Aroma compounds headspace sampling

The buds and the leaves (less often) of the Brussels sprout plant (Brasska olera-cea var. gemmifera) are eaten cooked with the main meal. In Brussels sprouts, breakdown products from glucosinolates are dominant and represent about 80-90% of the volatiles in headspace samples [176]. The residual volatiles are mostly sulfur compounds [176]. Compounds likely to be associated with the aroma of Brussels sprouts are 2-propenyl isothiocyanate, dimethyl sulfide, dimethyl disulfide and dimethyl trisulfide (Table 7.6) [35,176]. 

The highly volatile odorants are not detected or are underestimated when the screening method is applied to an aroma extract. These compounds are lost when the extract is concentrated or they are masked in the gas chromatogram by the solvent peak. To overcome this limitation, the screening has to be completed by GC-O of static headspace samples (GCOH Fig. 16.3) [59-61]. 

Aroma compounds are present in minute levels in foods, often at the ppb level ( ig/liter). In order to analyze compounds at these levels, isolation and concentration techniques are needed. However, isolation of aroma compounds from a food matrix, which contains proteins, fats, and carbohydrates, is not always simple. For foods without fat, solvent extraction (unit gu) can be used. In foods containing fat, simultaneous distillation extraction (SDE see Basic Protocol 1) provides an excellent option. Concentration of headspace gases onto volatile traps allows sampling of the headspace in order to obtain sufficient material for identification of more volatile compounds. A separate protocol (see Basic Protocol 2) shows how volatile traps can be used and then desorbed thermally directly onto a GC column. For both protocols, the subsequent separation by GC and identification by appropriate detectors is described in unitgu. 

For a compound to contribute to the aroma of a food, the compound must have odor activity and volatilize from the food into the head-space at a concentration above its detection threshold. Since aroma compounds are usually present in a headspace at levels too low to be detected by GC, headspace extraction also requires concentration. SPME headspace extraction lends itself to aroma analysis, since it selectively extracts and concentrates compounds in the headspace. Some other methods used for sample preparation for aroma analysis include purge-and-trap or porous polymer extraction, static headspace extraction, and solvent extraction. A comparison of these methods is summarized in Table Gl.6.2. 

Sampling volatile analytes from samples having complex matrices usually takes place in the HS-SPME mode. This variant yields decidedly better results in the determination of aroma compounds59 and other volatile components.60 Moreover, HS-SPME prolongs the life of the fiber because it is not in direct contact with the sample. On the other hand, the direct extraction of less volatile compounds from solution is possible using DI-SPME. But in this case, the fiber deteriorates more quickly, increasing the cost of analysis. Headspace sampling is therefore employed whenever possible. 

The atmosphere of cold stored Black Truffles is particularly rich in volatile compounds which impart the truffle aroma. We therefore developed a modified gas headspace sampling procedure for their isolation. 

The headspace sampling technique developed in the present study to collect volatiles from cold stored Black Truffles performed adequately. Indeed, the aroma Isolate obtained was described as typical, and 11 minor compounds could be described for the first time as Black Truffle aroma constituents. Moreover, these results allowed the formulation of the first Nature-Identical Black Truffle aromatizer. 

The volatiles of fresh pineapple (Ananas comosus [L] Merr.) crown, pulp and intact fmit were studied by capillary gas chromatography and capillary gas chromatography-mass spectrometry. The fnjit was sampled using dynamic headspace sampling and vacuum steam distillation-extraction. Analyses showed that the crown contains Cg aldehydes and alcohols while the pulp and intact fruit are characterized by a diverse assortment of esters, h rocarbons, alcohols and carbonyl compounds. Odor unit values, calculated from odor threshold and concentration data, indicate that the following compounds are important contributors to fresh pineapple aroma 2,5-dimethyl-4-hydroxy-3(2H)-furanone, methyl 2-methybutanoate, ethyl 2-methylbutanoate, ethyl acetate, ethyl hexanoate, ethyl butanoate, ethyl 2-methylpropanoate, methyl hexanoate and methyl butanoate. 

In the early 90s, a new technique called solid-phase-micro extraction (SPME), was developed (Arthur and Pawliszyn, 1990). The key-part component of the SPME device is a fused silica fiber coated with an adsorbent material such as polydimethylsiloxane (PDMS), polyacrylate (PA) and carbowax (CW), or mixed phases such as polydimethylsiloxane-divinylbenzene (PDMS-DVB), carboxen-polydimethylsiloxane (CAR-PDMS) and carboxen-polydimethyl-siloxane-divinylbenzene (CAR-PDMS-DVB). The sampling can be made either in the headspace (Vas et al., 1998) or in the liquid phase (De la Calle et al., 1996) of the samples. The headspace sampling in wine analyses is mainly useful for quantifying trace compounds with a particular affinity to the fiber phase, not easily measurable with other techniques. Exhaustive overviews on materials used for the extraction-concentration of aroma compounds were published by Ferreira et al. (1996), Eberler (2001), Cabredo-Pinillos et al. (2004) and Nongonierma et al. (2006). Analysis of the volatile compounds is usually performed by gas chromatography (GC) coupled with either a flame ionization (FID) or mass spectrometry (MS) detector. 

The rehumidified MlOO and Ml80 samples were stored at various temperatures (45, 50, and 60°C) above their Tg, resulting in the time-dependent collapse of matrix structure and simultaneous release of aroma compounds. Figure 61.2 shows that release of benzaldehyde occurred more rapidly when the storage temperature increased. Both static headspace-GC... 

Solid-phase microextraction (SPME) of wine was developed by both headspace (HS) (Vas et al., 1998) and liquid-phase sampling (De la Calle et al., 1996). Exhaustive overviews on materials used for the extraction-concentration of aroma compounds in wines were published from Ferreira et al. (1996), Cabredo-Pinillos et al. (2004), and Nongonierma et al. (2006). 

The primary interference with this basis of isolation is the water present in a sample. In most foods, the aroma components seldom make up more than 300 ppm (0.03%) of the product. Yet, the moisture content of a food, even a dry food, is generally above 2% and thus isolation methods based solely on volatility will produce a dilute solution of aroma componnds in water. The high boihng point of water precludes a simple concentration and analysis, i.e., the aroma compounds would be lost during concentration since they are present in low concentrations and are often more volatile than water. Thus, most dynamic headspace methods of aroma isolation involve some additional method to remove water from the isolate. 

Static headspace gas chromatography (SHGC) was used to study the effects of saliva volume on the retention of the five aroma compounds by the emulsion. Emulsion to saliva ratios included 100 0, 80 20, 60 40, and 40 60. The 2mL of the samples was transferred to 10-mL headspace vials, which were then incubated at 37°C and agitated at 750 rpm for 6 min, using an automated headspace unit (Combipal-CTC Analytics JVA Analytical Ltd., Dublin, Ireland). One milliliter of the headspace was injected and analyzed by using a gas chromatograph (GC Varian CP-3800 JVA Analytical Ltd.) equipped with a flame ionization detector (FID). The injector and detector... 

In the case of the model mouth, a general decrease is also observed in the amount of each compound released from the sample on dilution with saliva (Fig. 3). However, the decreases in amount of each compound released were not proportional to the decreases in the aroma compound concentration of the samples on dilution with increasing volumes of saliva. The amounts of 2-heptanone released from samples containing 20%, 40%, and 60% saliva expressed as a percentage of the amount released from the 100% emulsion sample were 66%, 58%, and 46%, respectively. The decreased effect of the values on the release of the aroma compounds observed in the model mouth may be partially explained by the differences in the operating procedures between the model mouth and the RAS. In the case of the RAS, the initial conditions before dilution of the headspace above the sample with nitrogen and collection of the aroma compounds onto Tenax were very close to those used in static headspace analysis unless initially flushed before sampling. In the model mouth, the sample flask caimot be sealed before collection of the aroma compounds thus equilibrium conditions in the flask are never achieved before headspace dilution. 

The sensory relevance of individual odorants can be estimated by injecting various headspace volumes. This is equivalent to AEDA of liquid samples. In contrast to AEDA, where aroma compounds are separated from the food matrix, static headspace GC-O provides data about the aroma above the food. This technique is suitable for studying the effect of the food matrix on the aroma profile. Therefore, AEDA and static headspace GC-O result in complementary data. 

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. 

A heterocyclic sulfur-containing compound, 2-methyl-thiophene, was identified in boiled crayfish tail meat and pasteurized crabmeat. Thiazole and 3-methylthiopropanal were identified in the crayfish hepatopancreas. Heterocyclic sulfur-containing compounds play important roles in generating meaty aromas in a variety of meat products and are considered important volatile aroma components of marine crustaceans (12— 14). The 2-methylthiophene could be an important flavor cemponent in boiled crayfish tail meat. Both thiazole find 3-methylthiopropanal were important contributors to the desirable meaty aroma associated with crayfish hepatopancreas. The 3-methyl-thiopropanal, identified in boiled crayfish hepatopancreas, is derived from Strecker degradation of methionine (15), and has been considered to be an important cemponent in basic meat flavor (16). Pyridine was detected in the headspace of the hepatopancreas from freshly boiled crayfish. Pyridine and 2-ethylpyridine have been previously reported as components in the atmospheric distillate from a sample of crayfish hepatopancreas frozen for three months (2). 

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