Aroma compounds solvent extraction

If an aqueous solution of a compound is shaken with another liquid that is immiscible (mutually insoluble) with water, some of the compound may dissolve in the other solvent. For example, molecular iodine, I>, is very slightly soluble in water but is highly soluble in tetrachloromethane, CC14, which is immiscible with water. When tetrachloromethane is added to water containing iodine, most of the iodine dissolves in the CC14. The solute is said to partition itself between the two solvents. Solvent extraction is used to obtain plant flavors and aromas from aqueous slurries of the plant that have been crushed in a blender. 

The resinoids described above should be distinguished from prepared oleoresins (e.g., pepper, ginger, and vanilla oleoresins), which are concentrates prepared from spices by solvent extraction. The solvent that is used depends on the spice currently, these products are often obtained by extraction with supercritical carbon dioxide [223a]. Pepper and ginger oleoresins contain not only volatile aroma compounds, but also substances responsible for pungency. 

In terms of specificity in isolation, one will also isolate food constituents that are not aroma compounds (e.g. pesticides, herbicides, PCBs, plasticisers, and some antioxidants). Since these compounds are typically present in foods at very low levels, they generally present few complications. The primary volatile that complicates the application of this methodology is water. In all cases, one obtains an aroma isolate that consists of volatiles in an aqueous solution . Thus, unless the amount of water is small and the subsequent analytical step is tolerant of some water, volatility-based techniques must include some water-removal process. This may be freeze-concentration, the addition of anhydrous salts, or solvent extraction. Distillation is often used to isolate aroma compounds from fat-containing foods. Since fat is not volatile (under isolation conditions), its presence does not prohibit the use of this methodology. 

Solvent extraction is an excellent choice for aroma-compound isolation from foods when applicable. Unfortunately, many foods contain some lipid material, which limits the use of this technique since the lipid components would be extracted along with the aroma compounds. Alcohol-containing foods also present a problem in that the choice solvents (e.g. dichloromethane and diethyl ether) would both extract alcohol from the product, so one obtains a dilute solution of recovered volatiles in ethanol. Ethanol is problematic since it has a high boiling point (relative to the isolated aroma compounds), and in concentration for analysis, a significant proportion of aroma compounds would be lost with the ethanol. As one would expect, the recovery of aroma compounds by solvent extraction is dependent upon the solvent being used, the extraction technique (batch or continuous), and the time and temperature of extraction. 

Adsorption (or absorption) involves passing an aroma-laden liquid (or gas) stream through a bed of adsorbent. Assuming that the adsorbent has a significant affinity for the aroma compounds of interest, they will be adsorbed onto the bed and concentrated. While for analytical purposes the bed is commonly thermally desorbed, it is more likely to be solvent-extracted in this application to recover the trapped volatiles. 

As mentioned in the introduction to this section, there is the opportunity to recover aroma compounds from baking or roasting exhaust gases. The patent literature contains numerous references to the recovery of aroma compounds using this approach, most commonly from cocoa, coffee, or tea processing. Aroma compounds from the roaster exhaust gases are either condensed in cryogenic traps [29-32] or collected on absorbents (e.g. charcoal [33]) and then solvent-extracted to obtain a concentrated aroma extract. The concentrated extract may be used to aromatise a similar product (e.g. soluble coffee) or may be used to flavour other products (e.g. coffee-flavoured ice creams). 

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. 

Because SPME extracts compounds selectively, the response to each compound must be calibrated for quantification. A specific compound can be quantified by using three GC peak area values from solvent injection, static headspace (gas-tight syringe), and SPME. The solvent injection is used to quantify the GC peak area response of a compound. This is used to quantify the amount of the compound in the headspace. The SPME response is then compared to the quantified static headspace extraction. These three stages are necessary because a known gas-phase concentration of most aroma compounds at low levels is not readily produced. A headspace of unknown concentration is thus produced and quantified with the solvent injection. Calibration must be conducted independently for each fiber and must include each compound to be quantified. 

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. 

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. 

Pierre, F.X., Souchon, I. and Marin, M. (2001) Recovery of sulfur aroma compounds using membrane-based solvent-extraction. Journal of Membrane Science, 187, 239. 

Bocquet, S., Viladomat, E.G., Nova, C.M., Sanchez, J., Athes, V. and Souchon, I. (2006) Membrane-based solvent extraction of aroma compounds Choice of configurations of hollow fiber modules based on experiments and simulation. Journal of Membrane Science, 281, 358. 

The availability of and improvement in membranes has rekindled some interest in dialysis in aroma research. Benkler and Reineccius (19, 20) initially published studies on the use of Nafion (Dupont) membranes for the separation of fat from flavor isolates. This would permit solvent extraction to be used in the isolation of aroma compounds from fat containing foods. Chang and Reineccius (21) later used a continuous tubular counter current flow system to accomplish this fat/aroma separation more efficiently. These membranes can be obtained commercially and have been improved in terms of membrane thickness and purity. While the aroma isolate obtained using this membrane may not perfectly reproduce the aroma being studied, this is an alternate technique for aroma isolation. 

Supercritical CO2 is a particularly good choice in aroma studies since it has an extremely low boiling point and leaves no off-odor residue to interfere in either analytical work or sensory evaluation. The fact that the solvent strength of a supercritical fluid depends on density is an additional factor which may be useful. One can vary solvent properties by changing density, thereby obtaining an effective extraction of a broad range of aroma compounds. 

Reaction of an aqueous solution of cystine with thiamin, glutamate, and ascorbic acid produces a complex mixture of compounds with an overall flavor resembling that of roasted meat. The reaction was carried out at 120 C for 0.5h at pH 5.0 in a closed system. The aroma compounds were isolated by means of the simultaneous steam distillation/solvent extraction method. The flavor concentrate was pre-separated by liquid chromatography on silica gel and subsequently analysed by GC and GC/MS. Unknown flavor components were... 

Aroma compounds from vanilla beans have been extracted using several extraction procedures, using alcohols and organic solvents (Galletto and Hoffman, 1978 Dignum et al., 2002), direct thermal desorption (Hartman et al., 1992 Adedeji et al., 1993) and solid-phase microextraction (SPME) (Sostaric etal., 2000), followed by identification of the compounds by gas chromatography-mass spectrometry (GC-MS). 

Silva et al. (2006) showed from sensory analysis that aromatic extracts obtained with a pentane/ether (1/1 v/v) solvent mixture, from cured vanilla beans, provided the flavour most representative of vanilla bean. They found clear differences between the numbers of aroma compounds identified in different organic aroma extracts 65 volatiles were identified in a pentane/diethyl ether extract by GC-MS analysis ether extraction gave 54 volatiles the pentane/dichlo-romethane solvent yielded only 41 volatiles. The volatile compounds identified included... 

Processes for production of ethanol and acetone-butanol-ethanol mixture from fermentation products in membrane contactor devices were presented in Refs. [88,89]. Recovery of butanol from fermentation was reported in Ref. [90]. Use of composite membrane in a membrane reactor to separate and recover valuable biotechnology products was discussed in Refs. [91,92]. A case study on using membrane contactor modules to extract small molecular weight compounds of interest to pharmaceutical industry was shown in Ref. [93]. Extraction of protein and separation of racemic protein mixtures were discussed in Refs. [94,95]. Extractions of ethanol and lactic acid by membrane solvent extraction are reported in Refs. [96,97]. A membrane-based solvent extraction and stripping process was discussed in Ref. [98] for recovery of Phenylalanine. Extraction of aroma compounds from aqueous feed solutions into sunflower oil was investigated in Ref. [99]. 

Rapp, A., MacNamara, K. and Hoffmann, A. (1996) Determination of wine aroma compounds from simple extracts using automated Large Volume Injection with PTV solvent splitting. Gerstel AppNote, 1(1), 1-8. 

Instead by solvent extraction [207], aroma compounds from aqueous media, e.g. fruit juices, can even be separated and enriched by techniques of solid phase micro extraction (SPME), preferably from the headspace [208] , corresponding devices can often be directly connected to GC systems. These techniques provide the complete spec-tmm of the individual compounds of an aroma. As it will normally not be possible and even not necessary to analyse all components of the complex mixture, the separation of its main compounds may demand a multi-dimensional (MD) gas chromatographic system [209[ as displayed in Fig. 6.14 [210[. Examples for the multi-ele-ment/multi-compound isotope analysis by such systems will be given later (6.2.2.4.4, [211[) they can even integrate the identification of the compounds by molecular mass spectrometry and a simultaneous determination of the enantiomer ratios of isomers [210, 211 [. The importance of enantiomer analysis as a tool for authenticity assessment is extensively treated in chapter 6.2.3. 

Simultaneous Steam Distillation/Extraction An elegant apparatus was described by Nickerson and Likens ( 5) for the simultaneous steam distillation and extraction (SDE) of volatile components. This device has become one of the mainstays in the flavor field. In this apparatus, both the aqueous sample and water-immiscible solvent are simultaneously distilled. The steam which contains the aroma chemicals and the organic solvent are condensed together, and the aroma compounds are transferred from the aqueous phase to the organic phase. Typical solvents used are diethyl ether, pentane or a mixture thereof normal extraction times are one to two hours. 

A number of modifications of this apparatus have been proposed. One such improvement is described by Schultz et al, (2 ) and is shown in Figure 10. In this paper, the authors describe the effects of pH, time, pressure and extracting solvent on the recovery of typical aroma compounds. Results showing percent recovery at various initial concentrations of aromas are shown in Table III. 

A micro version of the distillation extraction apparatus has been described by Godefroot t al, (2J). This apparatus uses heavier than water solvents, e.g., methylene chloride or carbon disulfide as the extractant. Because only one milliliter of solvent is used, no further concentration of solvent is required. The authors found 15 minutes distillation/extraction time sufficient for recovery of nonpolar compounds, e.g., mono and sesquiterpenes, while one hour was required for oxygenated and higher boiling compounds. This apparatus was evaluated by Nunez and Bemelmans (28) for low levels of aroma compounds in water. They reported that results were satisfactory for volatile levels greater than 1 ppm. 

If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing GC as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving ftrst the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fi agrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with GC. In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. 

In two publications, Stoffelsma and Pypker (1968) and Stoffelsma et al. (1968) (Polak s Frutal Works, Amersfoort Douwe Egberts, Utrecht, Holland. See Table 4.4) give a list of 158 compounds, 30 being reported for the first time in coffee. Among them figure five esters, five lactones and five furans. The aroma compounds were isolated from a solvent extract of the steam condensate of roasted and ground coffee. They were identified by comparison of their 1R spectra, of their retention times on two GC columns and, in a number of cases of their mass spectra, with those of reference samples. 

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