Gas chromatography-olfactometry in food flavour analysis

The application of gas chromatography-olfactometry (GC-O) in food flavour analysis represents to be a valuable technique to characterise odour-active, as well as character impact compounds, responsible for the characterizing odour of a food sample. The present article briefly reviews the use of GC-O in the flavour investigation of dairy products (milk and cheese), coffee, meat and fruits. Particular attention has been devoted to extraction techniques, GC-O hardware commonly utilised and olfactometric assessment methods, which can be applied to food analysis.     

Odour fingerprint acquisition by means of comprehensive two-dimensional gas chromatography-olfactometry and comprehensive two-dimensional gas chromatography/mass spectrometry

The analysis of complex matrices, such as perfumes, by means of gas chromatography-olfactometry (GC-O) can be rather imprecise due to the co-elutions, leading to a possible masking of odour-active trace-level compounds by major interferences or agglomeration of olfactive impressions resulting in unreliable olfactive characterization. To overcome these limits an innovative technique, comprehensive two-dimensional gas chromatography-olfactometry (GC x GC-O), was applied, revealing several relevant co-elutions, as in the linalool and linalyl acetate zones. A total of 177 compounds, out of these 135 odour-active, were detected by GC-O, while about 481 out of 818 compounds presented odour-activity through GC x GC-O analyses. In addition, GC/mass spectrometry (GC/MS) and GC x GC/MS analyses were also performed. Peak assignment was achieved by means of different information sources, such as GC/MS, GC x GC/MS, LRI, injection of standards and olfactive impressions.     

The combination of gas chromatography-olfactometry and multidimensional gas chromatography for the characterisation of essential oils

A research area of great interest to the flavour industry is the analysis of odour active compounds in essential oils. In this paper, a methodology is presented for the identification of character-impact odorants in essential oil samples using (a) gas chromatography-olfactometry (GC-O); (b) comprehensive two-dimensional gas chromatography (GC x GC) combined to time-of-flight mass spectrometry (TOFMS) and (c) heart-cut multidimensional gas chromatography-olfactometry (MDGC-O). The specific advantages and limitations of each technique are discussed. The advantage of combining these techniques in a strategy to identify character-impact odorants is demonstrated using examples from coriander leaf (Coriandrum sativum) and hop (Humulus lupulus) essential oils. In particular, resolution of co-eluting regions of compounds and evaluation of their individual odour activity is discussed. In coriander leaf, only E-2-dodecenal was found to contribute to a co-eluting odour region, E-2-dodecen-1-ol and 1-dodecanol being present below detection threshold. Using MDGC on a hop essential oil sample, eight significant peaks were resolved from an 18 s heart-cut where a potent odorant was perceived during GC-O.     

Investigating the coffee flavour in South African Pinotage wine using novel offline olfactometry and comprehensive gas chromatography with time of flight mass spectrometry

Pinotage wine from several South African wine cellars has been produced with a novel coffee flavour. We have investigated this innovative coffee effect using in house developed solventless sampling and fractionating olfactometric techniques, which are unique in their ability to study synergistic aroma effects as opposed to traditional gas chromatography olfactometry (GC-O) which is designed to, ideally, evaluate single eluting compounds in a chromatographic sequence. Sections of the chromatogram, multiple or single peaks, were recaptured on multichannel open tubular silicone rubber (polydimethylsiloxane (PDMS)) traps at the end of a GC column. The recaptured fractions were released in a controlled manner for offline olfactory evaluation, and for qualitative analysis using comprehensive gas chromatography coupled to time of flight mass spectrometry (GC × GC–TOFMS) for compound separation and identification, thus permitting correlation of odour with specific compounds. A combination of furfural and 2-furanmethanol was responsible for a roast coffee bean-like odour in coffee style Pinotage wines. This coffee perception is the result of a synergistic effect in which no individual compound was responsible for the characteristic aroma.     

Identification of potent odourants in wine and brewed coffee using gas chromatography-olfactometry and comprehensive two-dimensional gas chromatography

Volatile constituents in wine and brewed coffee were analyzed using a combined system incorporating both GC-olfactometry (GC-O) and comprehensive two-dimensional GC-flame ionization detection (GC×GC-FID). A column set consisting of a 15m first dimension ((1)D; DB-FFAP (free fatty acid phase)), and a 1.0m (2)D column (DB-5 phase) was applied to achieve the GC×GC separation of the volatile extracts isolated by using solid phase extraction (SPE). While 1D GC resulted in many overlapping peaks, GC×GC allowed resolution of co-eluting compounds which coincided with the odour region located using GC-O. Character-impact odourants were tentatively identified through data correlation of GC×GC contour plots across results obtained using either time-of-flight mass spectrometry (TOFMS), or with flame photometric detection (FPD) for sulfur speciation. The odourants 2-methyl-2-butenal, 2-(methoxymethyl)-furan, dimethyl trisulfide, 2-ethyl-5-methyl-pyrazine, 2-octenal, 2-furancarboxaldehyde, 3-mercapto-3-methyl-1-butanol, 2-methoxy-3-(2-methylpropyl)-pyrazine, 2-furanmethanol and isovaleric acid were suspected to be particularly responsible for coffee aroma using this approach. The presented methodology was applied to identify the potent odourants in two different Australian wine varietals. 1-Octen-3-ol, butanoic acid and 2-methylbutanoic acid were detected in both Merlot and a Sauvignon Blanc+Semillon (SV) blend with high aroma potency. Several co-eluting peaks of ethyl 4-oxo-pentanoate, 3,7-dimethyl-1,5,7-octatrien-3-ol, (Z)-2-octen-1-ol, 5-hydroxy-2-methyl-1,3-dioxane were likely contributors to the Merlot wine aroma; while (Z)-3-hexen-1-ol, β-phenylethyl acetate, hexanoic acid and co-eluting peaks of 3-ethoxy-1-propanol and hexyl formate may contribute to SV wine aroma character. The volatile sulfur compound 2-mercapto-ethyl acetate was believed to contribute a fruity, brothy, meaty, sulfur odour to Australian Merlot and SV wines.     

Gas chromatography-olfactometry analysis of the volatile compounds of two commercial Irish beef meats

The volatile flavour compounds of two commercial Irish beef meats (labelled as conventional and organic) were evaluated by gas chromatography-olfactometry and were identified by gas chromatography-mass spectrometry. The volatile compounds were isolated in a model mouth system. Gas chromatography-olfactometry was performed by a group of eight assessors using the detection frequency methodology. The odours of the detected compounds were described as well. Eighty-one volatile compounds were identified, 11 compounds of which possessed odour activity in the first beef sample and 14 of which in the second meat sample. Ten volatile flavour compounds were common to both: methanethiol, dimethyl sulphide, 2-butanone, ethyl acetate, 2- and 3-methylbutanal, an unknown compound, 2-octanone, decanal and benzothiazole. Two unknown compounds were only detected in the first sample while 2,3-pentanedione, 4-methyl-3-penten-2-one, 2-heptanone, dimethyl trisulphide and nonanal were only perceived in the second beef. Significant differences in terms of detection frequency, odour characteristics and in nature of the volatile flavour compounds were emphasised between the two samples.     

Comparison of Different Analytical Methods for Volatile and Odour-Active Substances in Polyolefins

Emissions of volatiles of polymeric materials are an important parameter for materials characterisation, and gain more and more importance either by their odour activity or by other undesired behaviour. Currently, numerous methods are applied for the determination of volatiles, but all of them use gas chromatography (GC) with different detectors. The information gained by flame ionisation detection (FID) is the total emitted volatiles expressed as a sum value, but no further deeper information is provided. Additionally, due to different sample amounts, preparation, way of sampling and GC parameters, results cannot be compared to each other. Also, determination of single sum values by integrating the total area of volatiles is of little help for material development due to the lack of detailed information about chemical composition, and other methods have to be applied. Other compounds of interest such as odour-active ones turn up in only very small amounts and cannot be detected by these methods as an extensive analytical sample preparation is necessary. We compare results obtained by different sample preparation techniques used in industrial standards (VDA 277 and VDA278) to scientific alternatives such as Solid Phase Microextraction (SPME) coupled to GC with mass spectrometry (MS) and Simultaneous Distillation/Extraction SDE coupled to GC-MS to demonstrate capabilities and applicability of each method.     

Chemical causes of the typical burnt smell after accidental fires

The components responsible for the typical burnt smell that occurs after accidental fires (e.g. in buildings) were identified. For this purpose, samples of odorous materials were taken from different real fire sites. Their volatile fractions were analysed by means of thermal desorption, headspace analysis and solid-phase microextraction (SPME) combined with gas chromatography–mass spectrometry (GC/MS). Measurements performed with SPME gave the highest number of analytes as well as the highest signal intensities. A divinylbenzene/carboxen/polydimethylsiloxane SPME fibre was found to be the most suitable for this task. To distinguish the odour-active compounds from the ca. 1,400 identified volatiles concentrated by SPME, an olfactory detection port was attached to the GC/MS and the column effluent was assessed by panellists. The results revealed that eleven odorous compounds were present in most of the investigated samples: acetophenone, benzyl alcohol, 4-ethyl-2-methoxyphenol, 2-hydroxybenzaldehyde, 2-hydroxy-5-methylbenzldehyde, 2-methoxyphenol, 2-methoxy-4-methylphenol, 2-methylphenol, 3-methylphenol, 4-methylphenol and naphthalene. Their odour activities were confirmed in additional olfactory experiments, and the relative ratios of these eleven compounds were determined. Based on these ratios, standard solutions that presented an intense odour with typical characteristics of the burnt smell were produced.     

Characterisation of free and glycosidically bound odourant compounds of Aragonez clonal musts by GC-O

To evaluate the potential aroma of Aragonez clonal red musts, several free and glycosidically bound odourant compounds were extracted. Then, the gas chromatography-olfactometry (GC-O) posterior intensity method was used to evaluate their odour intensity and the compounds were identified by gas chromatography-mass spectrometry (GC-MS). A group of eight sniffers evaluated free and bound fractions of Aragonez musts and perceived forty-three and twenty-two odourant peaks respectively. Furaneol (burnt sugar, candy-cotton) and vanillin (vanilla, sweet) were identified in both free and bound fractions of Aragonez musts, indicating their grape-derived origin. Principal component analysis (PCA) was applied to the posterior intensity method data and a relationship between the different odourant compound variables and the free fractions was established. Two principal components (PCs) were found which together explained 100% of the total variance. A large number of potentially important but yet unknown odourants was detected by the GC-O analysis.     

Comparison of volatile components in Chinese traditional pickled peppers using HS-SPME-GC-MS, GC-O and multivariate analysis

Volatile compounds of Chinese traditional pickled peppers (CTPPs) were extracted by solid-phase micro-extraction (SPME) and analysed by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GC-O) to visually compare their volatile compositions by applying principal component analysis (PCA). A total of 67 volatile components were identified by GC-MS, including 7 acids, 6 alkanes, 14 alcohols, 9 esters, 11 terpenes, 3 aldehydes, 5 ketones, 7 phenols and 5 miscellaneous compounds, tentatively identified or identified by comparing with mass spectra and retention indices of the standards or from literature. Of 45 volatile compounds detected in the sniffing port of GC-O, the majority of odour-active components included acetic acid, 2-ethyl phenol, L-linalool, tridecane, butyl butanoate, δ-3-carene. The individual concentrations of the volatile compounds such as acetic acid, ethanol, 1-propanol, L-linalool, hexyl 2-methyl butyrate and hexyl pentanoate corresponded well to the intensities of related attributes in the correlation analysis. Due to their high concentration level and low threshold value, these compounds played an important role in the final aromatic profile of the pickled peppers. The differences in flavours were observed by applying PCA to GC-MS data sets. From the PCA results, samples were primarily separated along the first principal component.     

A Comparison of One-Dimensional and Comprehensive Two-Dimensional Gas Chromatography for Decomposition Odour Profiling Using Inter-Year Replicate Field Trials

Decomposition odour analysis involves the chemical profiling of volatile organic compounds produced by decomposing remains. This is important for areas of forensic science that rely on the detection of decomposition odour such as insect attraction to carrion, positive alerts of cadaver dogs to decomposing remains, and the development of field instrumentation for search and recovery procedures. Traditionally decomposition odour analysis has been performed using gas chromatography–quadrupole mass spectrometry (GC–qMS); however, the use of comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry (GC×GC–TOFMS) is rapidly becoming more prevalent. The objective of this study was to compare GC–qMS and GC×GC–TOFMS for decomposition odour profiling based on inter-year replicate field studies using decomposing porcine remains. The increased peak capacity, sensitivity and selectivity afforded by GC×GC–TOFMS allowed peak co-elutions, chromatographic artefacts, and dynamic range to be more easily addressed and managed. Furthermore, the software associated with GC×GC–TOFMS provided several additional benefits including improved peak alignment between samples and increased consistency of reported results, overall allowing for additional statistical tests to be applied following data processing. Future GC–qMS results could be improved by implementing some of these software-associated procedures, potentially reducing the magnitude of variation observed between GC–qMS and GC×GC–TOFMS studies. One-dimensional GC analysis may also benefit substantially from coupling with TOFMS detection to provide an indirect increase in peak capacity using deconvolution. However, the wealth of information gained by using GC×GC–TOFMS in decomposition odour profiling is undoubtedly an asset in this field of research.

     

Application of zNosetrade mark for the analysis of selected grape aroma compounds

A novel and portable gas chromatograph (GC, zNose™) has been evaluated for the measurement in grape berries of selected six-carbon compounds; namely, hexanal, cis-2-hexen-1-ol, cis-3-hexen-1-ol and trans-2-hexenal. The zNose™ is a handheld GC which uses purge and trap for concentration, and has a surface acoustic wave (SAW) sensor as a detector. Operation of the zNose™ using direct aspiration of the sample failed to detect the compounds at the reported odour threshold values. Pre-concentration by Tenax^® trapping and solid-phase microextraction (SPME) were investigated to improve the zNose™ sensitivity. Use of a Tenax^® pre-trap with the zNose™ allowed detection of the compounds at concentration levels in the order of their threshold values. Excessive bleed from the SPME fibre prevented the use of SPME with zNose™.     

A Narrative Review of the Current Knowledge on Fruit Active Aroma Using Gas Chromatography-Olfactometry (GC-O) Analysis

Fruit aroma, a mixture of chemical compounds with odor, is a strong attractant derived from a complex mixture of different amounts and intensities (threshold) of chemical compounds found in fruits. The odor-producing compounds of fruit aroma are derived from carbohydrates, lipids, phenolic compounds, and mono- and sesquiterpenes, among others. The identification of compounds responsible for fruit aroma is usually conducted using gas chromatography coupled with olfactometry (GC-O). This technique separates the chemical compounds from the aroma of foods using a chromatographic column and divides the resultant outflow between the physical detector and a testing outlet (sniffing port). Trained judges describe the perceived odor in terms of the intensity of the odor zones perceived according to their training method. Moreover, the use of GC-O coupled with a mass detector (GC-MS-O) allows for the retrieval of chemical information such as identification and quantification of compounds, which can be correlated to sensory information. This review aimed to demonstrate the application of GC-MS-O in the identification of precursor compounds in fruit aroma, considering important factors for the application, main results, and most recent advances in this field.     

Comparison of two extraction methods for evaluation of volatile constituents patterns in commercial whiskeys Elucidation of the main odour-active compounds

An analytical procedure based on manual dynamic headspace solid-phase microextraction (HS-SPME) method and the conventional extraction method by liquid-liquid extraction (LLE), were compared for their effectiveness in the extraction and quantification of volatile compounds from commercial whiskey samples. Seven extraction solvents covering a wide range of polarities and two SPME fibres coatings, has been evaluated. The highest amounts extracted, were achieved using dichloromethane (CH₂Cl₂) by LLE method (LLECH₂Cl₂) and using a CAR/PDMS fibre (SPME_CAR/PDMS) in HS-SPME. Each method was used to determine the responses of 25 analytes from whiskeys and calibration standards, in order to provide sensitivity comparisons between the two methods. Calibration curves were established in a synthetic whiskey and linear correlation coefficient (r) were greater than 0.9929 for LLECH₂Cl₂ and 0.9935 for SPME_CAR/PDMS, for all target compounds. Recoveries greater than 80% were achieved. For most compounds, precision (expressed by relative standard deviation, R.S.D.) are very good, with R.S.D. values lower than 14.78% for HS-SPME method and than 19.42% for LLE method. The detection limits ranged from 0.13 to 19.03 μg L⁻¹ for SPME procedure and from 0.50 to 12.48 μg L⁻¹ for LLE.A tentative study to estimate the contribution of a specific compound to the aroma of a whiskey, on the basis of their odour activity values (OAV) was made. Ethyl octanoate followed by isoamyl acetate and isobutyl alcohol, were found the most potent odour-active compounds.     

Development and validation of an SPE-GC-MS/MS taste and odour method for analysis in surface water

The goal of this research was to develop a robust method for taste and odour compounds that can be implemented by laboratories with mass spectrometers lacking chemical ionisation capabilities or specialised sample introduction hardware that are commonly used for taste and odour methods. Development, optimisation, and validation of a solid-phase extraction method using liquid injection and gas chromatography – tandem mass spectrometry detection with electron impact ionisation are described. Camphor was used as an internal standard, and through method development and robustness testing it was shown to extract similarly to other taste and odour compounds, making it a cost-effective alternative to deuterated analogs. The instrumental parameters and extraction procedure were fully optimised prior to assessing the method’s linearity, precision, and accuracy. Using a 2000-fold enrichment factor, method recoveries for priority compounds geosmin (GSM) and 2-methylisoborneol (2-MIB) were >90%. Excellent linearity was obtained from the reportable detection limits up to 200 ng L^?1 and precision %relative standard deviations were 8.5% and 10.9% for GSM and 2-MIB, respectively. Detection limits of 0.9 and 5.5 ng L^?1 for GSM and 2-MIB respectively were deemed fit-for-purpose in comparison to their odour thresholds. Validation data were also obtained for other commonly analysed taste and odour compounds, including 2,4,6-trichloroanisole, 2-isopropyl-3-methoxypyrazine, and 2-isobutyl-3-methoxypyrazine. The validated method was used to screen surface waters in Nova Scotia, Canada for presence of taste and odour compounds, highlighting the presence of GSM on the east coast of Canada.     

Optimization and validation of organochlorine compounds in adipose tissue by SPE‐gas chromatography

Scientific evidence has shown an association between organochlorine compounds (OCC) exposure and human health hazards. Concerning this, OCC detection in human adipose samples has to be considered a public health priority. This study evaluated the efficacy of various solid‐phase extraction (SPE) and cleanup methods for OCC determination in human adipose tissue. Octadecylsilyl endcapped (C₁₈‐E), benzenesulfonic acid modified silica cation exchanger (SA), poly(styrene‐divinylbenzene (EN) and EN/RP₁₈ SPE sorbents were evaluated. The relative sample cleanup provided by these SPE columns was evaluated using gas chromatography with electron capture detection (GC–ECD). The C₁₈‐E columns with strong homogenization were found to provide the most effective cleanup, removing the greatest amount of interfering substance, and simultaneously ensuring good analyte recoveries higher than 70%. Recoveries > 70% with standard deviations (SD) < 15% were obtained for all compounds under the selected conditions. Method detection limits were in the 0.003–0.009 mg/kg range. The positive samples were confirmed by gas chromatography coupled with tandem mass spectrometry (GC‐MS/MS). The highest percentage found of the OCC in real samples corresponded to HCB, o,p′‐DDT and methoxychlor, which were detected in 80 and 95% of samples analyzed respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Identification of volatiles from pineapple (Ananas comosus L.) pulp by comprehensive two-dimensional gas chromatography and gas chromatography/mass spectrometry

Combining qualitative data from the chromatographic structure of 2-D gas chromatography with flame ionization detection (GC×GC-FID) and that from gas chromatography-mass spectrometry (GC/MS) should result in a more accurate assignment of the peak identities than the simple analysis by GC/MS, where coelution of analytes is unavoidable in highly complex samples (rendering spectra unsuitable for qualitative purposes) or for compounds in very low concentrations. Using data from GC×GC-FID combined with GC/MS can reveal coelutions that were not detected by mass spectra deconvolution software. In addition, some compounds can be identified according to the structure of the GC×GC-FID chromatogram. In this article, the volatile fractions of fresh and dehydrated pineapple pulp were evaluated. The extraction of the volatiles was performed by dynamic headspace extraction coupled to solid-phase microextraction (DHS-SPME), a technique appropriate for slurries or solid matrices. Extracted analytes were then analyzed by GC×GC-FID and GC/MS. The results obtained using both techniques were combined to improve compound identifications.