Comparison of Flavor Compounds in Fresh and Pickled Bamboo Shoots by GC-MS and GC-Olfactometry

Abstract

Volatile compounds from fresh and pickled bamboo shoots were extracted using a headspace-solid phase microextraction (HS-SPME), and analysed with a gas chromatography-mass spectrometry (GC-MS) and a gas chromatography olfactometry (GC-O). Result showed that a total 68 major volatile compounds were identified in fresh and pickled bamboo shoots by GC-MS analysis; 38 volatile compounds in fresh bamboo shoots were identified. Among them, 2-pentylfuran, hexanal, benzaldehyde, 1-hexanol and (E)-2-nonadienal were the main volatile compounds. As for pickled samples, 52 major volatile compounds were identified, the main of which included ethanol, 1-hexanol, hexanal, methoxy-phenyl-oxime and acetic acid. Moreover, 17 flavor-active compounds in fresh samples were identified by GC-O analysis. Grass and astringent flavor were the key flavor-active compounds. As for pickled samples, 19 flavor-active compounds were identified by GC-O analysis. Pungent and rancid flavor were the main flavor-active compounds. The differences of flavor features between fresh and pickled bamboo shoots suggested that flavor components in fresh bamboo shoots had obviously changed after the pickling process.

Introduction

Bamboo shoots are the immature and edible bamboo plants that just emerge from the ground, and they often constitute a range of traditional Asian dishes (Choudhury et al., 2012). The bamboo shoot being low in fat, high in dietary fibre and rich in mineral content, as an ideal vegetable has been used traditionally in many Asian countries (Chongtham et al., 2011). Bamboo shoots have high nutritive values and play essential roles in promoting the peristalsis of the stomach and the intestine (Park and Jhon, 2009), helping digestion and preventing cardiovascular diseases (Park and Jhon, 2010).

Several processing methods have been proven to be effective to preserve fresh bamboo shoots, such as pickling, roasting, boiling, blanching and canning (Satya et al., 2010). Many research reports have shown that different processing methods have varying degrees of influence on nutritional contents and flavor of bamboo shoots (Kumbhare and Bhargava, 2007; Nirmala et al., 2008; Giri et al., 2000). Meanwhile, some studies were conducted on the volatile flavor profiles of different kinds of bamboo shoots. The volatile aromatic components of Phyllostachys pubescens shoots were examined by Toshiyuki et al. (2010) and Chung et al. (2012) respectively, while the aroma-active components in fermented bamboo shoots were studied by Fu et al. (2002). However, little investigation is available on the volatile compounds of pickled bamboo shoots (Dendrocalamus latiflorus). Therefore, identification of the volatile compounds and clarification of the flavor compounds in pickled bamboo shoots are necessary to be investigated.

In recent years, headspace-solid phase microextraction (HS-SPME) has been widely used as an effective extraction technique for different samples (Barra et al., 2007; Farag and Wessjohann, 2012), and combination of HS-SPME with gas chromatography olfactometry (GC-O) techniques have been developed to study on characterization of aroma active compounds in food (Falcao et al., 2008; Kang et al., 2012; Resconi et al., 2012). Gas chromatography with olfactometric detection is based on sensory evaluation of the active odor components. Some intensive studies have been carried out regarding the sensory activity of the individual flavor components of foodstuffs, and the dependence between the odor and the chemical composition of the volatile fraction of these products (Perez-Silva et al., 2006; Cullere et al., 2011). The majority of the accomplishments within this area can be attributed to the combination of gas chromatography with olfactometric detection.

In this study, HS-SPME combined with GC-MS, was developed and applied to identify and evaluate and the volatile compounds in fresh and pickled bamboo shoots. GC-O technique was used to analyze the odor characterization of fresh and pickled bamboo shoots. The main objectives of this research were to clarify the nature of the flavor compounds of the fresh and pickled bamboo shoots and compare their difference in the volatile compounds, and to reveal the changes in flavor compositions of bamboo shoots after the pickling process. The results will benefit the scientific knowledge about the effect of the pickling process on the volatile and flavor of bamboo shoots, and provide test data for the special flavor quality assessment of pickled bamboo shoots.

Materials and Methods

Sample preparation    Bamboo shoots (Dendrocalamus latiflorus) were obtained commercially from a local market in Chongqing City in October, 2011. Fresh bamboo shoots with complete, better color, lustre and same size were selected and then cut into the shape of sheet about 4 cm long, 3 cm width, 0.3 cm thick before pickling process. The ethyl decanoate (ampule of 5 ml; Sigma-Aldrich China Inc., Shanghai, China) as an internal standard to enable assessment of the retention time precision for the volatile component peaks.

The pickled bamboo shoots were prepared in the traditionally homemade way. The fresh bamboo shoots (5 kg) were washed with clean water and blanched by boiling water, then placed in a 20 L plastic container and mixed with 400 g pure and granulated pickling salt after the bamboo shoots draining and cooling. Finally, the samples were filled into a 10 L earthen jar and sealed against the rim with plastic rope and placed indoors for natural fermentation at temperature ranging from 10 to 25°C. The bamboo shoots pickles were taken out from the jar and sampled after 3 months storage.

Headspace solid-phase microextraction    The extraction of volatile compounds was carried out by a HS-SPME method. A manual SPME device and SPME fibers were obtained from Supelco Co (Bellefonte, PA, USA). According to our previously experimental research about SPME fiber choice, a fused silica fiber, coated with a 65 mm layer of DVB/PDMS, was ultimately chosen to extract the volatile compounds of the selected samples.

Hundred grams of fresh bamboo shoots and their pickles were grinded, then 10 g samples and 10 µL ethyl decanoate (50 µg/mL in ethyl ether) were placed in a 40 mL vial closed by a PTFE/silicone septum (Supelco) and immediately kept at 50°C to equilibrate for 5 min in a water-bath. The headspace solid-phase microextraction technique was used as follows: Selected sample vials were placed into a water-bath at 50°C. Then, the SPME fiber was exposed to the HS 1 cm above the sample surface to adsorb the analytes for 40 min while maintaining the sample at 50°C. After this time the fiber was withdrawn into the needle and then introduced into a heated chromatograph injector for desorption and sanalysis. The fiber was in the injection port to desorb volatile compounds for 3 min and the plunger depth was set at 3 cm to allow the hottest part of the injection port.

GC-MS analysis    GCMS-QP2010 Plus (Shimadzu, Japan) was used to perform the GC-MS analysis. Volatile compounds were separated using a capillary chromatographic column (DB-5MS, 30 m × 0.25 mm i.d., 0.25 µm film thickness). Thermal desorption of the compounds from the fiber coating took place in the GC injector at 250°C in a splitless mode. the oven temperature was programmed as follows: 40°C held for 2 min, then raised to 80°C at a rate of 6°C /min, and to 230°C at a rate of 10°C/min, then held for 10 min. The carrier gas was helium (percentage purity > 99.999%) at a constant flow velocity of 1 mL/min. The mass spectra were acquired with a source temperature of 250°C under a 70 eV ionization potential. Interface temperature was 250°C. The ionization mode was EI, emission current was 200 µA, and detector voltage was 830 V. The mass scan range was 30 ∼ 350 AMU.

Retention indices (RI) were calculated using an alkane mix (C8-C20) and compared with the literature. Each compound was identified using the National Institute of Standards and Technology (NIST) library (2005L), the linear retention indices (LRI) and authentic standard (Similarity Index > 80). The identifications of some volatile compounds were only performed using mass spectrometry data because the retention index was unavailable.

Because internal standard was used in the present work, results were described in terms of relative peak area abundance calculated against the internal standard peak area (as 1.00) eluted on the chromatogram. For every vial sample, volatile compounds were extracted by SPME and analyzed by GC-MS at three repetitions under identical experimental conditions. Therefore, the results described in the present work are representative of a characteristic run of each sample.

GC-O analysis    The GC-O analysis was conducted using GC-2010 equipped with an FID detector (Shimazu, Japan) and sniffing port (Sniffer 9000, Brechbühler, Made in Switzerland). The column and analysis conditions were same as those described for GC-MS. The gas chromatography effluents of odor extract were split between the sniffing port and FID (1:1). The sniffing test on the odor compounds in Bamboo shoots pickles was performed by three trained persons with reference compounds.

They responded to the odor intensity of the stimulus by using a 3-point scale ranging from 1 to 3, 1 = weak, 2 = moderate and 3 = extreme. Each sample was sniffed twice by each trainer, and the smell intensity values were averaged for all six analyses. Retention times of alkane standards were analyzed by GC-FID prior to GC-O analysis. This enabled the conversion of flavor compounds peak retention times to standard RI. Standard values could then be related to indices from GC-MS sample analysis and compared with published values from compounds of known identity.

Statistical Analysis    All values provided were the average of triplicate (or more than triplicate) experiments. Analysis of variance and Tukey's test were carried out, with confidence level of 95% (p ≤ 0.05), using the software Microcal Origin 7.5 (Microcal Software, Inc., Northampton, U.S.A) to determine the significant difference of the results.

Results and discussion

Composition of volatile compounds    The GC-MS chromatograms of the flavor profile of fresh and pickled bamboo shoots are shown in Fig. 1. According to the identification of the chromatographic peaks, GC-MS data for the analysis of volatile compounds are shown in Table 1. From the results of GC-MS analysis, we identified 68 major volatile compounds: 38 in fresh bamboo shoots and 52 in pickled bamboo shoots, with 16 in both. The test results are obvious different in the two samples by comprehensive analysis those detected volatile components. The main volatile compounds from fresh samples include 2-pentylfuran (11.95), hexanal (9.84), benzaldehyde (9.22), 1-hexanol (6.49) and (E)-2-nonadienal (4.94), which accounted for 59.0% of the total composition. Meanwhile, the main volatile compounds from pickled samples include ethanol (15.85), 1-hexanol (12.15), hexanal (7.97), methoxy-phenyl-oxime (6.30) and acetic acid (6.08), which accounted for 63.7% of the total composition. The results showed that volatile components from bamboo shoots had obviously changed after pickle fermentation (Fig. 2). Alcohols, ketones, esters and acids content increased significantly (p < 0.05), whereas aldehydes and hydrocarbons content reduced obviously (p < 0.05), which may be associated with preserved microbial metabolic activities.

Fig. 1.

Representative chromatograms of GC-MS analysis of fresh (A) and pickled (B) bamboo shoots.

Table 1. The volatile compounds in fresh and pickled bamboo shoots by GC-MS.

No.	Compounds	MIa	Peak area abundantb,c,d
			Fresh	Pickled
	Alcohols
1	Ethanol	MS	2.04 ± 0.42	15.85 ± 1.13
2	(E)-4-methyl-Cyclohexanol	MS	1.69 ± 0.37	ND
3	3-methyl-1-Butanol	MS, KI	ND	0.47 ± 0.19
4	1-Pentanol	MS, KI	1.48 ± 0.53	3.01 ± 0.68
5	(Z)-2-Penten-1-ol	MS	0.57 ± 0.27	0.64 ± 0.23
6	1-Hexanol	MS, KI	6.49 ± 0.89	12.15 ± 1.21
7	(E)-3-Octen-2-one	MS	ND	0.46 ± 0.19
8	(E)-2-Hexen-1-ol	MS	0.41 ± 0.23	ND
9	1-Octen-3-ol	MS, KI	0.48 ± 0.16	1.85 ± 0.40
10	(E)-2-Hepten-1-ol	MS, KI	ND	0.36 ± 0.17
11	1-Heptanol	MS, KI	0.81 ± 0.35	ND
12	2-Heptanol	MS, KI	ND	0.13 ± 0.12
13	1-Octanol	MS, KI	1.85 ± 0.47	0.33 ± 0.16
14	Farnesol	MS	0.21 ± 0.15	ND
15	(E)-2-Octen-1-ol	MS, KI	0.56 ± 0.19	0.58 ± 0.25
16	1-Nonanol	MS, KI	0.59 ± 0.20	0.43 ± 0.21
17	(E)-2-Nonen-1-ol	MS, KI	1.44 ± 0.61	0.21 ± 0.13
18	2,6-Nonadien-1-ol	MS	0.49 ± 0.18	ND
19	Benzyl alcohol	MS	ND	1.06 ± 0.52
	Aldehydes
20	Acetaldehyde	MS	0.21 ± 0.16	0.16 ± 0.14
21	3-methyl-Butanal	MS, KI	ND	0.08 ± 0.03
22	Pentanal	MS, KI	ND	0.46 ± 0.16
23	Hexanal	MS, KI	9.84 ± 1.09	7.97 ± 1.18
24	2,4-Heptadienal	MS	ND	0.19 ± 0.12
25	(Z)-2-Heptenal	MS	0.62 ± 0.36	0.65 ± 0.42
26	Nonanal	MS, KI	2.85 ± 0.49	0.35 ± 0.15
27	(E)-2-Octenal	MS	0.83 ± 0.43	0.41 ± 0.19
28	Benzaldehyde	MS	9.22 ± 1.09	2.92 ± 0.82
29	(E)-2-Nonenal	MS	4.94 ± 0.76	0.25 ± 0.11
30	(E)-2-(Z)-6-Nonadienal	MS, KI	1.86 ± 0.09	ND
	Ketones
31	Acetone	MS	0.26 ± 0.16	0.43 ± 0.21
32	2-Butanone	MS,KI	0.42 ± 0.18	ND
33	4,4-dimethyl-2-Pentanone	MS	ND	1.05 ± 0.44
34	2-Octanone	MS, KI	ND	0.25 ± 0.15
35	(E,E)-3,5-Octadien-2-one	MS	ND	0.67 ± 0.23
36	2-Tridecanone	MS	ND	0.28 ± 0.11
37	2,10-Dimethyl-5,9-undecadien-2-one	MS	ND	0.45 ± 0.22
38	Acetophenone	MS	ND	0.31 ± 0.19
39	β-Ionone	MS, KI	0.29 ± 0.14	ND
	Acids
40	Acetic acid	MS	0.21 ± 0.12	6.08 ± 0.97
41	Butanoic acid	MS	ND	0.57 ± 0.21
42	Hexanoic acid	MS	ND	1.86 ± 0.86
43	Octanoic acid	MS	ND	0.23 ± 0.16
	Esters
44	Ethyl Acetate	MS, KI	ND	0.76 ± 0.58
45	Thiocyanic acid, methyl ester	MS	ND	0.20 ± 0.16
46	Methyl salicylate	MS	0.33 ± 0.15	0.31 ± 0.14
47	γ-Amylbutyrolactone	MS	ND	0.17± 0.12
48	γ-Nonalactone	MS	ND	0.23 ± 0.13
	Hydrocarbons
49	Octane	MS, KI	0.57 ± 0.19	ND
50	1,3-dimethyl-Benzene	MS	0.96 ± 0.38	ND
51	(E)-7-methy-l-1,6-dioxaspiro[4,5]-Decane	MS	3.64 ± 0.68	ND
52	dodecane	MS, KI	ND	0.18 ± 0.13
53	Pentadecane	MS, KI	ND	0.24 ± 0.15
54	Hexadecane	MS, KI	ND	0.21 ± 0.11
55	(E)-1,-3-Nonadiene	MS, KI	0.47 ± 0.27	ND
56	methoxy-Benzene	MS, KI	0.64 ± 0.31	ND
57	1-ethyl-2,3-dimethyl-Benzene	MS	0.72 ± 0.29	ND
58	Hexanenitrile	MS	ND	0.54 ± 0.30
59	Naphthalene	MS	0.56 ± 0.29	0.37 ± 0.24
	Phenolic compounds
60	Phenol	MS	0.76 ± 0.43	ND
61	2-methoxy-Phenol	MS, KI	ND	0.21 ± 0.18
62	p-Cresol	MS	ND	0.87 ± 0.43
	Others
63	dimethyl Sulfide	MS	0.20 ± 0.11	0.26 ± 0.13
64	dimethyl Disulfide	MS	ND	1.06 ± 0.74
65	2,3-dihydro-3-methyl-Furan	MS	ND	0.31 ± 0.19
66	2-ethylic-Furan	MS	0.53 ± 0.21	ND
67	2-pentyl-Furan	MS	11.95 ± 1.23	0.68 ± 0.38
68	methoxy-phenyl-Oxime	MS	ND	6.30 ± 0.91

^a Method of identification: MS, mass spectrum comparison using Wiley and NIST libraries; KI, Kovats index in agreement with literature values.

^b The numbers indicate relative peak area abundance calculated against the internal standard peak area (as 1.00) eluted on the chromatogram, results are expressed as means (n = 3) from total analyzed samples. Errors expressed as estimates of standard deviation for three parallel measurements, P ⩽ 0.05.

^c ND notated the results “not detected”.

^d Fresh stands for fresh bamboo shoots, Pickled stands for pickled bamboo shoots.

Fig. 2.

Comparison of volatile compositions between fresh and pickled bamboo shoots

Alcohols were the most abundant compounds isolated in the pickled bamboo shoots. 14 alcohols were identified from the pickled bamboo shoots, which accounted for 49.4% of the total flavor components. Among the alcohols, ethanol (15.85) was the most abundant one, followed by 1-hexanol (12.15), 1-pentanol (3.01) and 1-octen-3-ol (1.85). Steinhaus et al. (2009) demonstrated that alcohols have pleasant aromas and sweet flavor, thereby alcohols may be play an important role in the flavor formation of the pickled bamboo shoots. 1-octen-3-ol with the mushroom flavor was also detected in fermented bamboo shoots (Fu et al., 2002). Meanwhile, alcohols were the second abundant compounds in the fresh bamboo shoots. 14 alcohols were detected in the fresh bamboo shoots, which accounted for 26.5% of the total flavor components. Among the alcohols, 1-hexanol (6.49) was the most abundant alcohols, followed by ethanol (2.04), 1-octanol (1.85) and (E)-4-methyl-cyclohexanol (1.69). Compared with the fresh bamboo shoots, the alcohols content increased significantly from 19.11 to 37.53 in the pickled samples that are probably due to alcoholic fermentation and producing a large number of alcohols during the pickling process. Among the alcohols, ethanol content increased significantly (p < 0.05), which is mainly due to lactic acid fermentation process for producing large amounts of ethanol, acetic acid, carbon dioxide, and other substances in the ferment initial period. Meanwhile, the yeast fermentation process also produced ethanol; thereby ethanol content is relatively abundant (Zhang, Chen, & Yu, 2010). In addition, 1-Hexanol content also increased significantly (p < 0.05) in pickled samples compared with the fresh samples. 1-Hexanol is a common and important flavor component for some vegetables, and is the precursor for the straight-chain esters (Rowan et al., 1999).

Aldehydes were the most abundant compounds isolated in the headspace of the fresh bamboo shoots and they would contribute considerably to the total favor of fresh samples due to their low odor thresholds. 8 Aldehydes were detected from the fresh bamboo shoots, which accounted for 42.2% of the total flavor components. Among the aldehydes, hexanal (9.84) was also the most abundant compounds, followed by benzaldehyde (9.22), (E)-2-nonenal (4.94), nonanal (2.85) and (E,Z)-2,6-nonadienal (1.86). 10 Aldehydes were identified from the pickled bamboo shoots, which accounted for 17.7% of the total flavor components, were the most second abundant compounds in the pickled samples. Among them, hexanal (7.97) was the most abundant compounds, followed by benzaldehyde (2.92). This result showed that the amount of aldehydes in pickled samples increased compared with fresh samples, but the content reduced obviously from 30.37 to 13.44, which may be aldehydes were partly oxidated for acids during the process of pickling. Hexanal has oil and green apple aroma, naturally exists in many fruits and vegetables, which possesses volatile substances that contribute the most to the aroma of the fruits and vegetables (Janes et al., 2009). Benzaldehyde exists extensively in fruits and flowers (Krist et al., 2004; Radulovic et al., 2009), mainly in the form of glycosides in plants' stems, leaves or seeds, which has bitter almond flavor (Chu and Yaylayan, 2008). (E)-2-nonenal was identified in moso-bamboo stems and estimated to have a bamboo-like aroma (Takahashi et al., 2010). Among the aldehydes, benzaldehyde, as well as (E)-2-nonenal and nonanal content in pickled samples reduced significantly (p < 0.05), especially the content of benzaldehyde reduced from 9.22 to 2.92, which may be explained by that benzaldehyde in fresh samples was partially reduced to benzyl alcohol during the pickling process, thereby benzyl alcohol was detected in the pickled samples but not detected in fresh samples. Furthermore, (E)-2-nonenal and nonanal were possibly partially oxidated for acids, causing their content to decline in pickled samples. The results of research can be demonstrated during potherb mustard pickle fermentation (Zhao et al., 2007). In addition, the content of hexanal and (E,Z)-2,6nonadienal in pickled samples also reduced significantly (p < 0.05), and that (E,Z)-2,6-nonadienal was not detected in pickled samples.

Four acids were identified from the pickled bamboo shoots, which accounted for 11.5% of the total flavor components. Among the acids, acetic acid (6.08) was the most abundant acid, followed by hexanoic acid (1.86), butanoic acid (0.57) and octanoic acid (0.23). However, only acetic acid (0.21) was detected in fresh samples. Acids commonly exhibit pungent and unpleasant odor thus they may be have significantly influence on the flavor of pickled bamboo shoots. Acetic acid content significantly increased (p < 0.05) after pickling fermentation which is in accordance with Ur-Rehman et al. (2000) who reported that acetic acid originates from different reactions by the action of lactic acid bacteria. The other major acids detected were hexanoic and butanoic acid, and they provides a rancid and sweaty odor (Delgado et al., 2011), could importantly contribute to the rancid odor of pickled bamboo shoots. Compared with the fresh bamboo shoots, the acids content increased significantly (p < 0.05) in the pickled samples. Acids content in pickled samples increased from 0.21 to 8.74 compared with the fresh samples. It shows that acids are probably produced through the biosynthetic pathway in bamboo shoots cell tissue during pickling fermentation. Acetic acid content significantly increased (p < 0.05) after pickling fermentation and it was the major compound isolated in the pickled bamboo shoots. Butyric acid, hexanoic acid and octanoic acid provided the rancid and olid flavor in pickled bamboo shoots. However, those compounds were not detected in fresh bamboo shoots.

Seven ketones were identified from the pickled samples, which accounted for 4.5% of the total flavor components. Among the ketones, 4,4-dimethyl-2-pentanone (1.05) was the most abundant compound, followed by (E, E)-3,5-octadien-2-one (0.67) and acetone (0.43). 3 ketones were detected in the fresh samples, which accounted for 0.9% of the total flavor components. The total content of ketones and esters in pickled samples also increased significantly (p < 0.05) compared with the fresh samples. Ketones content of pickled samples is more abundant than fresh samples possibly due to the β-oxidation of the carbon chain of polyunsaturated fatty acids and subsequent decarboxylation generated, such as acetone, pentanone, octanone (Tanchotikul and Hsieh, 1989).

Five esters were identified in this investigation, which accounted for 2.1% of the total flavor components. The most abundant esters were ethyl acetate (0.76) and methyl salicylate (0.31). Methyl salicylate (0.33) was only one ester detected in fresh bamboo shoots. The content of ethyl acetate is less as well as their perception threshold is very high. Esters are probably produced through the biosynthetic pathway in bamboo shoots cell tissue during pickling fermentation.

p-Cresol (0.87) and 2-methoxyphenol (0.21) were two of the most predominant phenolic compounds detected from the pickled samples, and they have been associated with certain smoky and pungent flavor (Hollenbeck, 1994). p-Cresol as the main flavor-active compounds in fermented bamboo shoots also was identified by Fu et al. (2002). Selmer and Andrei (2001) reported p-cresol was the main fermentation by-product of tyrosine, the major free amino acid in bamboo shoots.

In addition, 2-pentylfuran (11.95) content in fresh samples was abundant. Furans is a common flavor component, which exists naturally in coffee, potatoes and other plants and their aroma are similar to flavor of bean, fruit and vegetable (Ang and Boatright, 2003; Beaulieu and Lea, 2006). Furthermore, some two methyl sulfides were identified from the pickled samples, which were dimethyl disulfide (1.06) and dimethyl sulfide (0.26) respectively. Although two methyl sulfides content are lower, due to their low odor thresholds, they have important impact on the flavor of pickles. The results were consistent with the previous studies on Korean kimchi (Cha et al., 1998).

Identification of flavor-active compounds    The flavor extract obtained by HS-SPME extraction of the representative fresh and pickled bamboo shoots were subjected to GC-O analysis. 17 flavor-active compounds in fresh samples were detected by GC-O, including 7 alcohols, 5 aldehydes, 1 hydrocarbon, 1 ester, 1 ketone, 1 phenolic compound and 1 dimethyl sulfide. Meanwhile, 19 flavor-active compounds from pickled samples were identified by panellists, including 5 aldehydes, 4 acids, 3 alcohols, 3 esters, 2 ketones, 1 phenolic compound and 1 dimethyl sulfide, as shown in Table 2.

As flavor components, the aldehydes were well studied in some fruits, vegetables and their processed products. 5 Aldehydes were identified in fresh samples by GC-O, including hexanal, benzaldehyde, (E)-2-octenal, (E)-2-nonenal, (E,Z)-2,6-nonadienal, and they would contribute significantly to the total favor of fresh bamboo shoots due to their low odor thresholds. GC-O identified hexanal and benzaldehyde as the most important flavor-active compounds in terms of their flavor intensities and their low odor thresholds (4.5 µg/kg), and they provided the grass and astringent flavor. Meanwhile, (E,Z)-2, 6-nonadienal had also contributed significantly to the grass flavor due to its higher intensity. Moreover, (E,Z)-2,6-nonadienal with the green leaves and cucumber flavor was also identified in bamboo shoots (Takahashi et al., 2010). In addition, Takahashi et al. (2010) also reported that (E)-2-nonenal exhibited the bamboo-like odor and play an important role in the aroma of fresh bamboo shoots.

Table 2. The GC _ O sniff results of flavor compounds in fresh and pickled bamboo shoots.

No.	RI (DB-5MS)	Aroma compounds	Odor description	MIa	intensity
					fresh	pickled
1	576	Acetic acid	Sour	MS, RT	ND	2
2	716	Thiocyanic acid methyl ester	Garlicky	MS, RT	ND	1
3	722	Dimethyl disulfide	Pickle	MS, RT	1	1
4	761	1-Pentanol	Fruity	MS,RT, RIL	1	ND
5	769	(Z)-2-Penten-1-ol	Medicine	MS, RT	2	2
6	775	Butyric acid	Rancid	MS,RT	ND	3
7	806	Hexanal	Grass	MS, RT, RIL	3	2
8	860	1-Hexanol	Astringent	MS, RT, RIL	1	1
9	901	Phenol	Astringent	MS, RT	1	ND
10	914	(E)-1,-3-Nonadiene	Fruity	MS, RT, RIL	1	ND
11	921	2,4-Heptadienal	Sour, Sweet	MS, RT	ND	1
12	952	2-Octanone	Green, Sour	MS, RT, RIL	ND	1
13	960	1-Heptanol	Fatty	MS, RT, RIL	1	ND
14	968	(E,E)-3,5-Octadien-2-one	Metal	MS, RT	ND	1
15	974	Hexanoic acid	Rancid, Olid	MS, RT	ND	3
16	982	Benzaldehyde	Pungent, Astringent	MS, RT	3	3
17	1013	(E)-2-Octenal	Coffee, Popcorn	MS, RT	2	2
18	1014	p-Cresol	Pungent, Olid	MS, RT	ND	2
19	1067	(E)-2-Octen-1-ol	Fruity	MS, RT, RIL	1	ND
20	1112	(E)-2-Nonenal	Green	MS,RT	2	1
21	1120	(E,Z)-2,6-Nonadienal	Grass	MS, RT, RIL	2	ND
22	1167	(E)-2-Nonen-1-ol	Green, Astringent	MS, RT, RIL	1	1
23	1173	Octanoic acid	Rancid, Sweat-like	MS, RT	ND	1
24	1175	2,6-Nonadien-1-ol	Fragrant	MS, RT, RIL	2	ND
25	1281	Methyl salicylate	Fragrant, Holly	MS, RT	1	1
26	1284	γ-Nonalactone	Sweet, Fruity	MS, RT	ND	1
27	1457	β-Ionone	Sweet, Fragrant	MS, RT, RIL	2	ND

^a Method of identification: MS, identified by MS; RT, identified by retention time of standard compounds; RIL, identified by retention index in agreement with literature values.

ND notated the results “not detected”.

Aldehydes were also the most abundant compounds in pickled samples detected by GC-O, including hexanal, 2, 4-heptadienal, benzaldehyde, (E)-2-octenal and (E)-2-nonenal. Among them, the order of intensity was benzaldehyde, hexanal, (E)-2-octenal and others. Benzaldehyde was also the highest flavor intensity in pickled bamboo shoots, thereby its pungent and astringen flavor had also significant effect on the flavor of pickled bamboo shoots.

Four acids with pungent odor from pickled samples were perceived by panellists, including acetic acid, butyric acid, hexanoic acid and octanoic acid. Among them, the intensity of butyric acid and hexanoic acid were higher than acetic acid and octanoic acid. Butyric acid and hexanoic acid as the most important flavor-active compounds in terms of their flavor intensities, and they provided the rancid and olid flavor. Meanwhile, acetic acid and octanoic acid were probably the main rancid odor in pickled bamboo shoots, due to their sour and sweaty flavor and their low odor thresholds.

In addition, p-cresol also had higher intensity in the pickled samples detected compounds. p-Cresol had exhibited the unpleasant and olid flavor and due to their low odor thresholds (55 µg/kg), which had also some contributed on the main rancid odor in pickled bamboo shoots. Furthermore, Fu et al. (2002) also reported that p-Cresol exhibited the barn-like odor and it was the most important aroma-active compound in fermented bamboo shoots.

The identification results of flavor-active compounds between fresh and pickled bamboo shoots are obvious different by GC-O analysis. In fresh bamboo shoots, aldehydes had the highest flavor intensity and were the main flavor compounds, as well as grass and astringent flavor were the key flavor-active. At the same time, acids had the highest flavor intensity and were the main flavor compounds in pickled bamboo shoots. Furthermore, pungent and rancid flavor was the key flavor-active in pickled bamboo shoots. This change may be attributed to some chemical and biochemical changes during the bamboo shoots pickle fermentation. Therefore, the formation mechanism of the rancid favor during pickle fermentation is needed to be investigated in future.

Conclusions

In this study, 38 major volatile compounds were identified in fresh bamboo shoots by HS-SPME combined with GC-MS analysis. Among them, 2-pentylfuran, hexanal, benzaldehyde, 1-hexanol and (E)-2-nonadienal were the main volatile compounds, which accounted for 59% of the total composition. As for pickled samples, 52 major volatile compounds were identified. Among them, ethanol, 1-hexanol, hexanal, methoxy-phenyl-oxime and acetic acid were the main volatile compounds, which accounted for 63.7% of the total composition. The results showed that volatile compounds between fresh and pickled bamboo shoots are obvious different and flavor components from bamboo shoots had obviously changed after the pickling process.

17 Flavor-active compounds in fresh samples have been identified by GC-O analysis. Grass and astringent flavor were the key flavor-active compounds. As for pickled samples, 19 flavor-active compounds have been identified by GC-O analysis. Pungent, rancid and olid flavor were the main flavor-active compounds. 3 Flavor compounds including butyric acid, hexanoic acid and p-cresol had significant effects on flavor of pickled bamboo shoots. To our knowledge, this is the first report using SPME-GC-MS and GC-Olfactometry to compare the volatile and flavor compounds between the fresh and pickled bamboo shoots. At the same time, a further sensory analysis by appraiser panel is needed for future research.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (XDJK2013C131).

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