ABSTRACT
Introduction ...................................................................................................... 94 Sorptive extraction ........................................................................................... 95
Commercial sample-enrichment techniques ........................................... 95 Novel multichannel sample-enrichment devices ................................... 96
Multichannel silicone (PDMS) rubber traps ....................................... 97 High-capacity headspace sorptive extraction of aroma compounds from milk followed by TDS-CIS-GC-FID ..................................................... 98
Sample enrichment ..................................................................................... 98 Chemical standards ..................................................................................... 99 Instrumentation ........................................................................................... 99
Novel heart-cutting fraction collection GC-based methods .................... 100 Capturing of single compounds and their combinations by off- line heart-cut GCFC ............................................................................ 100
Off-line olfactory evaluation of single compounds and their combinations ......................................................................................... 102
Slow release of the aroma of single compounds captured by GCFC on individual secondary MCTs ................................................... 105 Slow release of the total milk aroma pro€le captured by purge- and-trap on primary MCTs .......................................................... 105 Slow release of heart-cuts of combinations of single compounds captured by GCFC on individual secondary MCTs ............................. 105
Summary ......................................................................................................... 107 Acknowledgments ......................................................................................... 107 References ........................................................................................................ 107
Introduction Consumers consider the cooked cabbage-like, sulfurous and stale notes imparted by the ultrahigh-temperature (UHT) treatment of packaged long-life milk undesirable. (Contarini et al. 1997; Nursten 1997; Perkins et al. 2005; Simon and Hansen 2001; Vazquez-Landaverde et al. 2005; VazquezLandaverde, Torres, and Qian 2006). Analytical methods that have been used to study the aroma of dairy products are gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GCO) (Bendall 2001; D’Acampora Zellner et al. 2008; Friedrich and Acree 1998; Mahajan, Goddik, and Qian 2004; Moio et al. 1994; Moio and Addeo 1998; Moio, Piombino, and Addeo 2000; Qian, Nelson, and Bloomer 2002; Van Aardt et al. 2005). GCO is traditionally used to identify individual odor active gas chromatographic fractions; a human record of aroma perception, in real time, of the GC efuent at the olfactometer outlet. The rapid elution of compounds is problematic in terms of recalling appropriate odor descriptors. GCO requires utmost concentration and can cause nose fatigue. Typically one to two trained evaluators perform the sniffing. However, a group of evaluators is required for reliable GCO analysis, necessitating numerous analyses and multiple gas chromatographs equipped with sniff ports (Van Ruth 2001). Instrumental analyses are not always performed on the aroma-relevant compounds but also on nonodorous compounds that may include hazardous chemicals (Ampuero and Bosset 2003). Coelution of compounds is a common occurrence in separating complex mixtures, thus the evaluator may not realize that composite peaks, instead of pure compounds, are sniffed. Potential synergistic effects cannot be observed where single compounds are evaluated over time. Many synergistic effects are known to occur between single compounds in complex food matrices (Herrmann et al. 2010). Combinations of single substances can produce enhancing or masking interactive effects. Perceived synergistic sensory effects can arise not only from a blend of similar volatiles, but also from a blend of chemically unrelated compounds; for example, the sensory threshold for vanillin is lower in the presence of oak lactones (Perez-Coello, Sanz, and Cabezudo 1997). Interestingly, the end result of combining single substances in complex food matrices may be the emergence of a strikingly different sensory perception, completely unrelated to that of the individual compounds alone. Herrmann et al. (2010) evaluated the aging of beer and reported a change in the sensory perception of components when in combination. Here, E-2-nonenal was described by tasters as cardboard-like when the single substance was evaluated, whereas (E)-2-(Z)-6-nonadienal was described as cucumber-like. However, the combined effect of the two compounds produced a sweet fruity avor sensory perception distinctly
different from the single substances alone. This new sensory perception appears related to both the absolute and relative concentrations of the two compounds. Because the generation of new odors by addition of individual compounds is not yet fully understood, it is hard to determine how individual compounds of a product relate to the perception of its overall aroma when using traditional instrumental techniques (Ampuero and Bosset 2003).
