## 3 Discussion
### Asparagus Metabolome and Differences in Chemical Compositions of Green and White Spears  
This study is the first to compare green and white asparagus volatile profiles in a holistic way revealing greater detail of their biochemical composition. Some of the identified aldehydes have been detected earlier in asparagus, such as pentanal, hexanal, (_E_)-2-hexenal, heptanal, and octanal [12, 25, 26]. Other aldehydes detected in this study are reported in raw asparagus for the first time. Some of these, such as phenylacetaldehyde (benzeneacetaldehyde) and 3-methylbutanal are known vegetable VOCs, and their aroma attributes are green, fresh, floral, and fruity [27](www.foodb.ca, accessed on 13 September 2021). Among the unambiguously identified alcohols and ketones, 1-octen-3-ol, 1-penten-3-ol, 1-pentanol, and 2-butanone have been reported in (cooked) asparagus [8, 10, 12, 13, 25]. Furthermore, most of the monoterpenes and benzenoids, which are here reported for the first time in asparagus, are also already-known vegetable/fruit VOCs. For instance, styrene and limonene have been detected in blackcurant, cauliflower, apple, cabbage, celery, carrot, etc., and their aroma attributes contain several fruity, fresh, and floral notes [27]. Interestingly, in this study, a number of VOCs that have previously been reported in other asparagus materials (e.g., cooked white spears) were not detected. A few examples are 2,3-butanedione, 2,3-pentanedione, or 2,5-dimethylpyrazine [1]. These are volatile  
Figure 5: Relative abundance of selected identified volatiles in shoot sections of the asparagus varieties in their green (5 sections) and white (6 section) forms. Colours are based on the asparagus type. The numbers on the x axis correspond to the relevant spear sections as indicated on the right of this figure; each of the upper 3 sections represents 2 cm of spear, while the following 3 sections are each 4 cm in length. Green spears were shorter than the white as demonstrated in Figure 1, thus there were fewer green spear sections available for analysis. compounds usually formed via heat-induced reactions [28] and so this might explain their absence in the raw years. With regard to the non-volatiles, a large number of the identified compounds have been previously reported in asparagus. These include compounds such as rutin [22], 3-O-feruloylquinic acid [29], asparagine [30], protodioscin [31], as well as asparagusic acid [9] and its ester isomers [32]. However, it should be emphasised that here, when using a non-targeted LC-MS approach without (semi)automated MS/MS spectral analysis, the majority of the metabolites detected could not readily be identified, indicating that there is a wealth of chemical information still waiting to be discovered. One highlight of the study presented here is the list of discriminatory secondary metabolites for green and white asparagus, which was validated after analyses in two consecutive seasons (Table 1). In particular, some of the compounds that characterised the white spears were 1,2-dimethoxybenzene, (_E,E_)-2,4-decadien, shatavarin and protodioscin, while green spears were distinguished by significantly higher levels of 1-penten-3-ol, 3-pentanone, octanal, rutin and most of the identified phenolics (Tables 1, S3 and S6, Figure S3). These differences might primarily arise due to their growing environment and the contrasting developmental processes which the two types follow. Green asparagus grows above the ground, while white asparagus stays below ground and often under impermeable plastic foil to avoid any exposure to sunlight. The distinct chemical differences between the two asparagus types (Figures 3 and 4, Table 1) imply that biochemical routes are differentially active in green and white shoots. These contrasts may be directly connected to the light/dark conditions of growth as we are comparing photosynthetic and non-photosynthetic systems where many differences might be anticipated.