Chemical and Enzymatic Characterization of Leaves from Spanish Table Olive Cultivars

Abstract

Olive leaves are generated as by-products in the olive industry and contain substances with biological properties that provide health benefits. Although these compounds have been characterized in many leaves from olive cultivars devoted to olive oil extraction, few data are available on leaves from the processing of table olives. In this study, the concentration of polyphenols, triterpenic acids, sugars and enzymatic activities (polyphenol oxidase, peroxidase, β-glucosidase and esterase) were determined in the leaves of the olive tree (Olea europaea L.) of cvs. Aloreña, Cacereña, Empeltre, Hojiblanca, Manzanilla, Verdial, Gordal and Morona. The mean total phenolic content in olive leaves reached 75.58 g/kg fresh weight, and oleuropein was the main polyphenol identified (89.7–96.5%). The main triterpenic acid identified was oleanolic acid, and the main sugar was mannitol, with mean values of 15.83 and 22.31 g/kg, respectively. However, the content of these biocompounds was influenced by the type of cultivar and the orchards of origin. The highest oleuropein content was found in the Manzanilla variety, while the Gordal had the highest triterpene and mannitol content. In particular, the phenolic content could also be affected by endogenous enzymatic activities. High polyphenol oxidase, peroxidase and β-glucosidase activity and low esterase activity were detected, compared to the fruit. Similar to the phenolic compounds, enzymatic activities varied with the harvesting season. The lowest phenolic content corresponded to the highest polyphenol oxidase activity detected during spring. The rest of the enzymatic activities also varied throughout the year, but no common trend was observed.

Olive Leaf Processing for Infusion Purposes

“…The total phenolic concentration in the fresh leaf before dehydration was 13,276 mg/kg DW when it was analyzed immediately after harvesting and 14,575 mg/kg DW when left for 24 h at room temperature. As expected, the main polyphenol in the olive leaf was the bitter compound oleuropein, which constitutes more than 90% of the total phenols [17]. This substance, together with the rest of the phenolic compounds, decreased the concentration after receiving the dehydration treatments.…”

“…However, the concentration of phenolic and triterpenic compounds in leaves depends on several factors, including the origin, olive tree cultivars, climatic conditions, harvesting season, agricultural practices, etc. [2,4,14,16,17]. On the other hand, consumers are today demanding many different types of commercial herbal infusions (tea, peppermint, chamomile, etc.)…”

“…Several treatments for olive leaf dehydration include solar, room temperature, microwave, and freeze drying [20], but studies on infrared drying are not available, which could reduce the drying time and labor for a continuous system. The drying temperature has a great influence on the content of biocompounds and high variability has been observed among the results obtained by previous authors [7,17], although none of them have modeled the drying stage. The final objective of many researchers was to obtain concentrated olive leaf extracts rich in biocompounds, mainly by using ultrasound-assisted, microwave, supercritical fluids, pressurized liquid, and others [6,21,22], but none considered obtaining dried leaves for the preparation of an olive tea; thereby, the optimization of the drying process has not been previously studied.…”