E)-2-OctenalMedicinal16448ND
1488DecanalGreen1648816
1569(E,Z)-2,6-NonadienalCucumber2216432
1517BenzaldehydeAlmond3216323216
1620β-PhenylacetaldehydeFloral16216168
1697(E,E)-2,4-NonadienalFatty16ND8162
Esters
<900Ethyl acetatePineapple24248
1023Ethyl butanoateFruityND222ND
1232Ethyl hexanoateApple88884
1320Hex-2-enyl acetateFruity42484
1771Methyl salicylateHolly1688816
Alcohols
11561-ButanolFruity2NDND22
13581-HexanolFloral, grape888168
1373(Z)-3-Hexen-1-olGreen16168168
1405(E)-2-Hexen-1-olVegetable88888
14411-Octen-3-olMushroom168882
15581-OctanolFruity22222
1794α-Methylbenzyl alcoholGreenND2NDND4
1860Benzyl alcoholFloral161616816
1896β-Phenylethyl alcoholRosy881688
Terpenes
1169LimoneneFruityND2428
1551LinaloolCitrusy164161616
1633MentholMinty84828
1856GeranylacetoneFlowery1616321632
Acids
1435Acetic acidAcidic22222
16603-Methylbutanoic acidSweaty881688
1840Hexanoic acidAcidic42484
2051Octanoic acidFatty2ND222
Furans
12752-PentylfuranCaramel4NDND4ND
Ketones
13246-Methyl-5-hepten-2-oneSweety84884

C6 compounds seemed to be the most important aroma compounds in cherry. Hexanal, (E)-2-hexenal and (Z)-3-hexenal showed higher FD factors (FD ≥ 16) in all five cherry cultivars, whereas 1-hexanol, (E)-2-hexen-1-ol and (Z)-3-hexen-1-ol exhibited medium FD factors (FD ≥ 8). Compared to ‘Lapins’, ‘Rainier’, ‘Hongdeng’ and ‘Zhifuhong’, 1-hexanol (floral and grape odour) had the highest FD factor (FD = 16) in ‘Stella’. (Z)-2-Hexenal and hexanoic acid were detected in all samples with lower FD factors. They present green, acidic aromas. These C6 alcohols and aldehydes arise from the consecutive action of the lipoxygenase and alcohol deshydrogenase enzymes on polyunsaturated fatty acids.[25]

Nonanal had higher FD factors in ‘Stella’ and ‘Zhifuhong’ (FD ≥ 64) but lower values (FD = 16) in ‘Rainier’. Benzaldehyde and β-phenylacetaldehyde (FD = 32 and 16, respectively) was probably important to ‘Lapins’, ‘Hongdeng’ and ‘Stella’, giving almond and floral aroma, respectively. Mattheis et al. have reported relatively high concentrations of benzaldehyde in Prunus avium ‘Bing’[3]. Girard and Kopp have also found the importance of benzaldehyde by research on 12 sweet cherries.[4] Acetaldehyde, octanal and decanal had moderate FD factors (FD ≥ 4) in five cherries. Octanal (lemon-like odour) and decanal (grassy odour) can be perceived easily because of their low olfactory thresholds.[27]

2-Methylpropanal, 3-methylbutanal, (E)-2-octenal, (E,Z)-2,6-nonadienal and (E,E)-2,4-nonadienal had greater differences in FD factor in the five cherries. 2-Methylpropanal could be very important to ‘Zhifuhong’ (FD = 16), whereas its FD factor was only 2 in ‘Hongdeng’. 3-Methylbutanal had the highest FD factor (FD = 16) in ‘Lapins’ and ‘Stella’; however, it was not detected in ‘Rainier’ and ‘Hongdeng’. (E)-2-Octenal could be a potential aroma-active compound (FD = 16) in ‘Lapins’, but was not found in ‘Zhifuhong’. (E,Z)-2,6-nonadienal and (E,E)-2,4-nonadienal, giving cucumber and fatty aromas, respectively, have lower threshold levels (0.02 and 0.06 ppb, respectively, in water).[28] (E,Z)-2,6-Nonadienal had a higher FD factor (FD ≥ 16) in ‘Hongdeng’ and ‘Zhifuhong’, whereas (E,E)-2,4-nonadienal could be important in ‘Lapins’ and ‘Stella’ (FD = 16).

Benzyl alcohol (floral aroma) and β-phenylethyl alcohol (rosy aroma) were identified as potentially important aroma compounds in the five cherries, based on their AEDA values (FD ≥ 8). 1-Octen-3-ol, with a mushroom odour, contributed to the aroma (FD ≥ 8) in ‘Lapins’, ‘Rainier’, ‘Stella’ and ‘Hongdeng’. 1-Octanol was detected in all five cherry cultivars; it had lower FD factors and gave a fruity aroma. 1-Butanol was found in ‘Lapins’, ‘Rainier’ and ‘Zhifuhong’ with low FD factors (FD ≤ 4).

Several esters were detected in the five cherries. Among them, methyl salicylate, which generates a wintergreen odour, had a higher FD factor (FD ≥ 8) in the five cherries. Ethyl hexanoate exhibited potential importance in ‘Lapins’, ‘Rainier’, ‘Stella’ and ‘Hongdeng’, based on its FD factor (FD = 8). Ethyl acetate had the highest FD factor (FD = 8) in ‘Zhifuhong’, whereas hex-2-enyl acetate could be more important in ‘Stella’ (FD = 8).

As for the determination of acids, a total of four acids were detected by means of GC–O, acetic, 3-methylbutanoic, hexanoic and octanoic acids. 3-Methylbutanoic acid seemed to be the most significant acid in the five cherries (FD ≥ 8). Hexanoic acid had a moderate FD factor (FD = 8) in ‘Stella’. Generally, the contribution of acetic acid and octanoic acid to cherry aroma may be limited (FD ≤ 4), because their thresholds are quite high.

Four terpenes, limonene, linalool, menthol and geranylacetone, were detected in the five cherries. Among them, linalool, menthol and geranylacetone were common to all five samples, but limonene was absent from ‘Lapins’. Linalool, presenting as a citrusy aroma, seems to be an important odorant (FD = 16) in four cherries, with the exception of ‘Rainier’ (FD = 4). Menthol had middle FD factors (FD = 8) in ‘Lapins’, ‘Hongdeng’ and ‘Zhifuhong’, giving a minty aroma.

To sum up the results, the most significant aroma compounds in ‘Lapins’ were hexanal (FD ≥ 64); (E)-2-hexenal, nonanal and benzaldehyde (FD = 32); acetaldehyde, 3-methylbutanal, (Z)-3-hexenal, octanal, (E)-2-octenal, decanal, β-phenylacetaldehyde, (E,E)-2,4-nonadienal, methyl salicylate, (Z)-3-hexen-1-ol, 1-octen-3-ol, benzyl alcohol, linalool, and geranylacetone (FD = 16). In ‘Rainier’, hexanal and (E)-2-hexenal (FD = 32); (Z)-3-hexenal, nonanal, benzaldehyde, (Z)-3-hexen-1-ol, benzyl alcohol and geranylacetone (FD = 16) were determined as the most important odour-active volatiles. In ‘Hongdeng’, the most significant aroma compounds identified on the DB-Wax column involved hexanal, (E)-2-hexenal, nonanal, benzaldehyde and geranylacetone (FD = 32); (Z)-3-hexenal, (E,Z)-2,6-nonadienal, β-phenylacetaldehyde, benzyl alcohol, β-phenylethyl alcohol, linalool, and 3-methylbutanoic acid (FD = 16). In ‘Stella’, hexanal, (E)-2-hexenal and nonanal (FD ≥ 64); (Z)-3-hexenal and benzaldehyde (FD = 32); 3-methylbutanal, octanal, β-phenylacetaldehyde, (E,E)-2,4-nonadienal, 1-hexanol, (Z)-3-hexen-1-ol, 1-octen-3-ol, linalool and geranylacetone (FD=16) were found to contribute greatly to the aroma profile. Finally, in ‘Zhifuhong’, nonanal (FD ≥ 64); hexanal, (E)-2-hexenal, (E,Z)-2,6-nonadienal and geranylacetone (FD = 32); acetaldehyde, 2-methylpropanal, (Z)-3-hexenal, decanal, methyl salicylate benzyl alcohol and linalool (FD = 16) were determined as the most important odour-active volatiles.

Conclusion

In the present study, HS–SPME using DVB/CAR/PDMS fibre was employed to extract the volatiles in ‘Lapins’, ‘Rainier’, ‘Stella’, ‘Hongdeng’ and ‘Zhifuhong’. A total of 52 compounds were identified in five cherries on DB-wax and DB-5 columns. Of these, hexanal, (E)-2-hexenal, 1-hexanol, (E)-2-hexen-1-ol, benzaldehyde and benzyl alcohol were the main volatile compounds, according to their high comcentrations in five cherries.

A combination of HS–SPME and GC–O dilution analysis techniques provided a satisfactory assessment of the most volatile compounds that play a major role in odour perception. Hexanal, (E)-2-hexenal, (Z)-3-hexenal, nonanal, geranylacetone, benzaldehyde and benzyl alcohol were the most important aroma-active compounds in all five cherry samples. In addition, acetaldehyde, 3-methylbutanal, octanal, (E)-2-octenal, (Z)-3-hexen-1-ol, decanal, β-phenylacetaldehyde, (E,E)-2,4-nonadienal, methyl salicylate, 1-octen-3-ol, linalool and (E,Z)-2,6-nonadienal were also the important aroma compounds in certain cherry cultivars. From the present results, it was concluded that the aroma profiles were similar in the five cherry cultivars, but significant variations were found in the contributions of these compounds to each cherry. In future investigations, quantitative analysis and sensory work are needed to further characterize the differences between these cherry cultivars.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References

This work was financially supported by the doctoral foundation of Ludong university (LY2010002).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Acknowledgements
  7. References