GC-effluent sniffing

The composition of the volatile fraction of bread depends on the bread ingredients, the conditions of dough fermentation and the baking process. This fraction contributes significantly to the desirable flavors of the crust and the crumb. For this reason, the volatile fraction of different bread types has been studied by several authors. Within the more than 280 compounds that have been identified in the volatile fraction of wheat bread, only a relative small number are responsible for the different notes in the aroma profiles of the crust and the crumb. These compounds can be considered as character impact compounds. Approaches to find out the relevant aroma compounds in bread flavors using model systems and the odor unit concept are emphasized in this review. A new technique denominated "aroma extract dilution analysis" was developed based on the odor unit concept and GC-effluent sniffing. It allows the assessment of the relative importance of the aroma compounds of an extract. The application of this technique to extracts of the crust of both wheat and rye breads and to the crumb of wheat bread is discussed. 

Sensory examination of the effluent from the column of a gas chromatograph by "nasal appraisal" has been introduced into flavor analysis by Fuller et al. (22). Since that time this technique known as "GC-effluent sniffing" has been used in aroma analysis to locate the positions of odorants in a gas chromatogram. It was first... 

GC-effluent sniffing of wheat bread aroma concentrates has shown the presence of low level volatiles that smell like the fresh bread crust. As discussed in the preceeding sections, these compounds (3-7 in Figure 1) were proposed to be responsible for this odor note. 

The drawback of GC-effluent sniffing is that it does not allow a differentiation between those odor compounds that contribute intensely to a flavor of a food and those which are only components of the background flavor. 

The important odor compounds can be evaluated by the GC-effluent sniffing of a series of dilutions from the original aroma extract. Two variations of this technique were developed by Acree et al. (28, 29) and by us (30-37). 

Acree et al. (28, 29) used a video-terminal in addition to the gas chromatograph. They calculated CHARM-values on the basis of the duration of the sensory responses which were maintained during the GC-effluent sniffing of three-fold dilutions of the original extract. CHARM-values are directly proportional to odor units. 

Description of the odors recognized during GC-effluent sniffing of the crumb extract. 

Murray and Whitfield (11), in their extensive survey of 3-alkyl-2-methoxypyrazines in raw vegetables, reported finding 3-isobutyl-, 3-lsopropyl-, and 3-(Sec-butyl)-2-methoxypyrazines in red, or chili peppers. Combining gc effluent sniffing and gc-ms selected ion monitoring, Huffman e al. (12) detected 3-lsobutyl-2-methoxypyrazlne in both fresh and processed Jalapeno pepper volatiles. They attributed most of the fresh Jalapeno aroma to the presence of the bell pepper pyrazlne. 

Component Separation and Identification. Hewlett Packard 5830A and 5840A gas chromatographs fitted with glass, fused silica, or stainless steel capillary columns of various Inside diameters were used for separation of the volatiles fractions. Effluent splitters were used on occasion to permit gc effluent sniffing. Methyl silicone oil was the stationary phase of choice for most of the work, although Tween 20 and Carbowax 20M were used occasionally. 

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

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). 

To detect the odour-active volatiles. Fuller et al. [6] described a system for the sniffing of GC effluents which was improved and applied to food samples by Dravnieks and O Donnell [7]. The new technique, named GC olfactometry (GCO), was the starting point for the development of a systematic approach to the identification of the compounds which cause food aromas. As summarised in Table 6.23 the analytical procedure consists of screening for key odorants by special GCO techniques, quantification and calculation of OAVs as well as aroma-recombination studies. During the last decade these steps have been critically reviewed by Acree [8], Blank [9], Grosch [10, 11 ], Mistry et al. [12] and Schieberle /fi/. 

GC in combination with olfactometric techniques (GC-0) is a valuable method for the selection of aroma-active components from a complex mixture (7). Experiments based on human subjects sniffing GC effluents are described as GC-0. This technique helps to detect potent odorants, without knowing their chemical structures, which might be overlooked by the OAV concept (ratio of concentration to threshold) if the sensory aspect is not considered from the very beginning of the analysis. Experience shows that many key aroma compounds occur at very low concentrations their sensory relevance is due to low odor thresholds. Thus, the peak profile obtained by GC does not necessarily reflect the aroma profile of the food. 

Two techniques based on dilution have been developed CharmAnalysis by Acree and coworkers (6,12,13) and aroma extract dilution analysis (AEDA) by Grosch and his group (7,14,15). Both evaluate the odor activity of individual compounds by sniffing the GC effluent of a series of dilutions of the original aroma extract. Both methods are based on the odor-detection threshold. The dilution value obtained for each compound is proportional to its OAV in air, i.e., its concentration. Several injections are required to reach a dilution of the aroma extract in which odorous regions are no longer detected. 

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. 

After concentration of the extract by microdistillation [25] or by special procedures [26] to facilitate the identification of the odorants, an aliquot is separated by high-resolution GC and the effluent is split into a flame ionisation detector (FID) and a sniffing port [27]. The positions of the odorants in the gas chromatogram are assessed by sniffing the carrier gas as it flows from the port. This procedure is denoted GC-O. 

The extract of the volatiles is separated by high resolution gas chromatography (HRGC) and the odor of the compounds is assessed by sniffing the effluent of the GC column in parallel with the FID-detection. This technique allows the detection of odor-active volatiles, the determination of their odor qualities and, most important, the combination of these sensory data with an analytical parameter, the retention index (RI). In Figure 2, the results of... 

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. 

The GC profiles show that two compounds (peaks A and B) were newly produced by cooking. By sniffing the effluents from a gas chromatograph, it was determined that these two components have a characteristic seafood-like aroma, although not intense. 

Gc fractions which retained some pepper-like aroma lost this aroma after a few hours, even at low temperatures. Some success was achieved with larger bore glass capillary columns components with odors reminiscent of the pepper aroma partly survived separation in such columns, permitting gc sniffing of the effluent. However, attempts to collect several components were unsuccessful. 

Mineral water samples, stored in PE-lined aluminium/cardboard packages were incubated at 40 °C, and then volatiles in the mineral water were analysed by sniffing the effluent from a gas chromatographic column. The effluent was sensorially evaluated for the intensity of descriptors such as synthetic, sickly, musty, metallic or dry. Components detected by sniffing were subsequently identified by GC/mass spectrometry as aromatic hydrocarbons and carbonyls [135]. 

Olfactometry techniques can be classified into two categories dilution methods, which are based on successive dilutions of an aroma extract until no odour is perceived at the sniffing port of the GC, and the intensity methods, in which the aroma extract is injected and the assessor records the odour intensity and perception as a function of time. The technical solution is straightforward with a split at the end of a chromatographic column and a heated transfer line to a GC external sniffer port. The eluting compounds are splitted, for example, 1 50 to an FID or MS detector and the sniffing port. The column effluent is combined at the sniffer port with a laminar stream of inert make-up gas, which is heated to a constant temperature and additionally humidified. 

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