## 3 Discussion
Such a finding is consistent with the fact that onion is regarded as one of the major sources of dietary flavonoids [21]. Two major components, quercetin glucoside and quercetin di-glucose accounted for 80% of the total flavonoids in onions. It was reported that quercetin-3,4-O-glucose and quercetin monoglucide (quercetin 4-O-glucose) are the major flavonoids in onion [22]. Flavonoids are considered important factors in the overall antioxidant activity of dietary plants [23]. In particular, quercetin exhibits anti-HIV property and protects LDL cholesterol from oxidation ultimately reducing the risk of cardiovascular diseases [2]. With regards to _A. sativum_ the commercial product examined herein, our data revealed that its chemical composition varied to a large extent when compared to raw garlic (Figure 1), mostly attributedto the presence of diethyl phthalate. Phthalate esters were detected in several over-the-counter medicines from China [24] and their presence is likely to be derived either from gastrocnemius film coatings, plastic packing materials, or the _Allium_ plant itself. Phthalic acid volatile derivatives are major constituents of _A. atroviolaccum_[18] and the non-volatile acid form i.e. phthalic acid (L**14**) was also identified via UPLC-MS analysis (Table 2). The fact that SPME involves no solvent extraction step rules out solvent contamination during sample preparation especially as it was not detected in any of the other _Allium_ specimens analysed under the same conditions. Drying functional foods is one of the most expensive and critical processes. The cost varies depending on the employed drying method which must generally be of high efficiency and low cost. In this context, the influence of drying on _A. sativum_ and _A. cepa_ metabolism was further assessed using GC/MS and UPLC/MS. Considering that sulphur _Allium_ metabolites are thermolabile, it is expected that they can be transformed in response to the different processing steps [1]. Consistent with such a hypothesis, air-dried and microwave-dried garlic samples analysed using GC/MS were found to group together and separately from freeze dried samples (Figure 2A) suggesting degradation of the sulphur molecules to the same extent. In contrast, a freeze dried garlic sample conducted at a much lower temperatures ca. \(-50\,^{\circ}\)C was more enriched in sulphur compounds namely, diallyl disulphide, diallyl trisulphide, and 3-vinyl-1,2-dithiacyclohex-5-ene (Figure 2B). Such metabolite alteration is likely to affect the garlic flavour and or bioactivity. Some discrepancy in assessing the drying effect on garlic (Figure 2A) versus onion (Figure 2C) was observed from each respective PCA plot derived from the GC/MS dataset. Whereas air-drying produced a pronounced effect on _A. cepa_, microwave-drying retained most of the compounds still present in the freeze dried ones. PCA analysis revealed that air-dried _A. cepa_ exhibited higher 2-acetylpyrrole, methyl pentyl disulphide, and allyl methyl trisulphide levels (Figure 2D). Thiosulphinates in onions are affected by heating, however, to an extent that varies depending upon the processing type [25]. In contrast, examination of the drying effect on both _Allium_ drugs using the UPLC/MS platform revealed that both freeze and microwave-drying retained most of the compounds present in _A. sativum_ (Figure 5A) and _A. cepa_ (Figure 5C) as evident by their closer clustering along the PC1. Two glutamyl peptides namely, \(N\)-\(\gamma\)-glutamyl phenylalanine and \(\gamma\)-glutamyl-\(S\)-allyl thiocysteine were the most representative components in freeze dried and microwave-dried garlic samples (Figure 5B) compared to air-dried ones. These glutamyl peptides are considered to be storage products for nitrogen and sulphur with restricted occurrence in other plants [26]. Therefore, it was concluded that freeze drying followed by microwave-drying is less detrimental when compared to the air-drying method, likely due to the rapidness and reduced time of drying compared to air drying.