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Olives har­vested by hand and pressed within 36 hours yield oil which may remain fresh for longer peri­ods of time, accord­ing to new research from Namık Kemal University in Turkey.

Researchers tested six dif­fer­ent har­vest­ing meth­ods, press­ing each set of olives over the course of eight days. They found that olives har­vested by hand and processed within 24 to 36 hours oxi­dized more slowly than those har­vested by machines or over a longer period of time.

The study, which was pub­lished in Tehnicki Glasnik, a Croatian sci­en­tific jour­nal, iden­ti­fied three fac­tors in the result­ing olive oil that helped to deter­mine its over­all qual­ity: vis­cos­ity, ther­mal con­duc­tiv­ity and ther­mal resis­tiv­ity — reli­able pre­dic­tors of levels of lipid oxi­da­tion.

“Olive oil qual­ity and sen­sory prop­er­ties can be affected due to this lipid oxi­da­tion prob­lem,” Türkan Aktaş, a biosys­tems engi­neer who worked on the study, told Olive Oil Times. Oxidation plays a part in turn­ing olive oil rancid, affect­ing both its flavor and smell.

“The increas­ing of vis­cos­ity in olive oil shows the increase of the lipid oxi­da­tion,” he added.

The oxida­tive sta­bil­ity of olive oil was also deter­mined by ther­mal analy­ses of con­duc­tiv­ity and resis­tiv­ity. Thermal con­duc­tiv­ity mea­sures the abil­ity of a sub­stance to con­duct heat, while ther­mal resis­tiv­ity mea­sures the oppo­site.

“The increase of ther­mal con­duc­tiv­ity and decrease of ther­mal resis­tiv­ity shows a low ther­mal sta­bil­ity and an increase of lipid oxi­da­tion in oil,” the researchers wrote in the study.

Olive oils with low levels of ther­mal con­duc­tiv­ity and high levels of ther­mal resis­tiv­ity oxi­dize more slowly.

“The lowest vis­cos­ity and ther­mal con­duc­tiv­ity values and the high­est resis­tiv­ity values were found for oils that were obtained from olives har­vested by hand,” the researchers wrote in the study.

Olives har­vested by machine and beat­ing pole had higher levels of ther­mal con­duc­tiv­ity and lower levels of resis­tiv­ity. Spontaneously dropped olives had the high­est and lowest values of ther­mal con­duc­tiv­ity and resis­tiv­ity, respec­tively.

“Different har­vest­ing meth­ods can cause damage to dif­fer­ing levels and this damage can change the oil qual­ity,” Aktaş said. “The research showed that less bruis­ing occurs on olives that were har­vested by hand […] and less bruis­ing on the olives can pre­vent the oxi­da­tion.”

The olive oil’s vis­cos­ity and con­duc­tiv­ity increased the longer the inter­val between har­vest­ing the olives and press­ing them took place.

“The reason behind this sit­u­a­tion can be explained in the sense that the per­cent­age of fatty acids in oil sam­ples increased with the increase of the wait­ing period for olives after har­vest­ing as a result of oxi­da­tion due to the expo­sure of olives to air for a longer time,” the researchers wrote in the study.

Though the mea­sure­ments were all fairly sim­i­lar for the six dif­fer­ent har­vest­ing meth­ods, spon­ta­neously dropped olives, olives har­vested via beat­ing pole to plat­form and hand-picked olives had the lowest vis­cos­ity values. Olives har­vested by machine had the high­est values.

The study con­cluded that the opti­mum way to make olive oil with the lowest rates of oxi­da­tion may be to har­vest the olives by hand and press them within the first day. However, Aktaş warned that this may be an over­sim­pli­fi­ca­tion and more research is nec­es­sary to fully under­stand these rela­tion­ships.

“More detailed research per­formed on simul­ta­ne­ous mea­sure­ments of vis­cos­ity, ther­mal prop­er­ties and lipid oxi­da­tion analy­ses are required to make better sug­ges­tions to olive grow­ers and oil pro­duc­ers,” he said.

• Tehnicki Glasnik