Graham T. Eyres

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.

Method for small-molecule discovery based on microscale-preparative multidimensional gas chromatography isolation with nuclear magnetic resonance spectroscopy.

Absolute chemical identification requires obtaining a pure compound followed by structure elucidation using spectroscopic techniques, principally NMR spectroscopy and mass spectrometry. Classical isolation techniques suffer from insufficient resolution for complex samples, requiring time-consuming fractionation in multiple steps. Here, a novel preparative technique based upon capillary column multidimensional gas chromatography (MDGC) with 2D NMR to resolve, isolate, and identify pure volatile components from a complex sample is described. As a model application, geraniol was isolated from an essential oil matrix using MDGC and quantitatively resolved from 15 partially coeluting compounds from the first column. Geraniol was recovered from 10 (8.6 microg) and 100 injections (77.6 microg; purity >99%) for subsequent NMR analysis at 500 and 800 MHz (with cryoprobe). Proton and gCOSY NMR experiments were successfully performed at 12.3 microg/mL (10 injections), while gHSQC and gHMBC NMR experiments were obtained at 110.8 microg/mL (100 injections). This approach is applicable to the biodiscovery of volatile molecular species or, indeed, any volatile compound in a complex matrix that requires confirmation of component identity.

Application of microscale-preparative multidimensional gas chromatography with nuclear magnetic resonance spectroscopy for identification of pure methylnaphthalenes from crude oils

Mass spectrometry is often insufficient to distinguish between structural isomers, requiring confirmation using NMR spectroscopy. Here, a novel preparative technique based upon capillary multidimensional gas chromatography to isolate pure volatile components from complex samples is described. The method was developed through isolation of 1,4-dimethoxybenzene (5.2 microg, 10 injections) from a peppermint essential oil. Then isomers of 1- and 2-methylnaphthalene were isolated from a complex crude oil in sufficient amounts (3.1 microg, 38 injections and 5.0 microg, 35 injections) for discrimination using (1)H NMR spectroscopy. This methodology is applicable to identify any volatile molecule in complex matrices requiring confirmation using NMR spectroscopy.

Emerging opportunities for flavor analysis through hyphenated gas chromatography.

Important advances in chemical and flavor knowledge are possible through improved analytical separation and identification. Along with greater separation power, both a more robust analysis and an improved identified sample composition result. In this paper, a number of new integrated methods are explored that permit improved resolution and superior analyses for a range of studies. These methods focus on multidimensional gas chromatography (MDGC), comprehensive two-dimensional gas chromatography (GCxGC), olfactometry, mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. The overriding objective is to provide technical solutions that employ the best possible separation of compounds, allowing tools such as olfactometry, mass spectrometry, NMR spectroscopy, and other detectors to provide much better characterization of separated chemical species. Various novel strategies are demonstrated that provide the necessary increased separation power, integrated with specific detection steps. Case studies presented include the sensory-directed identification of a woody odorant in hop essential oil, correlation of compound identifications in coriander leaf, and development of new preparative isolation capabilities using MDGC with NMR spectroscopy.

Multiple component isolation in preparative multidimensional gas chromatography with characterisation by mass spectrometry and nuclear magnetic resonance spectroscopy

The preparative scale isolation of multiple components from an essential oil matrix is described using multidimensional gas chromatography (prep-MDGC) which allows their further characterisation by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Menthol, linalyl acetate, carvone and geraniol were isolated individually, and were also collected in various combinations. It was demonstrated to be possible to collect multiple selected components from numerous repeat injections of the sample, to permit increased mass recovery from an external cryotrap collection device. Peak retention times remained reproducible (<0.3s) over the repeated injections and switching events. This methodology may be utilised to confirm peak identity or to produce unique mixed-component reference standards, for instance to allow their identification in other samples using GC/MS, or identify them in comprehensive two-dimensional gas chromatography (GCxGC) analysis.

System design for integrated comprehensive and multidimensional gas chromatography with mass spectrometry and olfactometry.

An integrated system having the combined capability to perform gas chromatography (GC), comprehensive two-dimensional GC (GC × GC), and target heart-cut multidimensional GC (MDGC) using olfactometry (O), flame ionization (FID), and/or mass spectrometry (MS) detection is described. This combines a number of contemporary GC methods into a single instrument to provide very high resolution profiling of a sample. This provides initial assessment of volatile compound composition through GC × GC analysis with FID, which can be correlated with GC analysis using parallel O and FID detection. Subsequent microfluidic (Deans) switching selects regions (heart-cuts) of the chromatographic elution from the first dimension ((1)D) column for further resolution on a long second dimension ((2)D(L)) column for parallel detection of O and MS. Various (2)D(L) operational conditions, as well as the effect of different heart-cut (H/C) duration, were compared. The favored mode involves cryotrapping of heart-cuts, cooling the oven, and reducing carrier flow to offer greater efficiency. An analytical strategy that incorporates GC-FID/O, GC × GC-FID, and MDGC-MS/O analyses with cumulative solid phase microextraction (SPME) sampling for volatile sample enrichment is presented in this work. Excellent qualitative and quantitative performance was demonstrated with a Shiraz wine sample and an allergens mixture, with tentative identification of acetic acid, octen-3-ol, and ethyl octanoate as aroma contributors in Shiraz wine and determination of β-damascenone (floral odor) well separated from hexanoic acid (sweaty odor). A novel approach to obtain (2)D retention indices is reported, allowing matching of mass spectral, (1)I (retention index in (1)D) and (2)I (retention index in (2)D) data. The method employs the same olfactory detector at the end of the (1)D and (2)D(L) columns.

Cumulative solid phase microextraction sampling for gas chromatography-olfactometry of Shiraz wine

Solid phase microextraction (SPME) coupled to gas chromatography-olfactometry (GC-O) is now commonly used for determination of aroma-active compounds, but the method sensitivity and selectivity is restricted by the small volume and limited type of fibre coating phases. In an attempt to enhance the method performance, a cryogenic trapping (CT) approach was investigated in this study by coupling multiple SPME sampling events for wine headspace using GC-O analysis. By performing multiple SPME sampling employing different chemical polymer coatings, desorbed solute from the integrated sampling is accumulated by the CT at the front section of a Wax separation capillary column prior to chromatographic analysis. Results show that the CT was capable of retaining apolar alkane volatiles of decane and greater, and tested polar alcohols, including methanol. Chromatographic signals eluting later than the ethanol peak were found to progressively increase in response, and correlated well, with the cumulative number of SPME samplings. The approach was developed for GC-O screening of potent odorants in Shiraz wine collected from fibre coatings of polyacrylate (PA) and the triple-phase coated polydimethylsiloxane/divinylbenzene/carboxen (PDC). The aromagram for solute derived from a combined introduction of both PA plus PDC fibres (i.e. sequential fibre introduction into the injector; termed as PADC) compared well to the sum of those sampled by using a single fibre coating alone, which comprised of odorants derived from both fibre coatings. Accumulation in the CT of volatile solutes derived from up to 6 repeat PADC sampling events revealed a similar pattern of their aromagrams, though with stronger olfactory stimulus response. This study demonstrated a simple and effective way for enhancing SPME sensitivity and potentially less discrimination during the analysis of wine volatiles. However, the single dimensional GC separation method requires development of an improved separation strategy to better separate individual compounds.

22 – Hop Essential Oil: Analysis, Chemical Composition and Odor Characteristics

Abstract The essential oil of hops (Humulus lupulus L.) imparts odor and aroma characteristics to beer. Hops can influence beer aroma in terms of floral, spicy, herbal, woody and fruity characters. There are a large number of hop varieties commercially available with distinct odor characteristics, which can be attributed to the different composition of their essential oils. This composition is complex, potentially containing up to 1,000 compounds from a wide range of chemical classes. Fresh essential oil is dominated by terpene hydrocarbons, predominantly myrcene, α-humulene and β-caryophyllene. The composition varies depending on: intrinsic and extrinsic factors during growth, processing conditions, and the extraction method used to isolate the essential oil. In addition, oxidation and hydrolysis reactions occurring during storage alter the composition and further increase the chemical complexity. Despite more than 50 years of research, not all character-impact odorants in hop essential oil have been identified. Due to its abundance, myrcene is important for the odor of fresh hop essential oil. Linalool and geraniol have been determined to be important odorants contributing to the floral character of hop essential oil and beer. Other compounds such as β-ionone, β-damascenone, geranial, neral, trans-4,5-epoxy-(E)-2-decenal, 1,3(E),5(Z)-undecatriene, 1,3(E),5(Z), 9-undecatetrene, ethyl 2-methylpropanoate, methyl 2-methyl-butanoate, propyl 2-methylbutanoate, (Z)-1,5-octadien-3-one, nonanal and isovaleric acid have been implicated as potent odorants in hop essential oil. Hoppy aroma in beer is still not completely understood due to the physical, biochemical and chemical changes that occur during brewing and fermentation. Hop-derived odorants identified in beer but not present in hop essential oil include citronellol, γ-nonalactone, humuladienone, geranyl acetate and ethyl cinnamate. Oxidation and hydrolysis products of sesquiterpenes (e.g. humulene epoxides) have commonly been associated with “noble” hop characters in beer; however, the importance of these compounds remains controversial. The complexity of hop aroma in beer has led to increasing trends to add fractionated hop oils with specific odor characteristics to beer post-fermentation.

Effects of Agar Gel Strength and Fat on Oral Breakdown, Volatile Release, and Sensory Perception Using in Vivo and in Vitro Systems.

The density and composition of a food matrix affect the rates of oral breakdown and in-mouth flavor release as well as the overall sensory experience. Agar gels of increasing concentration (1.0, 1.7, 2.9, and 5% agarose) with and without added fat (0, 2, 5, and 10%) were spiked with seven aroma volatiles. Differences in oral processing and sensory perception were systematically measured by a trained panel using a discrete interval time intensity method. Volatile release was measured in vivo and in vitro by proton transfer reaction mass spectrometry. Greater oral processing was required as agar gel strength increased, and the intensity of flavor-related sensory attributes decreased. Volatile release was inversely related to gel strength, showing that physicochemical phenomena were the main mechanisms underlying the perceived sensory changes. Fat addition reduced the amount of oral processing and had differential effects on release, depending on the fat solubility or lipophilicity of the volatiles.

Comparison of four extraction methods for analysis of volatile hop-derived aroma compounds in beer

The volatile organic compound profile in beer is derived from hops, malt, yeast, and interactions between the ingredients, making it very diverse and complex. Due to the range and diversity of the volatile organic compounds present, the choice of the extraction method is extremely important for optimal sensitivity and selectivity. This study compared four extraction methods for hop-derived compounds in beer late hopped with Nelson Sauvin. Extraction capacity and variation were compared for headspace solid-phase micro extraction, stir bar sorptive extraction, headspace sorptive extraction, and solvent-assisted flavor evaporation. Generally, stir bar sorptive extraction was better suited for acids, headspace sorptive extraction for esters and aldehydes, while headspace solid-phase microextraction was less sensitive overall, extracting 40% fewer compounds. Solvent-assisted flavor evaporation with dichloromethane was not suitable for the extraction of hop-derived volatile organic compounds in beer, as the profile was strongly skewed towards alcohols and acids. Overall, headspace sorptive extraction is found to be best suited, closely followed by stir bar sorptive extraction.