Aroma analysis, SPME

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. 

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],... 

Schrampf, E. and Leitner, E. (11 A.D.) Monitoring of bacteria causing off-flavours in Bologna type sausage by SPME-GC-MS. pp. 147. Laboratory for Aroma Analysis and Enology (LAAE), University of Zaragoza, Spain. 

Mazidaa MM, SaUehb MM, Osman H (2005) Analysis of volatile aroma compounds of fresh chilli (Capsicum annuum) during stages of maturity using solid phase microextraction (SPME). J Food Comp Anal 18 427 37... 

Gas chromatography/olfactometry (GC/O) based on dilution analysis (e.g., CharmAna-lysis or Aroma Extraction Dilution Analysis) gives an indication of what compounds are most potent in the aroma of foods. The application of SPME to GC/O dilution analysis can be achieved by varying the thickness of the fiber phase and the length of exposure, resulting in various absorbant volumes. 

Sampling volatile analytes from samples having complex matrices usually takes place in the HS-SPME mode. This variant yields decidedly better results in the determination of aroma compounds59 and other volatile components.60 Moreover, HS-SPME prolongs the life of the fiber because it is not in direct contact with the sample. On the other hand, the direct extraction of less volatile compounds from solution is possible using DI-SPME. But in this case, the fiber deteriorates more quickly, increasing the cost of analysis. Headspace sampling is therefore employed whenever possible. 

In the early 90s, a new technique called solid-phase-micro extraction (SPME), was developed (Arthur and Pawliszyn, 1990). The key-part component of the SPME device is a fused silica fiber coated with an adsorbent material such as polydimethylsiloxane (PDMS), polyacrylate (PA) and carbowax (CW), or mixed phases such as polydimethylsiloxane-divinylbenzene (PDMS-DVB), carboxen-polydimethylsiloxane (CAR-PDMS) and carboxen-polydimethyl-siloxane-divinylbenzene (CAR-PDMS-DVB). The sampling can be made either in the headspace (Vas et al., 1998) or in the liquid phase (De la Calle et al., 1996) of the samples. The headspace sampling in wine analyses is mainly useful for quantifying trace compounds with a particular affinity to the fiber phase, not easily measurable with other techniques. Exhaustive overviews on materials used for the extraction-concentration of aroma compounds were published by Ferreira et al. (1996), Eberler (2001), Cabredo-Pinillos et al. (2004) and Nongonierma et al. (2006). Analysis of the volatile compounds is usually performed by gas chromatography (GC) coupled with either a flame ionization (FID) or mass spectrometry (MS) detector. 

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. 

Recently, rotundone was identified as a pepper aroma impact compound in Shiraz grapes (Siebert et al.,2008). Identification was achieved by performing GC-MS analysis of grape juice after purification by solid-phase extraction (SPE) using a styrene-divinylbenzene 500-mg cartridge and elution with n-pentane/ethyl acetate 9 1, followed by solid-phase microextraction (SPME) using a 65-pm polydimethylsilox-ane-divinylbenzene (PDMS/DVB) fiber immersed in the sample for 60 min at 35 °C. J5-Rotundone was used as an internal standard. The structure of the compound is reported in Fig. 4.5. 

More recently, headspace and HS-SPME-GC/MS approaches for analysis of aroma in must and grape extracts were also proposed (Lopez et al., 2004 Prosen et al., 2007 Sanchez-Palomo et al., 2005 Rosillo et al., 1999). 

Bonino, M., Schellino, R., Rizzi, C., Aigotti, R., Delfini, C., and Baiocchi, C. (2003). Aroma compounds of an Italian wine (Ruche) by HS-SPME analysis coupled with GC-ITMS, Food Chem., 80,125-133. 

Ho, C. W. Wan Aida, W. M Maskat, M. Y. Osman, H. Optimization of headspace solid phase microextraction (HS-SPME) for gas chromatography mass spectrometry (GC-MS) analysis of aroma compound in palm sugar (Arenga piimata, . Journal of Food Composition and AtMlysis,29, 19(8), 822-830. 

Sorptive extraction (Solid Phase Micro Extraction [SPME] and Stir Bar extraction) are relatively new techniques for the isolation of food aromas. Pawliszyn s group [41] was the first to develop the SPME method, and they applied it in environmental analysis. Since then, it has become a widely used technique for the analysis of volatiles in foods. Harmon [42] and Marsili [43] have provided a comprehensive review and a critical review, respectively, of this technique. 

Significant positive correlations were found between aroma release of esters and the three parameters of time-intensity for the fruity attribute (see Table 5). Linalol and y-octalactone were less correlated than esters but more than hexenol and mesifuran. These results were in agreement with SPME analysis indeed, the nature of fat seemed to have more influence on release of esters than of linalol and y-octalactone and than of hexenol and mesifuran. Diacetyl behaved differently than the other aroma compounds because it was the only hydrophilic compound. It was also the only one that was released in greater quantity from animal fat than from vegetable fats used in this study. 

Three commonly used extraction techniques for the analysis of aroma are simultaneous distillation/extraction (SDE), dynamic headspace adsorption on Tenax TA (Buchem N.V., Apeldoorn, The Netherlands), and solid-phase microextraction (SPME) [1,2]. All of these techniques have positive aspects and drawbacks, and these are described. In SDE the sample is boiled for 1 to 2 hr and so precooking may not be necessary, although the meat is usually minced to maximize surface area for the extraction process. The other techniques can be used to examine either a chopped or a whole piece of cooked meat. 

Figure 17 represents the analysis of a sample of curry powder that was thought to be lacking one of its spice components. Because the spice was known to contain a unique aroma chemical, it was an easy matter to transfer a small amount of the curry to a vial, perform a headspace extraction, and determine whether the spice had been added. The complete analysis required less than one hour from the time the sample was received in the laboratory. As a quality control measure, SPME can have a significant impact on the analysis of raw materials and finished products. 

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. 

Tenax and would be able to collect aroma chemicals with a wide range of polarities. Figure 9 shows the effect of collection time on the aroma chemicals collected. The Zenith trap is capable of collecting sufficient quantities of headspace material in around 5 minutes for quantitative GC/MS analysis. The Zenith trap combines the advantages of SPME and dynamic headspace. It overcomes the problems of fiber fragility, long extraction times, and the need to analyze and combine the results of several SPME fibers. 

SPME has become a valuable alternative to solvent extraction, purge-and-trap (dynamic), and static headspace methods (1 ). This is true for the analysis of flavors, fragrances, food aromas, and biological systems as is evidenced by... 

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