Development of the gas chromatography/mass spectrometry-based aroma designer capable of modifying volatile chemical compositions in complex odors

Abstract

Many volatile organic compounds (VOCs) are used to produce various commercial products with aromas mimicking natural products. The VOCs responsible for aromas have been identified from many natural products. The current major strategy is to analyze chemical compositions and aroma qualities of individual VOCs using gas chromatography/mass spectrometry (GC/MS) and GC-olfactometry. However, such analyses cannot determine whether candidate VOCs contribute to the characteristic aroma in mixtures of many VOCs. In this study, we developed a GC/MS-based VOC collection/omission system that can modify the VOC compositions of samples easily and rapidly. The system is composed of GC/MS with a switching unit that can change gas flow routes between MS and a VOC collection device. We first applied this system to prepare gas samples for omission tests, and the aroma qualities of VOC mixtures with and without some VOCs were evaluated by panelists. If aroma qualities were different between the 2 samples, the omitted VOCs were likely key odorants. By collecting VOCs in a gas bag attached to the collection device and transferring some VOCs to MS, specific VOCs could be omitted easily from the VOC mixture. The system could prepare omission samples without chemical identification, preparation of each VOC, and laborious techniques for mixing VOCs, thus overcoming the limitations of previous methods of sample preparation. Finally, the system was used to prepare artificial aromas by replacing VOC compositions between different samples for screening of key odorants. In conclusion, the system developed here can improve aroma research by identifying key odorants from natural products.

Keywords: aroma; gas chromatography/mass spectrometry; key odorants; omission test; volatile organic compound.

Fig. 1.

Outline of the developed GC/MS-based VOC collection/omission system. A) The system is composed of a GC autosampler (AS), GC/MS, a heated transfer line (TL), and a VCD. B) A switching device (SD) is installed in GC for change the gas flow routes from the separation column (a) to between MS (b) and VCD (c). C) A schematic image for preparing omission samples and for purifying compounds from 5 volatile organic compounds (a–e) using the system. The first step is to obtain reference TIC in the MS (D, reference) by transferring the 5 compounds to MS. To prepare the reference gas sample for sensory evaluation, all of compounds are collected in a gas bag (GB1) attached to an opened port of the VCD E). To omit compound c from the VOC mixture, it is transferred to the MS and others are collected in GB2. In MS monitoring, compound c is detectable (D, Omit). F) VOCs are collectable in Tenax TA absorbent tubes by attaching them in opened ports of VCD. G) VCD can be also used as a sniffer for sensory evaluation.

Fig. 2.

Peak resolution of VOCs in the developed system. A) MS monitoring in the developed system attaching Tenax TA in the opened port of the VCD detected 14 compounds by connecting the separation column to the MS (Reference), no peak by connecting it to the VCD (Whole), and 10 compounds by transferring the other 4 compounds to the MS (Omission). B) TICs of TD-GC/MS analyses of the Tenax TA tubes corresponding to Reference, Whole, and Omit. Peaks are corresponding to following chemicals. a) pentanoic acid, b) (E)-citral, c) 2-ethyl butanoic acid, d) 2-methyl pentanoic acid, e) 3-methyl pentanoic acid, f) 4-methyl pentanoic acid, g) p-cumic aldehyde, h) perilla aldehyde, i) hexanoic acid, j) 2-methyl hexanoic acid, k) 4-methyl hexanoic acid, l. 2-ethyl hexanoic acid, m) cis-jasmone, n) o-anisaldehyde.

Fig. 3.

Peak recoveries of 87 VOCs in the developed system. Bar graphs show recoveries (mean ± S.E.%) of 87 VOCs in the developed system. Recovery was expressed as 100% if the calculated percentage exceeded 100%. Orange, green, blue, black, and red indicate groups of aldehydes, fatty acids, alcohols/phenols, alkanes, and other compounds with nitrogen or sulfur, respectively. TICs of the VOCs are available in Supplementary Fig. 1.

Fig. 4.

Objective evaluation of omission samples prepared in the developed system by cats. A) Upper box: MS monitoring in the developed system attaching Tenax TA tubes in the opened ports obtained TICs by transferring all VOCs of the silver vine extracts to the MS (Reference) or to the VCD (Whole) or the 13 iridoids and the other VOCs to the MS and to the VCD, respectively (Omission). Lower box: Enlarge TICs of Reference and Omission. B) Upper box: TICs of TD-GC/MS analyses of the Tenax TA tubes corresponding to Reference, Whole, and Omission. (A and B) Lower box: Enlarged TICs of Whole and Omission. A and B) b, and c. candidates of trans-trans iridodial, f. cis-trans nepetalactol, h. isodihydronepetalactone, i. isoiridomyrmecin, k. dihydronepetalactone, l. candidate of isoepiiridomyrmecin, m. isoneonepetalactone, others. unknown candidate iridoids. C) Sensory evaluation in domestic cats presenting toward papers impregnated with all VOCs of silver vine extract and the VOCs eliminated the 13 iridoids. D and E) Duration of sniffing D) and rubbing and rolling E) toward Whole and Omission in 6 cats. P values from Wilcoxon matched-pair test, two-tailed. See also Supplementary Fig. 2.

Fig. 5.

Identification of key aroma compounds of lime using the developed system. A) Reference TICs of the lime and orange essential oils. B) Pattern diagrams of 5 samples obtained from lime and orange essential oils in the developed system. LLLL and OOOO were whole samples containing only lime and orange VOCs, respectively. In OLLL, OOLL, and OOOL, lime VOCs eluted before 12.3 min, 20.0 min, and 30.0 min, respectively, were replaced by orange VOCs eluted at the same retention time. C) TICs of TD-GC/MS analyses of Tenax TA tubes corresponding to LLLL, OLLL, OOLL, OOOL, and OOOO. D) Upper: Pattern diagram of LLLL and 2 omission samples (Omit 1 and 2). Lime VOCs eluted at 12.3–15.0 min and at 15.0–20.0 min were omitted by transferring into the MS in Omit 1 and Omit 2, respectively, and the rest of VOCs were transferred into the VCD. Lower: TICs of Omit 1 and 2 obtained in the GC/MS-VOC collection system. E) Enlarged TICs of the lime essential oil obtained in GC/MS-VCD, in which all VOCs were transferred to the MS (Reference) or 4 terpineols (a. 1-terpineol, b. 4-terpineol, c. β-terpineol, d. α-terpineol) (Omit 3) or lime VOCs eluted between 15 min and 20 min (excluding the 4 terpineols) (Omit 4) were omitted. F) Chemical structures of the 4 terpineols. Characters are corresponding to compounds shown in (E).