3.2.4.1 Visualization of phenolic compounds
Data visualization is an efficient analytical strategy to make it easier for readers to find hidden features in the data. The circos map plays a tremendous role in visualizing the relationships between multidimensional data which is widely used in the genomes. Recently, this method has been successfully applied to characterize the phytochemical profile of edible oils (Zhang et al., 2020). As shown in Table 3 and Fig. 2, the contents of phenolic compounds differed between cultivars and tree ages. It is clear that the presentation of graph is more intuitive compared with traditional tables. The samples and phenolic compounds were automatically divided into two parts (Fig. 2). Different samples were distributed uniformly in upper half circle, while the lower half circle presented the distribution of phenolic compounds at various intervals. The compounds with long intervals indicated high abundance in the samples. The contents of total phenolic compounds were higher in Coratina oils than in Koroneiki oils of all ages. In the other hands, the contents of polyphenols in oils differed greatly of same variety and the variation trend was the same with that of total tocopherols. The oils from 7-year-old trees contained the highest content of phenolic compounds.
In accordance with published data, secoiridoids (including tyrosol derivatives and hydroxytyrosol derivatives) were the major individual phenolic components in olive oil (Deiana et al., 2019). The proportion ranges of the secoiridoids in total polyphenols were from 70% to 94% in this study. However, the structures of secoiridoids in different samples were different. Interestingly, ligstroside aglycone (Try-EA) in both Coratina and Koroneiki oils from 2-year-old trees showed the highest in secoiridoids while decarboxymethyl oleuropein aglycone (Hy-EDA) was the highest in the remaining samples. Moreover, the diversification of the proportion of Try-EA in samples was opposite to that of the total polyphenol contents. Oils from 2-year-old trees contained the highest proportion of Try-EA, followed by 11-year-old and 7-year-old, but total polyphenol contents in oils from high to low were from 7-year-old, 11-year-old and 2-year-old trees. Owing to the low concentrations, quantity reported of phenolic acids, lignans and flavonoids in different literatures were inconsistent (Deiana et al., 2019; Fuentes et al., 2018). The trend of the amount of secoiridoids was consisting with that of total phenols, and the remaining phenolic components were slightly cultivar-dependent. The proportion of phenolic acids in Koroneiki oils was higher than that in Coratina oils. The contents of phenolic alcohols in Koroneiki and Coratina oils were similar. 11-year-old Coratina oil contained the highest concentration of lignans and the lowest concentration was existed in 7-year-old Coratina oil. The amount of flavonoids was marginally higher in Coratina oils than that in Koroneiki oils.