Odor-active compounds

Imagine that a friend has asked you to review a draft of a Methods section for a paper to be submitted to the Journal of Agricultural and Food Chemistry. Unfortunately, your friend has not had the benefit of a chemistry writing course hence, a few writing tips would be appreciated. The project involves the identification of odor-active compounds in 19 California chardonnay wines. The steps involved in the study are as follows ... 

Lee, S.-J. Noble, A. C. Characterization of Odor-Active Compounds in Californian Chardonnay Wines using GC-Olfactometry and GC-Mass Spectrometry. J. Agric. Food Chem. 2003, 51, 8036-8044. 

Determine the time and nitrogen flow rate so that breakthrough (no more retention for a given substance) of the trap does not occur. Do this by attaching a second trap in series and analyzing this trap. For aroma compounds, smell the end of the trap to detect odor-active compound leakage. Even if the vessel is heated, the trap should be kept at room temperature. 

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]). 

The identification experiments were focused on the twenty-three odorants detected in the FD-factor range 16 to 1024. The chemical structures of five of the most odor-active compounds in the neutral/basic fraction (nos. 4, 14, 17, 20, 22) are displayed in Figure 4. Additionally, the six most important odorants identified on the basis of AEDA results in the acidic volatile fraction of pale lager beer are shown (I-V Figure 4). 

For these reasons and to assure that the important odorants are included, the identification experiments should be focused on the odor-active compounds detected by AEDA in a wide FD-factor range of at least 1 to 100. 

But research over the past ten years reveals that in most cases there is no direct correlation between VOCs measured by routine emission tests and odor active compounds (Mayer and Breuer, 2000 Knudsen et al., 1999 Salthammer et al., 2004). Figure 8.1 shows an example of the time-dependent TVOC values of a flooring material in comparison to odor intensity scores (see later). 

This procedure allows the differentiation of odor active compounds from odorless substances within a complex mixture of volatiles. For decades this procedure has been successfully applied for aroma analyses of foods (Grosch, 1993). The mixture of volatile compounds either collected in a purified organic solvent extract or in a defined headspace volume is separated into its different components by means of GC and the effluent gas flow at the end of the GC capillary column is split between a FID and an experienced test person s nose. By sniffing the column effluent, the human nose is able to perceive the odor active compounds contained in a complex mixture and the test person can mark the corresponding spot in the FID chromatogram recorded in parallel and attribute a certain odor quality. A sample GC—O chromatogram of a solvent extracted material is shown in Figure 8.7. 

Polyphenylenoxide (PPO) Substituted phenols are used as monomers for the production of polyphenylenoxides, (PPOs) so they as well as phenolic degradation products can be found as emitted odor active compounds. In one case the odor of a PPO was predominantly caused by 2,6-dimethylphenol and trimethylanisol as well as by a tentatively identified substituted methoxypyrazine (Mayer and Breuer, 2004a). Another potent odorant derived from higher molecular phenolic compounds, antioxidants for example, by the influence of heat (>200 °C) and pressure is guaiacol (2-methoxyphenol) (Mayer and Breuer, 2006). 

More recently, Moyano et ah (2010) have evaluated the evolution of the odor-active compounds in amontillado sherries during the aging process. 

The nature of the Botrytis aroma compounds has been subjected to extensive research. In addition to the older findings about the importance of hydroxy-, oxo-, and dicarboxylic acid esters, acetals, and some special y- and 8-lactones, the role of volatile thiols has recently been elucidated. Nonetheless, additional research is needed to identify odor active compounds that are specific for botrytized wines. 

Citrus oil dominates this class of essential oil. It is obtained by the cold press method with the exception of lime oil, which is also prepared by steam distillation of essential oil separated during the production of juice.106,107 Aside from bergamot, these oils are primarily monoterpene hydrocarbon mixtures of which (if)-limonene (3) is usually the dominant compound. Since odor contribution of this monoterpene compound is low, it is often removed by distillation or repeated solvent extraction. The resulting oil rich in odor-active compounds is called terpeneless oil and is used extensively. In the case of bergamot and lemon oils, psoralen derivates like bergaptene (64) causing photosensitivity are problematic, and those for fragrance use are rectified to remove it (Table 8). 

Komes, D., Ulrich, D., and Lovric, T. (2006). Characterization of odor-active compounds in Croatian Rhine Riesling wine, subregion Zagoije. Eur. Food Res. TechnoL, 222, 1-7. 

Lee, S.J., and Noble, A.C. (2003). Characterization of odor-active compounds in Californian Chardonnay wines using GC-olfactometry and GC-mass spectrometry. J. Agric. Food Chem., 51, 8036-8044. 

Serot, T., Prost, C., Visna, L., and Burcea, M. (2001). Identification of the main odor-active compounds in musts from french and romanian hybrids by three olfactometric methods. J. Agric. Food Chem., 49, 1909-1914. 

Recently, AEDA and SHA-0 yielded 41 and 45 odor active compounds for Scheurebe and Gewurztraminer wines, respectively (P). Ethyl 2-methylbutyrate, ethyl isobutyrate, 2-phenylethanol, 3-methylbutanol, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, 3-ethylphenol and one unknown compound, named wine lactone, showed high flavor dilution (FD)- factors (Table I) in Gewurztraminer and Scheurebe wines. 4-Mercapto-4-methylpentan-2-one belongs to the most potent odorants only in the variety Scheurebe whereas cis-rose oxide was perceived only in Gewurztraminer (Table I). 4-Mercapto-4-methylpentan-2-one was identified for the first time in Sauvignon blanc wines (JO). The unknown compound with coconut, woody and sweet odor quality, which has not yet been detected in wine or a food, was identified as 3a,4,5,7a-tetrahydro-3,6-dimethylbenzofuran-2(3H)-one (wine lactone) (JJ). 

AEDA and SHA-0 are suitable tools for recognition of odor active compounds (13, 14), but the methods are afflicted with some simplifications no corrections were made for the losses of odorants during isolation procedure. By AEDA the complete amounts of the odorants present in the solvent extracts are volatilized during GC-0 and therefore ranked according to their odor thresholds in air, but the contribution of an odorant to the overall flavor in a food is strongly affected by its odor threshold in the food... 

Cabernet wine comparison. One of the objectives of the study was to identify the odoi active compounds of wines with "Brett" flavor through sensory analysis and gas chromatography-olfactometry (GCO). Wines identified by their respective winemakers as having "Brett" character were evaluated by a trained expert sensory panel also, using the technique CharmAnalysis (92-94) for GCO analysis, along with gas chromatography-mass spectrometry (GC-MS), odor-active compounds were identified by their respective Kovats retention indices (95). Contained below is a... 

Figure 2 details a GCO chromatogram for the "high Brett" wine. The 15 most odor active compounds were identified by GCO and GC-MS. The odor active compounds were ranked from 1-15, 1 being the most odor active compound. The numbers above each peak in the chromatogram correspond to the compound and associated descriptor in the table. 

Isovaleric acid (3-methyl butanoic acid) was found to be the dominant odorant in the "high Brett" wine as detected by CharmAnalysis. The odor described by the GCO sniffer was rancid the chemical identity of the odorant was confirmed by GC-MS. This acid is produced in wine by yeast as a metabolic byproduct of protein (99). Volatile phenolic compounds, such as 4-ethyl guaiacol, guaiacol, and 4-ethyl phenol, were also among the dominate odor active compounds in this wine however, the individual contribution by each of the three phenolics was half or less than the odor activity of isovaleric acid. 

Odor active compound Odor descriptor Retention index Charm value Spectral value... 

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". 

Acknowledgments Special thanks are due to Ed Lavin and Peter Ong for their enthusiastic interest and collaboration in the quest to identify the odor active compounds by GCO and GC/MS and to Jeanne Samimy of the Cornell NYSAES Geneva library for always making our literature searches easier. 

Figure 3. Odor spectrum gas chromatograms on an OVIOI column - the top 15 odor active compounds (a) "no Brett", (b) "medium Brett", (c) "high Brett". Numbers on the chromatogram refer to the chemical structures in figure 4.

LANDY p, NiCKLAUS s, SEMON E, MIELLE p and GUICHARD E, Representativeness of extracts of offset paper packaging and analysis of the main odor-active compounds , J. Agric. Food Chem, 2004 52 2326-2334. 

Ketones Aliphatic ketones formed by autoxidation of lipids also contribute to the flavor of oils and food products. For example, Guth and Grosch (13) identified l-octen-3-one as one of the odor-active compounds in reverted soybean oil. This compound was described as metallic and mushroom-like. The reaction pathway for the formation of l-octen-3-one from the linoleate-10-hydroperoxide via the p-scission route is illustrated in Figure 2. 10-Hydroperoxide of linoleate is not the usual hydroperoxide formed by autoxidation of linoleate however, it is one of the major hydroperoxides formed by the photosensitized oxidation (singlet oxygen reaction) of linoleate (14). 

Table 5 shows the sensory evaluation by Schieberle et al. (30) of the different kinds of butter, namely, Irish sour cream (ISC), cultured butter (CB), sour cream (SC), sweet cream (SwC), and farmer sour cream (ESC). It revealed ISC butter and ESC butter with the highest overall odor intensities. Table 5 shows that 19 odor-active compounds were detected by aroma extract dilution analysis (AEDA) in a distillate of the ISC butter. The highest flavor dilution (ED) factors have been found for 5-decalactone, skatole, i-6-dodeceno-y-lactone, and diacetyl followed by trany-2-nonenal, cw,c -3,6-nonadienal, c/i-2-nonenal, and l-octen-3-one. 

Perhaps the most important compounds identified in the roasted sesame oils are 2-furfurylthiol and guaiacol. Using aroma extract dilution analysis method, these two compounds have been characterized by Schieberle (92) to be the most odor-active compounds in roasted sesame seeds. 2-Furfurylthiol, having an intense coffee-like odor, increased from 16 ppb in roasted oil processed at 160°C for 30 min to 158 ppb in the oil processed at 200°C for 30 min (Table 12). Guaiacol has a burnt and smoky odor with an extremely low-odor threshold of 0.02 ppt in... 

TABLE 12. Changes in the Content (ppb) of Selected Odor-Active Compounds in Sesame Oils with Sesame Seeds Roasted at 160, 180, 200, and 220°C for 20 min (88, 89). 

Gas chromatography-olfactometery (GC-O) provides a sensory profile of odor active compounds present in an aroma extract by sniffing the GC effluent. Several techniques have been developed to collect and process GC-O data and to estimate the sensory contribution of individual odor active compounds, including dilution analysis (29, 30), time intensity (31), and detection frequency (32) methods. GC-O has successfully been used to evaluate the odor active compounds of olive oil (33), soybean oil (34), and fish oil enriched mayonnaise (35). 

Egolf, L.M. and Jurs, P.C. (1993b). Quantitative Structure-Retention and Structure-Odor Intensity Relationships for a Diverse Group of Odor-Active Compounds. AnaLChem., 65, 3119-3126. 

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