Aroma compounds analytical methods

In retrospect, there are no totally new techniques for the isolation of thermally generated aroma compounds. The developments we have seen in recent years have been modifications of techniques which have existed for several years. As in the past, each method has its own unique strengths and weaknesses. The choice of method is determined by the food product to be analyzed, the volatiles of interest and the analytical methods to be appl ied. 

By GC-MS analysis, peaks 36, 37 and 39 were estimated to be 3-hydroxy-4,5-dimethyl-2(5H)-furanone, acetate of hydroxymethyl furfural and 4-pentyl-2-pentenolide, respectively. At this stage, the sample was too small to apply other analytical methods therefore, we tried to synthesize all the possible compounds using the synthetic approaches described in the following section. None of thethree synthetic products showed the characteristic sugary aroma that we had recognized in each separated fraction however, the yield of fraction 11-GC TRAP from molasses was calculated to be ca. 

Various chemical processes of limonene, which lead to the obtainment of useful chemicals and some analytical methods, are based on these reactions. Many flavor chemicals are synthesized from limonene by reaction with water, sulfur and halogens, or hydrolysis, hydrogenation, boration, oxidation and epoxide formation (Thomas and Bessiere, 1989). Hydroperoxides have also been studied and isolated because of their effect on off-flavor development in products containing citrus oil flavoring agents (Clark et al., 1981 Schieberle et al., 1987). Hydration of d-limonene produces alpha-terpineol, a compound that gives off an undesirable aroma in citrus-flavored products. It is also possible to produce alpha-terpineol and other useful value-added compounds... 

In the last 15-20 years, several analytical methods have been reported for extraction of aroma compounds from wine finalized to replace the time-consuming (10-24 hours), continuous liquid-liquid methods. Liquid-liquid extraction was usually performed either with dichloromethane/pentane (2 1 v/v) (Drawert and Rapp, 1968) or Freon 11, sometimes in a mixture Freon 11/dichloromethane 9 1 (v/v) (Hardy, 1969 Rapp and Hastrich, 1978 Marais, 1986), both approaches being... 

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. 

Ferreira, A.C.S. and de Pinho, PG. (2003). Analytical method for determination of some aroma compounds on white wines by solid phase microextraction and gas chromatography, 1 Food Sci., 68(9), 2817-2820. 

The free volatile compounds of wines were extracted with dichloromethane. Representative wine aroma extracts for chemical and olfactory analysis were obtained using this solvent. Fig. 1 is the TIC of free volatile compounds of Rojal wines detected by SPE-GC-MS. Quantitative data of the volatile compormds found in free aroma fraction of the young red wines from Rojal grape variety are shown in Tables 1 and 2. The data are expressed as means (pg/1) of the GC-MS analyses of duplicate extractions and they correspond to the average of the analyzed wines. Improvement in the analytical method used to extract the volatile compounds from these wines has allowed us to identify and quantify 80 free volatile compounds in Rojal red wines including alcohols, esters, acids, terpenes, C13 noiisoprenoids, Ce compounds and benzenic compounds. They have been positively identified and quantitatively determined. 

As has been previously said, 2,3-butanodione (diacetyl) is an important aroma of alcoholic beverages, it has not been studied and measured extensively in the past because of analytical difficulties in the quantitation caused by its highly volatile nature, chemical instability, and interference of other compounds. Colorimetric methods to measure diacetyl have been widely used in the past. These methods involve steam distillation to isolate diacetyl from the matrix. However, distillation has the disadvantage of incomplete isolation of diacetyl from other closely related compounds that will result in an overestimation of its concentration. A fluorometric method was developed to improve upon the lengthy distillation methods that involve derivatization. Although acetaldehyde and its acetal can be determined by direct injection GC-FID in spirit drinks (EU reference method for spirits), most chromatographic methods for minor aldehydes implicate also derivatization. While a very sensitive and accurate method based on SMPE without derivatization and MS detection has been developed, it requires the use of... 

This chapter will discuss the basis of the methods used in the isolation and analysis of food aroma components. It will be pointed out repeatedly that there is no single method of isolation or analysis that provides a complete view of the aroma compounds found in a food. The goal is to find an analytical method that can measure those components that are of interest to the analyst. They may be, for example, the compounds that give an off-flavor or those that give a fresh note to a food. Unfortunately, any aroma profile will be a partial view of the overall picture. The reader is encouraged to obtain a more complete discussion of this topic than can be provided in this text and suggests references [5-8] as sources. 

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. 

Historically, modem aroma research began with the isolation and identification of aroma compounds in foods. It was thought that if we could identify all of the aroma compounds in foods, we would be able to reproduce the aroma of that food by formulating a flavor based on the analytical data. This did not prove to be the case. Researchers found that there were very large numbers of aroma compounds present in foods and not all could possibly be contributors to the aroma of a food. Thus, an era began where researchers attempted to determine which aroma compounds were traly needed to recreate the aroma of a food. It was postulated that somewhere between 20 and 30 compounds should be adequate to reproduce the aroma of a food. The question then was, which compounds were needed Several approaches were developed to meet this challenge. These methods will be briefly discussed. 

The gas chromatographic determination of individual carbonyl compounds appears to be a method suitable for conparison with findings of sensory panel tests. Analytical methods for the odorants causing aroma defects is still in the early stages of development because only a few fats or fat-containing foods have been examined in such detail that the aroma substances involved are clearly identified. 

Analytical methods involving exhaustive extraction of flavor compounds (i.e., liquid/liquid extraction, dynamic headspace) do not take these matrix effects into account. However, new instrumentation and methodologies are yielding improved information on the mechanisms involved in flavor/matrix interactions and the effects on flavor perception. For example, spectroscopic techniques, such as nuclear magnetic resonance (NMR), can provide information on complex formation as a function of chemical environment and have been used to study both intra- and intermolecular interactions in model systems [28,31]. In addition, NMR techniques, initially developed to study ligand binding for biological and pharmaceutical applications, were applied in 2002 to model food systems to screen flavor mixtures and identify those compounds that will bind to macromolecules such as proteins and tannins [32]. Flavor release in the mouth can be simulated with analytical tools such as the retronasal aroma simulator (RAS) developed by Roberts and Acree [33]. These release cells can provide... 

The aim of GC-0 techniques in food aroma research is to determine the relative odor potency of compounds present in the aroma extract. This method gives the order of priority for identification and thus indicates the chemical origin of olfactory differences (7). The value of the results obtained by GC-O depends directly on the effort invested in sample preparation and analytical conditions. Analysis of an aroma extract by dilution techniques (AEDA, Charm) combined with static headspace GC-O provides a complete characterization of the qualitative aroma composition of a food. However, this is only the first step in understanding the complex aroma of a food. 

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. 

Aroma analysis is most often performed utilizing GC-MS. This demands separation of volatile constituents from nonvolatile matrices. Additionally, higher concentrations of analytes are favorable to allow detection of trace key compounds.16,64,65 82 Therefore, various preparation methods derived from aroma extract production were developed. The composition of the concentrate may differ depending on the method used and thus selected to accommodate the aim. 

Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxy-acid esters, and the corresponding jf- and -lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. 

Applications of the Kaltron method and GC-MS in wine aroma analysis were reported by Rapp et al. (1996). The study of the data repeatability in the analysis performed using both Kaltron (Rapp et al., 1994) and polystyrene XAD-2 resin (Gunata et al., 1985 Versini et al., 1988 Voirin et al., 1992) extraction, has already been performed in the cited papers. Besides this, a test with eight repetitions of the whole analytical process using XAD-2 resin has been performed both on a non-floral (Pinot blanc) and a floral (Morio-Muskat) varietal wine for the quantification of 25 varietal and fermentation compounds (Carlin, 1998) for each wine sample the mean CV % values ranged from about 7.0 to 7.5 with a standard deviation from about 3 to 4.7 %, respectively, depending on the different level of some compounds, mostly monoter-penes, in each wine type. 

In general, aroma enrichment by using both polyhydroxylated styrene-divinylbenzene/ENV+ and C18 cartridge or polystyrene/XAD-2 resin are less time-consuming, require less solvent vapour protected working space, are generally more healthy and consume less organic solvent than traditional methods of continuous liquid-liquid extraction. Analytical precision, referred to the most representative wine compounds included... 

Quantification of aroma-impact components by isotope dilution assays (IDA) was introduced in food flavor research by Schieberle and Grosch (1987), when trying to take into account losses of analytes due to isolation procedures. The labeled compounds have to be synthesized, the suitable fragments have to be chosen, and calibration has to be effected. A quantitative determination of ppb levels of 3-damascenone (Section 5,D.38) in foods, particularly in roasted coffee (powder and brew), was developed by Sen et al. (1991a). Semmelroch et al. (1995) quantified the potent odorants in roasted coffee by IDA. Hawthorne et al. (1992) directly determined caffeine concentration in coffee beverages with reproducibility of about 5 % using solid-phase microextraction combined with IDA. Blank et al. (1999) applied this combined method to potent coffee odorants and found it to be a rapid and accurate quantification method. They also concluded that the efficiency of IDA could be improved by optimizing the MS conditions. 

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