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.