3.2.4.2 Effects of cultivar and tree age on phenolic compounds
The phenolic compounds are a characteristic category substance in olive
oils due to its antioxidant activity, nutritive value and sensory
characteristics. It is an important index in identifying quality of
olive oils. Furthermore, it has been demonstrated that the antioxidant
efficiency and the taste threshold of different phenolic fractions were
different (Dierkes et al., 2012). ‘Coratina’ and ‘Koroneiki’ are widely
regarded as the olive cultivars with high content of polyphenol (Vossen,
2007). Though the contents of polyphenols in the studied oils were
remarkably lower than those in the same varieties from Mediterranean
regions, the contents were similar with those from non-Mediterranean
regions (Fuentes et al., 2018; Vossen, 2007). Compared with the other
olive cultivars in China, the studied samples contained higher
polyphenol contents (Wang et al., 2018).
According to the published reports, endogenous olive polyphenol oxidase
and peroxidase played a key role in shaping the phenolic profile of
olive oils, particularly for secoiridoid compounds (García-Rodríguez et
al., 2011). The increase of total polyphenols in the oils from
2-year-old trees to 7-year-old trees consisted with the findings carried
out by Ayton et al., who reported that the polyphenol content rose with
tree age (Ayton et al., 2007). The decline of total polyphenols in the
oils from 7-year-old trees to 11-year-old trees may also due to the
state of the trees.
3.3 Antioxidant capacity assay
In terms of antioxidant capacity, there are two approaches for measuring
the antioxidant capacities according to the mechanisms of deactivate
radicals: single electron transfer (SET) and hydrogen atom transfer
(HAT). The DPPH, FRAP and ABTS methods belong to the assays based on the
SET. Both SET and HAT happened in the ORAC test (Fernandez et al.,
2014). The results of antioxidant capacities of samples evaluated by the
four different methods are summarized in Fig.3. Koroneiki oil from
7-year-old trees had the greatest antioxidant capacity evaluated by all
three SET methods while 7-year-old Coratina oil showed the strongest
antioxidant activity in ORAC test. The differences of the experimental
results might be explained by the principles of the methods, as HAT
method contained active oxygen whereas SET method did not. On the other
hand, 2-year-old Koroneiki oil showed the lowest antioxidant capacity in
all experiments.
The correlation between the major chemical compounds and the antioxidant
capacity assessed by four methods of all studies oils is presented in
Table 4. Correlation analysis revealed that the influences of phenolic
compounds to the antioxidant capacity of samples were the most
noticeable. The content of total phenolic compounds was extremely
significantly positively correlated with all of the four antioxidant
capacities. The contents of Hy-EDA and oleuropein aglycone (Hy-EA) were
extremely positively correlated with four kinds of free radical
scavenging abilities, and the content of tyrosol and p-coumaric acid was
extremely negatively correlated with them. In addition, the contents of
hydroxytyrosol and decarboxymethyl ligstroside aglycone (Try-EDA) were
significantly or extremely significantly correlated with all kinds of
antioxidant capacities. Besides, the content of hydroxytyrosol acetate
(Hy-AC) was negatively correlated with DPPH and ORAC antioxidant
capacity, while the content of o-coumaric acid was positively correlated
with ABTS antioxidant capacity. Previous papers have demonstrated that
the structure of phenolic content modulates their antioxidant
activities, o-diphenols show stronger antioxidant properties due to the
formation of intramolecular hydrogen bonds in the detoxification of free
radicals (Carrasco-Pancorbo et al., 2005). As a result, the abilities to
scavenge free radicals of hydroxytyrosol derivatives are stronger than
tyrosol derivatives. Artajo et al. have studied the antioxidant
efficiency of the phenolic fraction and found Hy-EDA and Hy-EA emerged
the greatest antioxidant ability among VOO phenols (Artajo et al.,
2006). This result is suggested to explain the relationship between
individual phenolic compounds and the antioxidant capacities of the
oils.
Tocopherols are other well-known lipid radical scavengers in VOOs.
However, various parameters affect their antioxidant properties. The
content of total tocopherols was extremely positively correlated with
all antioxidant capacities. Among tocopherol components, the content of
α-tocopherol was positively correlated with DPPH and ORAC antioxidant
capacities. The content of β-tocopherol was positively correlated with
DPPH, FRAP and ORAC antioxidant capacities. The content of δ-tocopherol
was positively or extremely positively correlated with DPPH, FRAP and
ABTS antioxidant capacities.
Furthermore, the content of squalene was negatively correlated with FRAP
and ABTS antioxidant capacities, the percentage of palmitic acid was
negatively correlated with the ABTS antioxidant capacity, and UFA/SFA
was positively correlated with ABTS antioxidant capacity. Other fatty
acid and minor compounds showed no effects on the antioxidant capacity
of the oil. Nonetheless, due to insufficient numbers of samples, the
conclusions need further verification.