Determination of fresh onion (Allium cepa L.) volatiles by solid phase microextraction combined with gas chromatography-mass spectrometry
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Solid-phase microextraction (SPME) was used for the extraction of the volatiles of yellow onions (Allium cepa L.), where the primary volatiles are produced enzymatically after rupture of the plant cells. The SPME-GC analysis of successive samples at timed intervals provided information comparable with that obtained previously by headspace techniques; however, SPME was more convenient and faster to perform. Moreover, the SPME-GC-MS system employed permitted easy monitoring of the fast changes in the volatile composition. Because ambient temperatures were used in the analysis, the method described should produce only minor artefacts during the absorption and desorption steps of SPME. The most important compound found in the headspace of sliced onions by SPME-GC-MS after 1 min emission of volatiles was thiopropanal S-oxide. Also, some dipropenyl disulphides and propenyl propyl disulphides were identified. After 30 min, most of the thiopropanal-S-oxide disappeared, and other sulphur and non-sulphur compounds appeared in the GC chromatograms. The major constituents were diprop(en)yl disulphides. Similar constituents were also found in the water-slurred onions by headspace and direct SPME.
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