Abstract
Olive leaves are still considered waste in the oil industry; however, the leaves have a content of oleuropein and other bioactive compounds that gives them great potential to be transformed into by-products. The most appropriate moment along an agronomic cycle (November 2019 to October 2020) has been evaluated to take advantage of this potential of the olive leaves varieties of Picual, Cornicabra and Manzanilla. In addition, factors that affect the content of phenolic compounds such as absolute maximum and minimum temperatures, relative humidity, sunshine hours, rainfall, differential of temperatures and mineral nutrition have been studied. The results show that the pruning season was the best time to take advantage of the olive leaf due to its high oleuropein content, especially in Picual and Manzanilla. The variety was the factor that most affected all the phenolic compounds studied, while the absolute minimum temperature factor notably affected verbascoside. Particular mineral nutrients, such as Mg and Fe, turned out to be most useful for differentiating locations. The content of verbascoside and hydroxytyrosol was also affected by agronomic conditions (location/conventional or ecological).
Highlights
Olive-growing (Olea europaea L.) to produce olives and olive oils generates high amounts of by-products, such as olive stone, and waste
Similar trends to other studies were observed during the different seasons, in winter a decrease in oleuropein content of Picual and Cornicabra was observed, which was similar to that observed by Romero et al (2017) [11] in olive leaves from Picual trees in the south of Spain during the 2012/2013 season, where the oleuropein content decreased from 72.12 mg/g in November to 54.02 mg/g in January
In the autumn the lowest contents were obtained in all varieties, this behavior could be associated with the lower production of young leaves in this season in combination with the degradation of oleuropein due to the decrease in the enzymatic activity of L-phenylalanine ammonia-lyase, which is involved in the metabolism of phenolic compounds in olive trees [13–15]
Summary
Olive-growing (Olea europaea L.) to produce olives and olive oils generates high amounts of by-products, such as olive stone, and waste. Olive leaves are burned or used for animal feed, though this kind of waste is the most susceptible to being transformed into a byproduct due to its composition, being rich in phenolic compounds such as secoroids. This group is represented mainly by oleuropein, which is the majority compound of olive leaf; simple phenolic alcohols, such as hydroxytyrosol and tyrosol; flavonoids, such as apigenin 7-O-glucoside and diosmetin-7-O-glucoside; hydroxycinnamic acid derivatives, such as verbascoside. There is a great potential to obtain aqueous extracts rich in bioactive compounds from olive leaves due to the water-solubility of these compounds, making this waste very interesting for its transformation in the oil mills themselves [6]
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