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

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.

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.”

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.

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.”

• Food Chemistry