The effect of exogenous abscisic acid spray on phenolics and scavenging free radical activity of olives during storage

Abstract

The olive tree (Oleo europaea) is largely popular for the production of both oil and table olives, which are positively considered in nutrition studies. Apart from fatty acids, olives also contain some minor components such as phenolics which are regarded as highly beneficial to the health because of their antioxidant properties. Phenolics occurring in olive pulp are also present in processing by-products. However, phenolics decrease drastically during the storage in such a way that postharvest techniques guaranteeing oil quality are essential when oil processing is delayed. In the present study we investigated abscisic acid (ABA) spray treatment as a postharvest procedure to minimize losses of phenolics during olive storage. With this purpose, we evaluated the effect of ABA on olive pulps and stones on days 7, 15 and 30 after the treatment. The total phenolic contents were measured in ABA treated olives by the Folin-Ciocalteu method whereas the 1,1-diphenyl-2-picrilhydrazyl free radical (DPPH) scavenging assay was used to evaluate the scavenging free radical capacity The values obtained were compared with those of the pulps and the stones obtained from untreated olives. The results indicate that ABA treated olive pulps had higher total phenolic content and DPPH scavenging capacity as compared with untreated samples. A similar effect was also observed for the stones, although the increases in both total phenolic content and scavenging free radical activity were not as pronounced as in the pulps. For both olive pulps and stones, exogenous ABA spray was more effective after prolonged storage periods. Concerning phenolics, we focused our study on oleuropein and hydroxytyrosol for their abundance in olive fruit and their pharmacological properties. The effect of ABA on contents of oleuropein and hyrdroytyrosol was evaluated by HPLC-UV. The identification was carried out on the basis of retention times and UV spectra while calibration curves were performed for their quantification. Oleuropein and hydroxytyrosol exerted significantly (p < 0.05) higher contents in olive pulps and stones after the ABA treatment. In line with the overall results on phenolics and scavenging free radical activity, the increments were more marked for the pulps than for the stones and particularly after longer storage periods. These findings indicate that the postharvest ABA spray treatment might be a promising approach to avoid the natural losses of the phenolic content and antioxidant properties of olive fruits during the storage. The postharvest method proposed allows olive pulps and stones with improved antioxidant properties to be obtained. Since the stones are a residue from oil processing, ABA treated olive stones are proposed as a high added value olive by-product. These results can be interesting to the olive oil industry, in particular when delay in oil processing occurs and olives have to be stored.