Oleuropein Degradation Kinetics in Olive Leaf and Its Aqueous Extracts

Abstract

Although olives leaves are currently considered a waste material from oil mills, they have great potential to be transformed into by-products due to their high oleuropein content. Oleuropein is a glycoside precursor of hydroxytyrosol, which is the phenolic compound with the highest antioxidant capacity in nature and which is associated with multiple health benefits. For this reason, the demand for oleuropein is growing in the pharmaceutical, cosmetic and food sectors. The objective of this study is to determine the stability of oleuropein in olive leaves from oil mills in solid and aqueous forms under different conditions of temperature, relative humidity and lighting. The results indicate that the degradation of oleuropein conforms well to first-order kinetics. The rate constants at the temperatures tested in the aqueous extracts indicate activation energies from RTl to 80 °C and from 7 °C to 14 °C, as the degradation reactions were different in these ranges. Furthermore, olive leaf powder stored at any temperature with an RH ≥ 57% showed greater stability after six months, which is an encouraging result for the storage and transformation of this waste in oil mills.

Quantitative Determination of the Main Phenolic Compounds, Antioxidant Activity, and Toxicity of Aqueous Extracts of Olive Leaves of Greek and Spanish Genotypes

“…Olive leaves were dried in the dark for seven days at room temperature (22 ± 2 • C) and at ambient relative humidity (45 ± 3%) and were then stored under the same conditions until use according to Martínez-Navarro et al [3]. Dried leaves were ground in a knife mill, sieved (35 mesh), and subjected to microwave extraction at 800 W for 30 s using water as an extractant according to the methodology described by Martínez-Navarro et al [12].…”

“…The interaction of these species with molecules of a lipidic nature produces new radicals, hydroperoxides, and different peroxides, which can interact with biological systems in a clearly cytotoxic way [2]. Many antioxidant compounds are known, but the ones that have the greatest antioxidant activity are gallic acid, followed by hydroxytyrosol [3].…”

Extractability of oleuropein, hydroxytyrosol, tyrosol, verbascoside and flavonoid-derivatives from olive leaves using ohmic heating (a green process for value addition)

“…Particular examples of the values reported in previous studies are: 18.5 mg g −1 using ultrasound-assisted extraction; 22.9 ppm using cloud point extraction based on the salting-out phenomenon; 3.90 mg g −1 in Picual leaf extracts using microwave extraction with water. 46–48…”

Chemical and Enzymatic Characterization of Leaves from Spanish Table Olive Cultivars

“…High β -glucosidase activity is related to a greater transformation of oleuropein into its aglycone, which is still a bitter compound. The presence of esterase enzymes could hydrolyze the ester bond of this molecule and release the hydroxytyrosol molecule (not bitter), a compound with a high antioxidant capacity [ 16 ]. The concentration of hydroxytyrosol in olive fruit, due to esterase activity, increases with the ripening of the fruit [ 17 ].…”

“…As has been commented on above, oleuropein is the major phenolic compound in olive leaves, and it is primarily responsible for the bitterness of the fruits and leaves. The oleuropein chemical structure includes a molecule of elenolic acid linked to hydroxytyrosol by an ester bond, and a molecule of glucose by a glycosidic bond [ 16 ]. Oleuropein can be decomposed under the action of light, acids, bases, and high temperature, and by the action of endogenous enzymes in fruit [ 17 ] and leaves [ 18 ].…”