Abstract
Plants of the Allium genus produce sulphur compounds that give them a characteristic (alliaceous) flavour and mediate for their medicinal use. In this study, the chemical composition and antimicrobial properties of Allium cepa red cv. and A. sativum in the context of three different drying processes were assessed using metabolomics. Bulbs were dried using either microwave, air drying, or freeze drying and further subjected to chemical analysis of their composition of volatile and non-volatile metabolites. Volatiles were collected using solid phase micro-extraction (SPME) coupled to gas chromatography–mass spectrometry (GC/MS) with 42 identified volatiles including 30 sulphur compounds, four nitriles, three aromatics, and three esters. Profiling of the polar non-volatile metabolites via ultra-performance liquid chromatography coupled to high resolution MS (UPLC/MS) annotated 51 metabolites including dipeptides, flavonoids, phenolic acids, and fatty acids. Major peaks in GC/MS or UPLC/MS contributing to the discrimination between A. sativum and A. cepa red cv. were assigned to sulphur compounds and flavonoids. Whereas sulphur conjugates amounted to the major forms in A. sativum, flavonoids predominated in the chemical composition of A. cepa red cv. With regard to drying impact on Allium metabolites, notable and clear separations among specimens were revealed using principal component analysis (PCA). The PCA scores plot of the UPLC/MS dataset showed closer metabolite composition of microwave dried specimens to freeze dried ones, and distant from air dried bulbs, observed in both A. cepa and A. sativum. Compared to GC/MS, the UPLC/MS derived PCA model was more consistent and better in assessing the impact of drying on Allium metabolism. A phthalate derivative was found exclusively in a commercial garlic preparation via GC/MS, of yet unknown origin. The freeze dried samples of both Allium species exhibited stronger antimicrobial activities compared to dried specimens with A. sativum being in general more active than A. cepa red cv.
Highlights
Onion (Allium cepa L.) and garlic (A. sativum L.) are among the oldest cultivated plants, used for culinary purposes in addition to their therapeutic effects [1]
The present study aims to assess the impact of three drying methods viz., shade-drying, freeze drying, and microwave-drying, on the chemical composition of onion and garlic cloves as analysed via solid-phase microextraction (SPME) coupled to (GC/MS) and in parallel to UPLC/MS
Several sulphur rearrangement products i.e., allyl compounds identified in peaks M1, M6, M10, M14, M16, M28, M29, M32, and M36 along with a cyclic, 3-vinyl-1,2-dithiacyclohex-5-ene (M21) were detected in A. sativum, and likely to have been formed at the high temperature of the gas chromatography–mass spectrometry (GC/MS) injection port
Summary
Onion (Allium cepa L.) and garlic (A. sativum L.) are among the oldest cultivated plants, used for culinary purposes in addition to their therapeutic effects [1]. Interest in garlic cancer chemopreventive effect is based on epidemiological studies showing a decrease of gastric cancer risk proportional to the increase of garlic intake. Such evidence has been related to the ability of garlic to reduce nitrite levels in the gastric tract [1,2]. “thiosulphinates”, typical of Allium and responsible for its characteristic pungent aroma and taste. These metabolites are relatively unstable which warrants the development of analytical methods with which changes in their structure can be monitored i.e., in response to processing methods
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