Olive Leaves Have Higher Levels of Phenols in the Summer, Research Shows

Summer harvesting might provide the best results for the growing number of farmers and companies investing in olive leaf-derived products.
Dec. 28, 2020
Paolo DeAndreis

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Scientists have shown that the chem­i­cal com­po­si­tion of olive leaves may depend on the time of year in which they are har­vested.

Researchers in Brazil focused their study on three well-known cul­ti­vars in a sin­gle orchard, mea­sur­ing their con­tent of polyphe­nols and other char­ac­ter­is­tics.

The sea­sons have a strong influ­ence on the con­tent of bioac­tive com­pounds, with higher con­tents being observed for most com­pounds in the leaves in the sum­mer.- Researchers, Federal University of Pelotas

Their find­ings show that some olive vari­eties might offer a sig­nif­i­cantly higher antiox­i­dant and phe­no­lic pro­file than oth­ers, and that sum­mer har­vest­ing might pro­vide the best results for the grow­ing num­ber of farm­ers and com­pa­nies invest­ing in olive leaf-derived prod­ucts.

We inves­ti­gated Arbequina, Manzanilla and Picual olive trees,” Alexandre Lorini and Deborah Murowaniecki Otero, two of the authors of the study, told Olive Oil Times. We spent approx­i­mately one year col­lect­ing the sam­ples from the trees, and one more year car­ry­ing out the analy­sis, both in the lab­o­ra­tory and with sta­tis­ti­cal analy­sis.”

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The sci­en­tists col­lected sam­ples of leaves from 50 trees of each of the three olive vari­eties at the end of each sea­son. They mon­i­tored the daily tem­per­a­tures of the orchard located in the south­ern­most Brazilian region of Rio Grande do Sul as well as the solar radi­a­tion to which the trees were sub­jected through­out the year. All exam­ined trees were grown under the same agro­nomic and envi­ron­men­tal con­di­tions, in the same olive groves on a pri­vate prop­erty.

The cli­mate that the trees are exposed to varies accord­ing to the sea­sons,” the two researchers said. In the hot sea­sons, we have tem­per­a­tures that might reach around 104 °F (40 ºC) in addi­tion to an aver­age solar radi­a­tion above 15 mega­joules per square meter each day, while in the cold sea­sons tem­per­a­tures can fall to 32 °F (0 ºC) and the solar radi­a­tion mea­sure below 15 mega­joules per square meter each day.”

In the dis­tinct con­di­tions of the dif­fer­ent sea­sons, sci­en­tists col­lected about five kilo­grams of leaves that were then gath­ered in a spe­cific reser­voir and taken to the chro­matog­ra­phy lab­o­ra­tory of the Federal University of Pelotas. The mate­r­ial was crushed and ground in a mill with the aid of liq­uid nitro­gen, stored in poly­eth­yl­ene pack­ag­ing and kept at −112 °F ( – 80 ºC).

The goal of the researchers has been to quan­tify the phe­no­lic com­pounds (api­genin, hydrox­y­ben­zoic acid, kaempferol, lute­olin, oleu­ropein, quercetin, rutin and tyrosol), deter­mine the total flavonoids, mea­sure the hydrolyz­able tan­nins and the con­densed tan­nin con­tent, as well as the total carotenoids and chloro­phylls.

We real­ize in this work that the cli­matic vari­a­tion that occurs in the sea­sons of the year in the place where the trees are planted affects the meta­bolic com­po­si­tion,” the pair of researchers said. The same effect does not always occur in all cul­ti­vars, which shows that each cul­ti­var can adapt in dif­fer­ent ways.”

Hot sea­sons influ­ence the syn­the­sis of polar com­pounds such as phe­no­lic acids and flavonoids in all the cul­ti­vars we tested,” they added. The non­po­lar com­pounds, such as carotenoids and chloro­phylls, are affected in dif­fer­ent ways, where in the cul­ti­var Arbequina there was greater syn­the­sis in the change to cold sea­sons, while in Manzanilla and Picual, there was an increase in the change to the hot sea­sons.”

The results show that for the total of phe­no­lic com­pounds, flavonoids and hydrolyz­able tan­nins in the leaves, there is no inter­ac­tion between the cul­ti­vars and the sea­sons. Thus, the sta­tis­tics of the sep­a­rate vari­ables showed that leaves of the cul­ti­var Manzanilla have higher con­cen­tra­tions of phe­no­lic com­pounds and hydrolyz­able tan­nins.

Still, when eval­u­at­ing only the sea­sons and their con­tri­bu­tion to the increase of cer­tain bioac­tive com­pounds, it was noticed that win­ter was the sea­son dur­ing which the low­est syn­the­sis of phe­no­lic com­pounds occurred.

In the sum­mer, it was pos­si­ble to observe a greater syn­the­sis of these com­pounds, which indi­cates that the tem­per­a­ture has a direct influ­ence on the mech­a­nism, where high tem­per­a­tures can result in the acti­va­tion of pro­tec­tive mech­a­nisms of the olive tree, and as a con­se­quence induce greater syn­the­sis of these com­pounds,” the researchers wrote in the study.

See Also: Panel Rejects Health Claim for Olive Leaf Extract

As for the con­tents of the inves­ti­gated phe­no­lic com­pounds, researchers noted that the Manzanilla leaves showed higher amounts of api­genin, hydrox­y­ben­zoic acid and tyrosol through­out the year.

The same was observed for the leaves of the Arbequina cul­ti­var, regard­ing the con­tents of kaempferol and lute­olin. The high­est oleu­ropein and rutin con­tents were observed in all sea­sons in the leaves of the cul­ti­var Picual.

The most antiox­i­dant activ­ity has been expressed by the extract of the Manzanilla leaves while indi­vid­ual phe­no­lics under­went sig­nif­i­cant inter­ac­tion between cul­ti­vars and sea­sons show­ing, that the sea­sons have a strong influ­ence on the con­tent of bioac­tive com­pounds, with higher con­tents being observed for most com­pounds in the leaves in the sum­mer,” the researchers wrote.

Finally, the hypoth­e­sis tested was proven, reveal­ing that the cul­ti­vars cho­sen for plant­ing, as well as the exist­ing cli­matic changes with the sea­son changes, affect the meta­bolic pro­file of olive leaves (Olea euro­peae L.),” they added.

While the results offer hints to local grow­ers, researchers do not know if those results could be true else­where, includ­ing in the Mediterranean basin, the evo­lu­tion­ary home of the olive tree.

We do not know the Mediterranean cli­mate very well, but in our read­ings, we saw that there is a dif­fer­ence between sum­mer and win­ter, just like in the region where our exper­i­ment was car­ried out,” the sci­en­tists said. Taking this into account, we believe that the results may be sim­i­lar, but more research is needed. We believe that this type of study would be very inter­est­ing.”

The research, explained the two authors, may offer new insights for con­sumers of olive leaf extracts and prod­ucts as well as to the phar­ma­ceu­ti­cal indus­try.

However, a sec­ond aspect, noted the sci­en­tists, is to show the oil extrac­tion indus­try that in the sea­son that they col­lect the olives, we have a cer­tain con­tent of com­pounds and that they can take advan­tage of the leaves to enrich their oils.”


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