Aroma compounds sample preparation

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. 

Quantification of aroma compounds using GC and internal reference has long been a problematic issue.81 Detection response factor, peak shape, discrimination phenomenon at the injector port, and, of course, disproportion during sample preparation were more or less unavoidable. Stable isotopes used as internal standards combined with an MS detector have realized reproducible and far more accurate quantification. The major drawback of this method is the tedious process of preparing the isotope-labeled standards. 

To verify whether the volatiles listed in Table 6.32 are actually the key odorants, an aroma model was prepared by using an unripened cheese (UC) as base ]53J. The odorants and in addition the compoimds showing high taste activity values ]54] were quantified in UC and in Swiss cheese ]53]. The differences in the concentration of these compounds in both samples were calculated, and, accordingly, the compounds were dissolved in water and/or sunflower oil and then added to freeze-dried UC. The flavour model obtained agreed in colour, pH, water, protein and fat content with grated Swiss cheese, only the texture was more grainy ]53]. 

The purpose of this presentation is to review procedures for the analysis of volatile compounds generated through biological processes. Numerous techniques have been proposed to separate the volatile chemicals from the nonvolatile materials and water, and to concentrate them. After sample preparation, the complex aroma sample can be separated into its individual components by high resolution gas chromatography and the aroma chemicals then structurally identified by spectral techniques. 

In E. foetidum a total of 13 aroma-active constituents with FD-factors > 9 were detected. These consisted of two n-aldehydes (nos. 2 and 6), seven 2-alkenals (nos. 3, 5, 7, 8, 9, 11, 28), (Z)-3-hexenal (no. 22) and three unknown compounds (nos. 29, 30 and 32)(Table V). All of these compounds were detected in C sativum with the exception of an unknown compound (no. 32) having a spicy, herbaceous note. ( )-2-Dodecenal was by far the predominant aroma component with an FD-factor of 2187. Unknown compound no. 30 had the second highest FD factor (=243). The musty, chlorine-like character of this compound may be important in the overall aroma of E. foetidum herb. Other aroma contributors include compounds with FD factors from 27 to 81, such as nos. 5, 6, 7, 11, 22, and 29. Despite having an FD factor of 27, (Z)-3-hexenal (no. 22) was probably derived via lipoxygenase action during sample preparation and may not be a characteristic aroma component of essential oil of E. foetidum herb. 

Figure 4.8 Set up for sampling volatile organic compounds from the coffee flow. Volatiles were introduced into the dilution lance by a flow created with a vacuum pump and were then diluted 7.5 fold using dried compressed air containing a standard for mass calibration [187]. Reprinted with permission from Sinchez-Lopez, J.A., Zimmermann, R., Yeretzian, C. (2014) Insight into the Time-resolved Extraction of Aroma Compounds during Espresso Coffee Preparation Online Monitoring by PTR-ToE-MS. Anal. Chem. 86 11696 11704. Copyright (2014) American Chemical Society...

The nature of flavor compounds creates challenges for analysis. Aroma compounds must be volatile. They are usually present at very low concentrations in foods. Despite the fact that hundreds of volatile compounds are often present in a food, only a few may be odor-active. Gas chromatography has been an invaluable tool for separation and subsequent identification of volatile compounds. Concentration of flavor chemicals is often necessary since the compounds are usually present at low levels. Some methods of sample preparation are described in this handbook, including solid-phase microextraction (see Chapters 16, 20-22, 30, and 31), sorptive stir bar extraction (Chapter 32), absorption on a porous polymer (Chapters 21, 22, and 27), super-critical CO2 extraction (Chapter 22), simultaneous steam distillation (Chapter 31), accelerated solvent extraction (Chapter 35), simultaneous distillation extraction (Chapters 21 and 31), and direct gas injection with cryofocusing (Chapter 20). Sampling conditions are considered in Chapters 20, 23, and 24, and comparisons of some chemical detector sensitivities are made in Chapters 18, 23, and 27-29. 

A number of the procedures described in Sec. VI will yield a material that is primarily lipid in nature. In addition, many samples available to the researcher are themselves lipids. A few materials that one may encounter are coffee oil, vegetable and nut oils, cocoa butter, lard, butter oil, lipids used for deep fat frying, and lipids used as the solvent for Maillard reaction systems. Such materials can be a relatively rich source of aromatic compounds because aroma compounds are typically lipid soluble. A number of procedures can be used to prepare a sample. In this section we will cover three useful ones. 

The aim of GC-0 techniques in food aroma research is to determine the relative odor potency of compounds present in the aroma extract. This method gives the order of priority for identification and thus indicates the chemical origin of olfactory differences (7). The value of the results obtained by GC-O depends directly on the effort invested in sample preparation and analytical conditions. Analysis of an aroma extract by dilution techniques (AEDA, Charm) combined with static headspace GC-O provides a complete characterization of the qualitative aroma composition of a food. However, this is only the first step in understanding the complex aroma of a food. 

Aroma models prepared on the basis of the quantitative data shown in Table 6.37 agreed very well with the original oil samples (Table 6.36). The similarity scores amounted to 2.6 (oil I) and 2.7 (oil S), respectively. In these experiments [64] an odourless plant oil was used as the solvent for the odorants. Reduction of the aroma model for oil I to only seven odorants (nos. 3, 6, 7, 12, 16, 17, 19) lowered the similarity score to 2.2 but the characteristic overall odour remained was still preserved. In the case of oil S, a mixture containing only odorants nos. 1, 2, 8, 9 and 10 did not differ in the aroma from that of the complete aroma model. This result indicates that the other compounds quantified in oil S (Table 6.37) are not important for the aroma. 

Solid Phase Microextraction (SPME) has become one of the preferred techniques in aroma analysis, offering solvent fi ee, rapid sampling with low cost and easy preparation. Also, it is sensitive, selective and compatible with low detection limits [18]. Placed in the sample headspace, SPME is a non-destructive and non-invasive method to evaluate volatile and semi-volatile compounds. In this sense, the extraction of volatile compounds released from a great number of foods has been carried out by using HS-SPME technique [29, 33],... 

As we saw with the early measurements of Biot, natural sources of chiral material such as lemon oil are complex mixtures. We will see shortly that not only are there differences in the smell of many enantiomers there are also large differences in threshold sensitivity, that is, the concentration in which the substance can be detected by the nose. So you might have a 99.9% pure sample, but the dominant aroma may be from the 0.1% impurity. It wasn t until the midtwentieth century that chemists were able to prepare "pure" compounds of enantiomers and then verify the different smells. One of the first examples of such compounds are P-(—)-carvone and S-(—)-carvone, which were studied by three different research teams and published independently in 1971 [5-7]. The structures of these enantiomers are shown in Figure 4.3. It is important to remember that these two compounds have identical physical properties unless they are being analyzed or reacted with... 

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