## 3 Discussion  
Onion (_A. cepa_) and garlic (_A. sativum_) have been used for centuries either as raw vegetables for culinary purposes, or as ingredients in traditional medicine worldwide. Several processing techniques are commonly adopted to preserve food products either to increase their shelf life or to reduce some unpleasant characteristics. Nevertheless, some scientific evidence points to the fact that several biochemical modifications and inter-conversions actually occur during processing steps [1]. The current study presents an untargeted comparative metabolomics approach utilizing SPME-GC/MS and UPLC/MS high-throughput analytical technologies to provide insights onto the effect of drying protocols on the two examined _Allium_ species. One commercial garlic preparation was included in this study as several garlic supplements are marketed worldwide and it is of interest to compare its composition with that of the native drug [19]. Thiosulphinates are the most examined secondary metabolite class found in the _Allium_ species which originate from _S_-alk(en)yl-l-cysteine-_S_-oxide, located in the cytoplasm, through an enzymatic reaction catalysed by allinase, a C-S lyase present in the vacuoles. In fact, thiosulphinates have been found in most _Allium_ species examined so far, although with either qualitative or quantitative differences [2]. Thiosulphinates are unstable metabolites that undergo several rearrangements giving rise to an array of sulphur compounds that still exhibit biological activity including thiosulfonates, di- and tri-sulphur compounds, 2-vinyl-2,4-dihydroxy-1,3-dithii, 3-vinyl-3,4- dihydro-1,2-dithii, and ajoene [2], some of which were detected herein using GC/MS (Table 1). Consistent with our findings, [PERSON] et al. previously reported on sulphur compounds in _Allium_ species analysed using GC/MS and in comparison to HPLC-UV. UPLC/MS was found more suited for sulphur metabolites profiling as substantial degradation of sulphur conjugates i.e., dipeptides occurred during GC/MS analysis (Table 1) [2]. While GC/MS is of great value in analysis of volatiles of moderate thermal stability, thiosulphinates from the _Allium_ species are known to decompose upon heating during GC analysis. For example, [PERSON] et al. showed that bis(1-propenyl)disulphide, a common component of _Allium_ distilled oil, rearranges at 85 \({}^{\circ}\)C to 2-mercapto-3,4-dimethyl-2,3-dihydrothiophene, leading to 3,4-dimethyl-2-thienyl disulphides. It should be noted that in our volatiles collection protocol, SPME was carried out at ambient temperature suggesting that rearrangement of sulphur compounds occurred during the GC/MS analysis of volatiles and not during the collection step. Compared to GC/MS, UPLC/MS presents a better platform for profiling the _Allium_ species and to further assess the drying impact on its metabolome [20]. Not only, was a much broader metabolite class unravelled including flavonoids, fatty acids, and sulphur compounds but additionally several novel metabolites were identified in _Allium_. In particular, UPLC/MS led to the identification of two non-sulphur containing dipeptides namely, _N_-(\(\beta\)-hexosyl)-\(\gamma\)-glutamyl-liqueucine (L9) in _A. cepa_ and _N_-(\(\beta\)-hexosyl)-glutamyl-phenylalanine (L12) in addition to a novel hydroxylated fatty acid identified as 9,12,13-trihydroxy-octadec-7-enoic acid (L39). Furthermore, a sulphated oxylipin tentatively identified as jasmonic acid hydroxy-\(O\)-sulphate (L34) and trimethyl-nonanol sulphate (L42). Both sulphated lipids are reported for the first time in _Allium_ species, suggesting that in _Allium_, occurrence of sulphur compounds is not restricted to peptides. In fact, the full complement of bioactive compound(s) in _Allium_ species has yet to be elucidated, a necessary step to better explain its medicinal use or food properties. Analysis of both _Allium_ species via UPLC/MS (Figure 4A) showed that onion encompasses higher levels of flavonoids compared to garlic.