Isolation of the Volatile Fraction

Recently, the procedures that are suitable to isolate the volatile fraction of a sample under mild conditions have been reviewed [1]. Three techniques—solvent extraction, distillation and solid-phase microextraction (SPME)—will be presented here. 

Solid samples are extracted with low-boiling solvents. As the polarity of the volatiles is different, a two-step extraction procedure is recommended, e.g. methylene chloride as the first solvent and diethyl ether as the second solvent [13]. The yield of the odorants is enhanced when the dry sample is soaked in water before the extraction procedure [14]. After filtration and drying, the extract is concentrated to approximately 50 mb and is then freed from the non-volatile material by using the solvent-assisted flavour evaporation (SAFE) method (Sect. 16.2.2.2). 

The compact distillation unit shown in Fig. 16.1 has been designed for the rapid and careful isolation of volatiles from the non-volatile food components [15]. This technique, denoted SAFE, is suitable for solvent extracts, aqueous samples, or matrices with high oil content. 

The procedure is as follows. After application of high vacuum (approximately 5 mPa) to the apparatus, the distillation procedure is started by dropping aliquots of the sample into distillation flask no. 4 (Fig. 16.1). The volatiles, includ- 

2 Oxidative cleavage of unsaturated fatty acids by lipoxygenase and hydroperoxide lyase 

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Reports a standardized method for direct determination of TBA value in oils, fats, and lipid extracts. Unlike distillation and extraction methods, this technique determines total reactive substances without previous isolation of the volatile fraction. Furthermore, the solvent of choice is organic (l-butanol) rather than aqueous. This protocol is reported as the official (AOCS) method to determine TBA value (direct method) of animal and vegetable fats and oils, fatty acids and their esters, partial glycol esters, and similar materials. 

METHODS FOR IDENTIFICATION OF KEY ODORANTS 2.1. Isolation of the volatile fraction... 

The analysis of aroma compounds starts with the isolation of the volatile fraction from the food. Techniques used in the preparation of flavor extracts from foods have recently been reviewed [7-9], The most important task in the choice of the isolation procedure is to test whether the flavor of the extract is identical or at least similar to the flavor of the food itself. This has to be checked by a sensory evaluation of the food extract (e.g., after dilution with an appropriate medium like water or oil) before a time consuming chemical analysis is performed. 

The evaluation of only one GC run has an important drawback. Since it is not possible to exactly evaluate the intensity of the odor during sniffing, the results cannot be used to decide whether a compound is a key odorant in beer flavor or contributes little to the overall odor. Furthermore, the number of the compounds detected depends on accidental factors, e.g., the amount of die food used for the isolation of the volatile fraction or the degree of the concentration. 

Froline (2 mM) and dihydroxyacetone (DHA 1 mM) were combined in 100 ml of 0.1 mol/1 phosphate buffer (pH 7.0) and boiled at back-flush for 2 hours. The volatiles were isolated by ether extraction and Acp was found to constitute only 0.1 % of the volatile fraction. 

Very recently, Cantergiani et al. (2001) (Figure 2,11) investigated the composition of the volatile fraction of a Mexican green coffee with a pronounced earthy/mouldy off-flavor. The three components responsible were determined by GC-olfactometry, isolated, concentrated and finally characterized by GC/MS as geosmin (Section 5,B.46), 2-methylisoborneol (Section 5,B.44) and 2,4,6-trichloroanisole (Section 5,H.82), The concentrations were lower in the reference than in the defective samples. The... 

A different example of the potential of GC-MS for the analysis of flavors and off-flavors is represented by a series of studies dealing with the characterization of volatile compounds in cork used for the production of wine stoppers [43-45]. The approach chosen for these investigations was the application of DHS for aroma sampling and the use of GC-MS for the analysis of the volatile components of different cork samples in various analytical problems (1) characterization of the volatile fraction in raw material (bark) and new and used cork wine stoppers [43], (2) characterization of volatile compounds produced by microorganisms isolated from cork [44], and (3) study of the effects of electron-beam irradiation on cork volatile compounds in a sterilization process [45]. 

Prior to radiochemical analysis the samples were ashed and separated into size fractions by means of procedures described by Nathans et al. The determinations of the fraction weights and of the mean diameters of the particles in the fractions have also been described extensively in the same paper. An aliquot of each size fraction was dissolved and subjected to a separation procedure to isolate Sr, Ru, Sb, Cs, Ce, Pm, U, and Pu fractions. The procedure is sketched in Figure 1. Further decontamination of Ru and Ce was carried out only with the Johnie Boy sample. The Sb and Pu fractions were set aside for later analysis. After complete analysis of the Cs fractions, anomalies were found in the data for the coral samples. These samples had been ashed at about 475°C. Apparently some Cs had volatilized at this temperature. Such a behavior explained the anomalies, and this was confirmed by Heft by more extensive experimentation (4). Thus, Cs data are reported only for the Johnie Boy sample, which was ashed at low temperature in a Tracerlab low temperature asher. 

The following procedure is a modification of the first reported synthesis of the trimethylindium adduct.9 An important feature in the method is the removal of diethyl ether, which forms no isolable adduct with trimethylindium but is very difficult to remove completely from neat trialkyl. With trimethylgallium this is even more difficult because the adduct bond is stronger and the adduct is isolable. In the present case, it is achieved by displacement of the diethyl ether by dppe and concomitant formation of the isolable dppe adduct. Even here the diethyl ether has to be removed by careful fractionation, to avoid loss of the volatile trialkyl. The original solvent for the preparation was benzene, but this has been replaced successfully with toluene. 

The oleoresinous exudate or "pitch of many conifers, but mainly pines, is the raw material for the major products of the naval stores industry. The oleoresin is produced in the epithelial cells which surround the resin canals. When the tree is wounded the resin canals are cut. The pressure of the epithelial cells forces die oleoresin to the surface of die wound where it is collected. The oleoresin is separated into two fractions by steam distillation. The volatile fraction is called gum turpentine and contains chiefly a mixture of monoterpenes but a smaller amount of sesquiterpenes is present also. The nonvolatile gum rosin 5 consists mainly of llie dilerpenuid resin acids and smaller amounts of esters, alcohols and steroids. Wood turpentine, wood rosin and a fraction of intermediate volatility, pine oil are obtained together by gasoline extrachon of the chipped wood of old pine stumps. Pine oil is largely a mixture of the monoterpenoids terpineol. borneol and fenchyl alcohol. Sulfate turpentine and its nonvolatile counterpart, tall oil, 5 are isolated as by-products of the kraft pulping process. Tall oil consists of nearly equal amounts of saponified fatty acid esters and resin acids. 

Once an internal standard is available, the analysis can be easily performed the food sample is spiked with an appropriate amount of the labelled standard and then the volatile fraction is isolated by solvent extraction and sublimation under a high vacuum (cf. Figure 1). If required, the odorants and the internal standards are enriched by liquid chromatography... 

Paralleling the studies of the volatile products of roasted cacao beans and of baked cereal products, and using the same techniques, a great deal of effort has gone into the determination of the compounds present in the volatile fractions of cooked meat. Most of these have been concerned only with beef, either roasted or boiled, but chicken has also received appreciable attention (21). Several lists of compounds isolated from the volatiles of cooked beef have been published (22-24), both cumulative and newly isolated ones. The totals for chicken (as of 1972) and for beef (as of 1977) are more than two hundred each. It... 

Two oat varieties were studied with respect to their oil content. The composition of these SCCO2 extracted oils, with regard to fatty acids, free fatty acids, phosphorus and thermal stability has previously been reported (Fors and Eriksson, submitted for publication 1988). Volatile compounds were isolated from the oat oils by molecular vacuum distillation. The fractions obtained were transferred to aqueous alkali and extracted by CH2CI2. The adjustment in pH was made to remove fatty acids which could otherwise interfere with the later work. Moreover, it is well established that many heterocycles are important flavor compounds in heated food items. These compounds are normally isolated in the basic fraction. The isolates were analysed by chemical and sensory methods. 

Star anise volatile oils can also be isolated by supercritical C02 extraction coupled to a fractional separation technique. Gas chromatography-mass spectrometry analysis of the various fractions obtained in different extraction and fractionation conditions allowed the identification of the best operating conditions for the isolation of essential oil. A good extraction performance was obtained operating at 90 bar and 50°C (for 630min) for both treated materials. Optimum fractionation was achieved in both cases by operating at 90 bar and -10°C in the first separator and at 15 bar and 10°C in the second (Della Porta eta/., 1998). 

Figure 3. Concentrations of five categories of volatile compounds, observed as a result of acid hydrolysis of the glycoside fractions isolated from juice and skin extracts of Napa Merlot (NMJ, NMS) and Cabernet Sauvignon (NCJ, NCS) fruit.

Coal tar creosote components are slowly released from treated wood products by oil exudation, rainwater leaching, and by volatilization of the lighter fractions (Henningsson 1983). USDA (1980) reported that the major components of creosote were not detected in soil samples taken to a depth of 6 inches within 2-24 inches from treated poles, presumably as a result of biotransformation of mobilized components by soil microorganisms. Creosote components released to soils in waste water effluents have been found to be biotransformed by soil microbes under aerobic conditions (Middleton 1984). Bacteria of the genus Pseudomonas isolated from a creosote-contaminated waste site have been reported to degrade creosote-derived quinoline (Bennett et al. 1985). Acclimation to creosote phenolic constituents by soil microorganisms has also been demonstrated (Smith et al. 1985). 

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