Even Dark Glass Bottles Leave EVOO Susceptible to Oxidation, Study Finds

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Under cer­tain con­di­tions, glass bot­tles may hin­der extra vir­gin olive oil’s resilience to oxi­da­tion, reduce its healthy prop­er­ties and mod­ify its fla­vor, accord­ing to new research out of Italy.

In the study, the researchers observed what hap­pened to extra vir­gin olive oil over time when stored inside dif­fer­ent kinds of bot­tles and pack­ages.

When sub­jected to super­mar­ket-like con­di­tions for more than a few weeks, both green glass bot­tles and dark­ened bot­tles did not ade­quately pro­tect their high-qual­ity con­tents.

“The time has come to put some more effort into extra vir­gin olive oil pack­ag­ing and pro­tec­tion while on sale or when shipped over­seas,” said Maurizio Servili, co-author of the research and food sci­ence and tech­nol­ogy pro­fes­sor at the University of Perugia.

In the study, the researchers exposed sev­eral sam­ples of extra vir­gin olive oil to dif­fer­ent con­di­tions over time. The sam­ples were bot­tled in green glass con­tain­ers, ultra­vi­o­let-grade absorb­ing glass and mul­ti­layer plas­tic-coated paper­board alu­minum foil.

They mea­sured the chem­i­cal changes that took place within each oil, focus­ing on the mod­i­fi­ca­tions in the quan­tity and qual­ity of its phe­nols, polyphe­nols, volatile com­pounds and sen­sory attrib­utes.

The researchers also mea­sured the spe­cific alter­ations caused by chloro­phyll.

“Chlorophyll is among the extra vir­gin olive oil com­po­nents and plays a big role in mod­i­fy­ing the qual­ity pro­file of a bot­tle over time,” Servili told Olive Oil Times. ​“It does not hap­pen with any other oil because it is prac­ti­cally absent in most other fats such as sun­flower or soy­beans seed oils.”

“Nowadays, it is unusual, but it once hap­pened quite often, to spot on a super­mar­ket shelf an extra vir­gin olive oil bot­tle turned some­what orange,” he added, explain­ing that this hap­pened because tra­di­tional glass bot­tles let the light through and trig­gered chloro­phyll.

The researchers exposed each of the sam­ples to 500 luxes for 12 hours each day. The sam­ples then spent the remain­ing 12 hours in the dark, con­di­tions sim­i­lar to those found in many super­mar­kets.

“Not only that,” Servili added, ​“but we have also enacted oper­a­tions that many food retail­ers do, such as peri­od­i­cally mov­ing the bot­tles on the shelves so that light does not directly impact on the same glass.”

After 150 days in those con­di­tions, the extra vir­gin olive oil pack­aged in dark green glass was dam­aged by the photo-oxi­da­tion.

“We began find­ing prod­ucts derived from an ongo­ing ran­cid­ity process,” Servili said. ​“The well-known K270 para­me­ter showed that between 160 and 180 days into the exper­i­ment, some sam­ple pro­files reached the lim­its pro­vided by the law to define extra vir­gin olive oil.”

After 240 days of expo­sure to those con­di­tions, all sam­ples in glass could no longer be clas­si­fied as extra vir­gin. However, the obser­va­tion sam­ples kept in absolute dark­ness showed that after two years of stor­age in glass, the con­tents had under­gone just minor changes.

“We used medium-qual­ity extra vir­gin olive oils usu­ally found in a super­mar­ket. None of the prod­ucts used had at the begin­ning an acid­ity level above 0.5 per­cent,” Servili said. ​“Phenols in the sam­ples were between 700 and 720 mil­ligrams per kilo­gram at the begin­ning.”

“But after 300 days, the olive oil in the green glass had lost 96 per­cent of its phe­nols while 87 per­cent was lost with dark­ened glass bot­tles,” he added.

In those same con­di­tions, mul­ti­layer plas­tic-coated paper­board alu­minum foil fared much bet­ter, with only a 25 per­cent aver­age reduc­tion of the bio-active phe­nols.

While the effects of tem­per­a­ture stress on extra vir­gin olive oil are well known, the study high­lights the pos­si­bly more sig­nif­i­cant impact of light.

“While the tem­per­a­ture may well affect the oil’s qual­ity and oxi­da­tion, the impact of light on extra vir­gin olive oil is crit­i­cal because of the chloro­phyll and the sub­se­quent photo-oxi­da­tion, which hap­pens quite quickly,” Servili said.

In that case, light trig­gers chloro­phyll which reacts with triplet oxy­gen to form an excited state of sin­glet oxy­gen.

“The very nature of photo-oxi­da­tion hin­ders the free rad­i­cals for­ma­tion so that pri­mary con­tents such as polyphe­nols can only par­tially con­tain the ran­cid­ity process,” Servili said. ​“We saw that green glass and ultra­vi­o­let grade absorb­ing glass did not per­form in a sig­nif­i­cantly dif­fer­ent way.”

“Multilayer plas­tic-coated paper­board alu­minum foil fared bet­ter because it pro­tected its con­tents from any light while also avoid­ing any per­me­abil­ity to oxy­gen,” he added.

The Italian researcher noted how rel­e­vant the glass cul­ture is in the olive oil world and empha­sized that solu­tions are already at hand.

“If we look at first-class pre­mium extra vir­gin olive oils, they usu­ally come in black-coated bot­tles or mul­ti­lay­ered coat­ing col­ored bot­tles, which are not only ele­gant but also offer strong pro­tec­tion to their con­tents,” Servili said. ​“The extra vir­gin olive oils that we tested are some of those pro­duced in large quan­ti­ties and sold at lower prices, mostly in green bot­tles.”

In Italy, explained Servili, an extra vir­gin olive oil bot­tle is unlikely to spend pro­longed peri­ods of time on a super­mar­ket shelf, given the high rate of extra vir­gin olive oil con­sump­tion in the coun­try.

“But those same bot­tles are also shipped abroad and inad­e­quate pro­tec­tion from light dur­ing trans­port and fol­low­ing dis­play on a shelf could quickly lower their qual­i­ties even more than the tem­per­a­ture stress due to the ship­ment itself,” Servili said.

That means that some bot­tles upon arriv­ing at their over­seas des­ti­na­tions might fail to meet the stan­dards of extra vir­gin.

Time spent on the shelf before being pur­chased by a con­sumer extends the period of light expo­sure.

“If such a bot­tle is dis­played for more than a few weeks, and often it is much more than that, maybe a new approach to photo-oxi­da­tion should be con­sid­ered,” Servili said, cit­ing how two of the extra vir­gin olive oil sam­ples were com­pletely ran­cid after 10 months in the exper­i­ment.

“Absence of light is a strange con­cept because when we talk about olive oil oxi­da­tion, we all think of tem­per­a­ture,” Servili said. ​“That hap­pens because we think of extreme con­di­tions, like those dur­ing ship­ping when extra vir­gin olive oil could reach 45 ºC or even 50 ºC.”

“Still, there are stud­ies accord­ing to which dur­ing the ship­ment by sea inside the con­tain­ers the tem­per­a­ture does not exceed 30 ºC,” he added.

The over­all results of the study con­firm what emerged in pre­vi­ous research that inves­ti­gated extra vir­gin olive oil pack­ag­ing in super­mar­ket con­di­tions.

In a 2018 study, sci­en­tists from the University of Pisa found that both tem­per­a­ture and light sig­nif­i­cantly affected the shelf-life of the extra vir­gin olive oil.

“This fact sug­gests that not only the stor­age con­di­tions can pre­vent oxi­da­tion processes from occur­ring, but they can even be use­fully used to slow down [and] almost to block them,” the researchers con­cluded.

• Food Chemistry
• Heliyon