Gas Chromatography-Olfactometry of Aroma Compounds

The aroma of foods is caused by volatile compounds which are perceived by the human nose. Many studies (reviews in [1, 2]) have indicated that only a small fraction of the hundreds of volatiles occurring in a food sample contribute to its aroma. To detect these compounds, a method proposed by Fuller et al. [3] is used. In this procedure, which is designated gas chromatography-olfactometry (GC-O), the effluent from a gas chromatography column is sniffed by an expert who marks in the chromatogram each position at which an odour impression is perceived. 

However, a single GC-O run only is usually insufficient to distinguish between the potent odorants that most likely contribute strongly to an aroma and those odorants that are only components of the background aroma. Therefore, to improve the results, two methods, combined hedonic aroma response measurements (CHARM) analysis [4] and aroma extract dilution analysis (AEDA) [5, 6] have been developed. As discussed in Sect. 16.4 in both methods serial dilutions of food extract are analysed by GC-O. 

Reviews published by Acree and Teranishi [7], Blank [8], Grosch [1, 2, 9], Mistry et al. [10] and Schieberle [11] agree that GC-O was the starting point for the development of a systematic approach for the identification of the compounds causing food aromas. The aim of this chapter is to discuss the potential and the limitations of GC-O. 

The analysis of aroma compounds begins with the preparation of a concentrate containing the volatiles that smell like the starting material. However, as odorants are substances with a wide variety of functional groups, there is no ideal 

In bioactive materials, enzymatic reactions (nos. 1-3 in Table 16.1) are inhibited by homogenising the sample in the presence of calcium ions that precipitate the enzymes [12]. A lower pH value enhancing reactions 4-7 should be buffered and a higher temperature is avoided by distilling off the volatiles under vacuum. Samples containing hydroperoxides derived from unsaturated acyl lipids are sensitive to temperatures above 40 °C (no. 8). 

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A relatively new methodology caEed aroma dEution analysis (ada), which combines aroma dEution and gas chromatography-olfactometry to gain a better understanding of the relative importance of aroma compounds, was recently done for coffee. In a roasted Colombian coffee brew, 41 impact compounds were found with flavor dEution threshold factors (FD) greater than 25, and 26 compounds had FD factors of 100 or above. WhEe the technique permits assessment of the impact of individual compounds, it does not evaluate synergistic effects among compounds (13). 

Gas chromatography/olfactometry (GC/O) based on dilution analysis (e.g., CharmAna-lysis or Aroma Extraction Dilution Analysis) gives an indication of what compounds are most potent in the aroma of foods. The application of SPME to GC/O dilution analysis can be achieved by varying the thickness of the fiber phase and the length of exposure, resulting in various absorbant volumes. 

A first approach to distinguish between the odor-active compounds and the many odorless volatiles present in such aroma extracts is the application of gas chromatography/olfactometry (GCO, formerly called "sniffing-technique" [13-17]). 

Diaz-Maroto, M. C., Guchu, E., Castro-Vazquez, L., de Torres, C., Perez-Coello, M. S. (2008). Aroma active compounds of American oak Quercus alba) and Prench, Hungarian and Russian oak woods Quercus petraea) studied by Gas Chromatography-Mass Spectrometry and Gas Chromatography-Olfactometry. Flavour Fragr. J., 23, DOI 10.1002/ffj.l858. 

Since all aroma molecules are more or less volatile, the technique that a priori is best suited to screen the odor active molecules from the rest of molecules is Gas Chromatography-Olfactometry (GC-O). This technique makes use of the human nose as detector for the compounds eluting out of the chromatographic column, typically a fused silica capillary column (Acree et al. 1984). There are several different approaches for GC-O differing in the way in which the olfactometric signals are... 

Lopez, R., Ezpeleta, E., Sanchez, I., Cacho, J., and Ferreira, V. (2004). Analysis of the aroma intensities of volatile compounds released from nuld acid hydrolysates of odourless precursors extracted from TempranUlo and Grenache grapes using gas chromatography-olfactometry. Food Chem., 88, 95-103. 

Concerning the impact of ethanol on aroma perception, Pet ka et al. (2003) showed that ethanol at low concentrations (under 10%) could decrease aroma compound detection threshold. Nevertheless, Grosch (2001) observed that the less ethanol present in a complex wine model mixture, the greater the intensity of the fruity and floral odours. Although this effect could be easily explained by the increased partial pressure of the odorants with reduced ethanol concentration, they showed in GC-0 (gas chromatography-olfactometry) experiments that ethanol strongly increased the odour threshold of wine volatiles. In fact the reduction in odour activity of the wine volatiles when ethanol was added was much larger than the reduction in their partial pressure. 

Brett flavor in wine The question still remains what is "Brett" flavor Results from our initial work indicates that "Brett" aroma in wine is a complex mixture of odor-active compounds, including acids, alcohols, aldehydes, ketones, esters, and phenolics. Analysis by gas chromatography-olfactometry revealed two predominate odor-active compounds responsible for the Brett flavor in the wines studied isovaleric acid and a second unknown compound other identified odor-active compounds included 2-phenyl ethanol, isoamyl alcohol, cis-2-nonenal, trans-2-nonenal, B-damascenone, ethyl decanoate, guaiacol, 4-ethyl guaiacol, 4-ethyl phenol. Our findings are a snapshot into the much larger picture know as Brett flavor. Ultimately this preliminary investigation requires the descriptive analyses of many more wines to know what odor active compounds describe the flavor know as "Brett". 

Model reaction trials and modem analytical methods (gas chromatography/mass spectrometry (GC/MS), gas chromatography/olfactometry (GC/0)) permitted the identification of key mechanisms responsible for flavour generation in process flavourings and some of the most important ones are detailed below. Often chemically complex precursor raw materials (vegetables such as onions, spices, yeast extracts, animal products) are used. Research work on these complex reactions is rare but necessary and allows the discovery of new key odorants and formation pathways. For example, Widder and co-workers [13] discovered a new powerful aroma compound, 3-mer-capto-2-methylpentan-l-ol in a complex process flavour based on onion. 

Buettner, A., Schieberle, P. (2001) Application of a comparative aroma extract dilution analysis to monitor changes in orange juice aroma compounds during processing. In Leland, J.V., Schieberle, P, Buettner, A. Acree, T.E. (eds.) Gas Chromatography-Olfactometry, ACS Symposium Series 782, pp. 33-45... 

In this study, the aroma compounds detected by Gas Chromatography Olfactometry (GC-O) and with OAV>l in at least one of the wine samples were considered. The use of OSs allows one to not only identify those series that contribute to the aroma profile of a wine, but also to rank them in terms of their odorant capacity and obtain its aromatic fingerprint. 

Qian, M., G.A. Reineccius, Identification of aroma compounds in parmigiano-reggiano cheese by gas chromatography/olfactometry, J. Dairy ScL, 85(6), p. 1362, 2002. 

The impact of these advances on our understanding of wine flavor has recently been reviewed [2 5], One of the most promising areas of current study is the use of gas chromatography mass spectrometry (GCMS) and gas chromatography olfactometry (GCO) combined with sensory aroma recombination studies to identify a small subset of aroma compounds that can mimic the aroma of varietal wines. These techniques have been successfully used to identify important aroma components of Gewiirtz-traminer, Scheurebe, and Grenache rose wines [6-9]. 

A. Plotto, J. P. Mattheis, D. S. Lundahl, and M. R. McDaniel, Validation of gas chromatography olfactometry results for Gala apples by evaluation of aroma-active compounds mixtures. Flavor Analysis (C. J. Mussinan and M. J. MoreUo, eds.), ACS, Washington, DC, 1998. 

In recent studies, potent aroma compounds have been identified using various gas chromatography-olfactometry (GCO) techniques, such as Charm Analysis and aroma extract dilution analysis (AEDA) (7,8). The flavor compounds that are identified by these methods are significant contributors to the sensory profile. In some cases, these sensory-directed analytical techniques have enabled the discovery of new character impact compounds. However, in other instances, key aroma chemicals have been identified that, while potent and significant to flavor, do not impart character impact. For example, in dairy products, chocolate, and kiwifmit, these flavor types appear to be produced by a complex blend of noncharacterizing key aroma compounds. 

The characteristic compound for raw carrot is 2-xec-butyl-3-methoxypyra-zine, which has an extremely low (2 ppt) threshold value. Its sniffing port aroma in gas chromatography-olfactometry has been described as raw carroty (34). Unsaturated aldehydes contribute to the flavor of cooked carrot, the most significant being ( )-2-nonenal ( fatty-waxy ) (34). 

Jirovetz, L., Buchbauer, G., Ngassoum, M.B. and Ceissler, M. (2002) Aroma compound analysis of Piper nigrum and Piper guineense essential oils from Cameroon using solid phase microextraction gas chromatography, solid phase microextraction gas chromatography mass spectrometry and olfactometry. 

The analytical method used to analyze an aroma isolate depends on the task at hand. If one wishes to determine the amount of an aroma compound(s) in a food, gas chromatography (GC) may suffice. If one is looking for odorous compounds in a food (desirable or undesirable), then one will use GC/Olfactometry. If one wishes to identify the aroma compounds in a food, this would require GC and mass spectrometry (or GC/Olfactometry/MS). While other instrumental methods may also be applied (e.g., IR or nmr), the bulk of aroma research is done by these three methods. 

R. T. Marsili and N. MUler, Determination of major aroma impact compounds in fermented cucumbers by sohd-phase microextraction-gas chromatography-mass spectrometry-olfactometry detection, J. Chromatogr. Sci. 38 307 (2000). 

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