Methyl sensory property

Alkylcarbonic acids have been separated into their enantiomers without any deriva-tization and their sequence of elution was assigned by co-injection with enantio pure references [17]. Latest results on stereoselective flavour evaluation revealed characteristic sensory properties for all the enantiomers of 2-alkylbranched acids, esters and corresponding alcohols. Tremendous differences between the mirror images of 2-methylbutanoic acid have been found. While the R-enantiomer exhibits a penetrating, cheesy-sweaty odour, the S-enantiomer emits a pleasant sweet and fine fruity note [87, 88]. All commercially available homologues of 2-methylbutanoic acid esters and 2-methylbutyl acetate are simultaneously stereoanalyzed, using heptakis (2,3-di-O-methyl-6-0-tert-butyldimethylsilyl)-P-cyclodextrin (DIME-P-CD) in PS 268 as the chiral stationary phase [88] (Eig. 6.33, Table 6.19). 

Several alkyl-branched fatty acids were found to be powerful and characteristic food flavors. 2-Methylbutyric acid is an important constituent of the aroma of cranberry Vaccinium vitis-idaea L.) (77). 3-Methyl-valeric acid and isovaleric acid were identified in tobacco leaves and found to contribute to the distinct sensory properties of Turkish tobacco smoke 590). Isovaleric acid, which has the lowest flavor threshold (0.7 ppm) 522) of all saturated fatty acids, is regarded as an essential flavor component of Limburger cheese (575). ( )-3-Methyl-2-hexenoic acid was recognized as the malodorous component of schizophrenics sweat (579). The undesirable odor of mutton can be attributed to branched-chain and unsaturated fatty acids having 8 to 10 carbon atoms. The 4-methyl-branched acids and in particular 4-methyloctanoic and 4-methyl-... 

As an example, 4-methyl-5-propyl oxazole has been characterized as having a green vegetable aroma. However, oxazoles which have a 4 or 5 carbon length chain on the oxazole ring and no alkyl group on carbon-2 or 4 have distinct bacon-fatty notes (e.g., 5-butyl oxazole). When a methyl or ethyl group is substituted on carbon-2 (e.g., 2-ethyl-5-butyl oxazole), the fatty aroma is reduced and sweet-floral aromas become more characteristic. The sweet-floral character is further enhanced by additional methyl or ethyl substitution on carbon-4 [69]. The oxazolines tend to have a wide variety of sensory properties. 2-isopropyl-4,5,5-trimethyl-3-oxazoline has a rum-like note while 2-isopropyl-4,5-diethyl-3-oxazoline has a typical cocoa aroma. 

Fig. 3.41. Fungal degradation of triacylglycerols to methyl ketones (according to Kinsella and Hwang, 1976) Table 3.47. Sensory properties of methyl ketones...

The influence of the sensitivity of the assessors on AEDA has been studied [11], with the result that the differences in the FD factors determined by a group of six panellists amount to not more than two dilution steps (e.g. 64 and 256), implying that the key odorants in a given extract will undoubtedly be detected. However, to avoid falsification of the result by anosmia, AEDA of a sample should be independently performed by at least two assessors. As detailed in [6], odour threshold values of odorants can be determined by AEDA using a sensory internal standard, e.g. ( )-2-decenal. However, as shown in Table 16.6 these odour threshold values may vary by several orders of magnitude [8] owing to different properties of the stationary phases. Consequently, such effects will also influence the results of dilution experiments. Indeed, different FD factors were determined for 2-methyl-3-furanthiol on the stationary phases SE-54 and FFAP 2 and 2 , respectively. In contrast, 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone showed higher FD factors on FFAP than on SE-54 2 and 2, respectively. Consequently, FD factors should be determined on suitable GC capillaries [8]. However, the best method to overcome the limitations of GC-O and the dilution experiment is a sensory study of aroma models (Sect. 16.6.3). 

Organoleptic Properties. Stereochemical differences of enantiomeric excipients may influence perception by sensory organs. Kutti [17] reported as early as 1886 that the interaction of stereoisomer with chiral receptors led to chiral discrimination as a consequence of the formation of diastereomers. He observed that the dextrorotatory asparagine has a sweet taste whereas the levorotatory form is tasteless. Greenstein and Winitz [18] and Solms et al. [19] reported such differences for many amino acids. Shallenberger et al. [20] reported that for some monosaccharides, both isomers have similar sweetness. In contrast, aspartame (A-aspartylalanine methyl ester) is marketed as the l,l isomer because it is more than 100 times as sweet as sucrose. However, the l,d diastereomer of aspartame is bitter [11], It should be noted that the individual differences of perception of these properties could vary. 

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