Harvesting by Hand Leads to Lower Levels of Oxidation, Study Finds

Researchers found that olives harvested by hand and processed with the first 36 hours had lower levels of oxidation.

By Daniel Dawson
Jul. 23, 2018 10:35 UTC

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.

Different har­vest­ing meth­ods can cause dam­age to dif­fer­ing lev­els and this dam­age can change the oil qual­ity.- Türkan Aktaş, Researcher

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.


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