Abstract
Volatile organic compounds (VOCs) in cut onions (Allium cepa L.) were continuously measured by PTR-MS during the first 120 min after cutting. The headspace composition changed rapidly due to the very reactive volatile sulfurous compounds emitted from onion tissue after cell disruption. Mass spectral signals corresponding to propanethial S-oxide (the lachrymatory factor) and breakdown products of this compound dominated 0–10 min after cutting. Subsequently, propanethiol and dipropyl disulfide predominantly appeared, together with traces of thiosulfinates. The concentrations of these compounds reached a maximum at 60 min after cutting. Propanethiol was present in highest concentrations and had an odor activity value 20 times higher than dipropyl disulfide. Thus, propanethiol is suggested to be the main source of the characteristic onion odor. Monitoring the rapid changes of VOCs in the headspace of cut onion necessitates a high time resolution, and PTR-MS is demonstrated to be a very suitable method for monitoring the headspace of freshly cut onions directly after cutting without extraction or pre-concentration.
Highlights
Different species of Allium are used worldwide as a common food or food ingredient
The dilution factor was controlled by the flow rate of the dry zero-air (200 mL/min for Proton-transfer reaction-mass spectrometry (PTR-MS) and 179 mL/min for gas chromatography (GC)-SCD) and the flow rate drawn by the instrument (220 mL/min for PTR-MS) or by a pump through the sample loop (199 mL/min for GC equipped with a sulfur specific detection (GC-SCD)) (Figure 2)
Real time PTR-MS was applied to follow the very rapid development of onion odor emitted from freshly cut onions
Summary
Different species of Allium are used worldwide as a common food or food ingredient. Cut onion (Allium cepa L.) immediately brings tears to our eyes, and during cutting, a distinct onion odor develops. The tears and the odor are caused by very volatile and reactive sulfur compounds released after rupture of the onion cell structure. When onions are cut and the tissue is damaged, chemical reactions occurs giving rise to the characteristic sulfurous note of the onion odor [1,2,3].
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