Volatile traps, aroma compounds

Methods of fixing the volatile aroma and flavor compounds separately from the instant coffee powder have been developed. The volatile mixture can be mixed with aqueous gelatin or gum arabic and spray dried. The oily droplets of the flavor and aroma compounds are coated with gelatin or gum arabic in a dry lattice. This powder can be mixed in with instant coffee powder and is relatively stable in the presence of air. Emulsification with sugar is also a highly effective way of trapping and preserving coffee volatiles, but is of limited use for instant coffees. 

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. 

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. 

Trap desorption. The choice of the thermal desorption apparatus is critical in order to avoid contamination and to be able to work with aroma compounds in a wide range of retention indices. In all systems, problems can be encountered due to reactive compounds or cold spots within the analyzer. It is recommended that all transfer lines, valves, or surfaces in contact with the volatile compounds be made of an inert material such as fused-silica or deactivated glass-lined stainless steel. Even more ideal are systems that do not have long... 

For volatile trapping, preparation of the traps and conditioning requires -1 hr. Trapping of the aroma compounds requires 15 min. Thermal desorption of the traps followed by GC analysis requires -1 hr. 

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. 

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. 

Distillation, as defined in this chapter, also includes direct thermal analysis techniques. These techniques involve the heating of a food sample in an in-line (i.e., in the carrier gas flow of the (jC) desorber. (jenerally, aroma compounds are thermally desorbed from the food and then cryofocused to enhance chromatographic resolution. This technique has been used for a number of years for the analysis of lipids and was later modified to include aqueous samples [32,33]. Aqueous samples were accommodated by including a water trap after the desorption cell. This general approach has been incorporated into the short path thermal desorption apparatus discussed by Hartman et al. [34] and Grimm et al. [35]. A schematic of this apparatus is shown in Figure 3.5. In the schematic shown, a sample of food is placed in the desorption tube and quickly heated. The volatiles are distilled into the gas flow that carries them into the cooled injection system where they are cryofocused prior to injection into the analytical colunm. 

The volatile aroma compounds, together with some water, are removed by vacuum distillation from an aqueous food suspension. The highly volatile compounds are condensed in an efficiently cooled trap. The organic compounds contained in the distillate are separated from the water by extraction or by adsorption to a hydrophobic matrix and reversed phase chromatography and then prefractionated. 

Table 1 lists volatiles identified in white and black truffle aromas by head-space SPME (lOO-pm PDMS) GC/MS, and Table 2 lists results by purge-and-trap (Tenax) GC/MS. Results obtained by HS-SPME-GC/MS agreed well with those obtained by headspace Tenax adsorption GC/MS for the volatile organic sulfur compounds, and the expected discrimination of the polar or very volatile compounds by HS-SPME was confirmed. Pelusio et al. concluded that HS-SPME-GC/MS is a powerful technique for analysis of volatile organic sulfur compounds in truffle aromas, but because HS-SPME (with PDMS fibers) strongly discriminates more polar and very volatile compounds, it is less suited for quantitative analysis. 

Sampling and Analysis. A frozen slice of bread was cut in pieces and stacked in an enlarged sample flask of an aroma isolation apparatus according to MacLeod and Ames (74). Volatile compounds were trapped on Tenax TA and afterwards thermally desorbed and cold trap injected in a Carlo Erba GC 6000 vega equipped with a Supelcowax 10 capillary column (60 m x 0.25 mm i.d.) and a flame ionisation detector. Similar GC conditions were used for GC-MS identification of volatile compounds by dr. M.A. Posthumus (Dept. Organic Chemistry, VG MM7070F mass spectrometer at 70 eV El, 75). 

The volatile compounds in the atmosphere of cold stored Black Perigord Truffles (Tuber Melanosporum) were adsorbed onto a Tenax trap by means of a vacuum pump. The efficiency of the sampling method was sensorially validated. The volatiles eluted from the trap by heat desorption were analysed by capillary gas chromatography - mass spectrometry. A total of 26 compounds was identified. Their contribution to the final aroma impression was discussed. 

The analysis of natural compounds in foods is also assisted by the use of the purge and trap technique in methods for distinguishing strawberry varieties [100] the aroma of unprocessed foods including gherkin [101], durian [102], garlic [103] and meat [104-107] cheese [108,109] and other dairy products [110] tobacco, tea and coffee [111,112] and peanuts [112]. Food additives including sulphur dioxide [113,114] and food contaminants such as VOCs [115-126], have been recovered by PT, particularly from table-ready foods. Animal [127,128] and plant tissues [129,130] have also been subjected to PT for separation of volatile compounds. 

An example of a DHS application is the determination of aroma-active compounds in bambuu shoots. In this study, compoimds such as p-cresol, methional, 2-heptanoI, acetic acid, ( ,Z)-2,6-nonadienal, linalool, phenyl acetaldehyde, were extracted from the bambuu shoot samples and analysed by GC. The required sample amount was 10 g, and the extraction temperature was 60°C, using a 30 min extraction time. The stripped analytes were first trapped into a cooled adsorbent tube (VOCARB 3000, at 0 °C), and then thermally desorbed to GC. In DTD, the sample amount required for the analysis is typically smaller than in solid head-space (SHS). In the determination volatile components such as camphor, 1,8-cineoIe and 2,3,S,S-tetramethyl-4-methylene-2-cyclopenten-l-one, from Lavandula luisieri, only 10-20 mg of (dry) plant sample was required for the analysis. The volatiles were desorbed fi om the sample under a helium flow and then cryofocused on a Tenax TA trap at -30 °C. The trap was then quickly heated and the desorbed volatiles were transferred directly to the GC column through a heated fused-silica line (85). 

The human sense of taste can detect only four flavors - sweet, sour, bitter, and salty. The remaining flavor notes detected are, in reality, aromas. Therefore, many flavor-producing compounds are volatile, making them amenable to purge and trap GLC. This is particularly applicable to the study of alcoholic beverages, which are perfect examples of a food that relies heavily on both aroma and taste for product acceptance. There are dozens of compounds involved in flavor. Some of those are propanol, ethyl butyrate, isoamyl acetate, ethyl caproate, ethyl caprylate and ethyl caprate. 

Identified by Merritt and Robertson (1966) in their coffee aroma (see Section 5.B), by Ho etal. (1993) in the volatile compounds of a roasted Colombian coffee (headspace trapping with short-path thermal desorption GC/MS), the concentration given is 0.1 ppm. 

Andersen, R.A., T.R. Hamilton-Kemp, P.D. Fleming and D.F. Hilderbrand. 1986. Volatile compounds from vegetative tobacco and wheat obtained by steam distillation and head-space trapping. In Biogenesis of aromas (T.H. Parliment and R. Croteau, eds.). Amer. Chem. Soc. Symp. Series S17i 99-111. 

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