Effect of the fruit ripening on the antioxidant capacity and total phenolics content on walnuts and almonds.

Walnuts (Juglans regia L.) and almonds (Prunus amygdalus) are a valuable crop traditionally cultivated for their wood and their largely consumed fruits. Not only dry fruits (nuts and almonds) are used, but also green fruits, shells, kernels, bark, green husks (epicarp) and leaves have been used in both cosmetic and pharmaceutical industry [1]. In recent decades an increasing tendency towards the use of natural substances in place of the synthetic ones has been observed in order to reduce environmental pollution, and natural antioxidants, such as phenolic compounds, are gaining importance as a good alternative to chemical preservatives [2]. Polyphenols are a large and diverse group of phytochemicals in plants, known for their antioxidant activity; several studies demonstrated polyphenols benefits for human health, decreasing the risk of degenerative diseases reducing the oxidative stress, inhibiting macromolecular oxidation and having antimicrobial activity [3]. Different works assessed the total polyphenolic content and demonstrated the potential antioxidant of walnut and almond products, especially fruits but also leaves and liqueurs produced by green fruits, bark and specific compounds like juglone. However, despite the abundant literature about this topic, very little is known about the effects of the ripening stage on the changes in the phenolic content and antioxidant activity in husk, kernel and seed. In the present work, almonds and walnuts were collected manually from no pesticides treated trees located in San Venanzo di Castignano, AP, Italy, on the geographic rid reference longitude 42°57'20.4"N and latitude 13°38'39.2"E. The fruits were harvested periodically from May to October 2015 and polyphenols were immediately extracted with acidified ethanol from the husk, kernel and seed separately. The total phenolics were assayed colorimetrically by means of the Folin-Ciocalteu method and their content was expressed as gallic acid equivalents. The antioxidant power of extracts was evaluate by their ability to scavenge the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical; it was calculated from the decrease in absorbance of the DPPH radical caused by antioxidants, due to the scavenging of the radical by hydrogen donation. The degree of discoloration indicates the scavenging potential of the antioxidant extract and ascorbic acid was used as reference compound. Both fruits extracts were found to be important sources of phenolic antioxidants. The average polyphenol content ranged from 322.5 mg/GAE to 1687.3 mg/GAE in seeds and kernels respectively for walnuts and from 175.1 mg/GAE to 1569.6 mg/GAE in the seeds and green husk respectively for almonds. Generally the highest polyphenol content was observed in any case between the 40th and the 50th day of harvesting. Highest radical scavenging capacity (RSC) was observed in walnuts at the late ripening stages in husks, kernels and seeds: after 75 days from the first harvesting, the seeds showed the greatest RSC inhibiting the 87.7% of the DPPH. In almonds, the same radical scavenging behaviour was also observed at the late stages of fruits ripening and almond kernels displayed the highest antiradical activity being able to scavenge the 90.8% of DPPH. The results showed in this study may be valuable with regard to the utilization of by-products of walnuts and almonds production as a functional food material.