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.