Frying in Virgin Olive Oil Adds Healthy Compounds, Researchers Find

New research reveals that deep fry­ing cer­tain foods, such as French fries, with vir­gin olive oil, may enhance their nutri­tional pro­file.

According to a new study pub­lished in Food Chemistry, dur­ing the fry­ing process, some of the healthy prop­er­ties of vir­gin olive oil are absorbed by the food.

Separate research from the Higher Council for Scientific Research’s (CSIC) fat insti­tute in Spain pre­vi­ously found that food fried in olive pomace oil absorbed some of its healthy com­pounds as well.

Virgin olive oils are obtained from the fruit of the olive tree with no chem­i­cal alter­ation or heat treat­ment and dif­fer from extra vir­gin olive oil due to their higher free acid­ity lev­els.

The researchers found that even repeated use of the same vir­gin olive oils for deep fry­ing would still ben­e­fit the fries. After sev­eral uses, top­ping up the fryer with fresh olive oil replen­ished the fry­ing oil with vir­gin olive oil’s antiox­i­dants.

“Consequently, part of the com­po­si­tion of this oil will be trans­ferred to the fried food,” Alexandre Guedes Torres, a pro­fes­sor of nutri­tional bio­chem­istry and food sci­ence at the University of Rio de Janeiro, told Olive Oil Times.

“Among the most rel­e­vant com­pounds that were trans­ferred, we saw plant sterols and toco­pherols, the intake of which is desir­able due to their poten­tial to lower cho­les­terol and because they present vit­a­min E activ­ity, respec­tively,” he added. ​“Besides these two com­pounds, other poten­tially bioac­tive ones were also trans­ferred to the fries, such as lig­nans and triter­penic com­pounds.”

Using ana­lyt­i­cal chro­matog­ra­phy tech­niques, researchers were able to iden­tify and assess 56 vir­gin olive oil com­pounds that would be trans­ferred to food dur­ing the dry­ing process.

The researchers’ find­ings con­firmed their hypoth­e­sis that some healthy bioac­tive com­pounds from vir­gin olive oils are lost by degra­da­tion due to heat­ing the oil. Still, some resist the fry­ing con­di­tions and are trans­ferred to the fried food.

“Our study focused on the changes occur­ring in the meta­bolic pro­file of the vir­gin olive oil dur­ing deep fry­ing,” the researchers wrote. ​“Our results showed that sev­eral of these com­pounds resist the heat applied, 190 °C, in 30 deep fry­ing cycles of French fries… enrich­ing the fries with bioac­tive com­pounds from the vir­gin olive oil.”

According to the sci­en­tists, the research shows that such fry­ing tech­niques and vir­gin olive oils might sig­nif­i­cantly enhance the nutri­tional pro­file of fries.

“Virgin olive oil is a nutri­tion­ally rich oil,” the researchers wrote. ​“It adds value to the prepa­ra­tion because it is a source of oleic acid and bioac­tive com­pounds, that would not be present nor trans­ferred to the food by employ­ing refined edi­ble veg­etable oils com­monly used for fry­ing.”

“Thus, we can say that among the choices we have for fry­ing oil, olive oil is pos­si­bly one of the most inter­est­ing ones due to its appeal­ing nutri­tional pro­file,” they added.

The researchers con­ducted their tri­als in con­di­tions that mimic home food prepa­ra­tion, where the cook­ing is done on a small scale, and there is no exces­sive reuse of the fry­ing olive oil.

“Further work is needed to deter­mine if these results will be repro­duced under indus­trial oper­a­tions,” the researchers wrote. ​“It seems promis­ing, but we sense that an inte­grated food sci­ence approach would be needed, con­sid­er­ing the design of the friers and the over­all scale, among other fac­tors that impact heat trans­fer dur­ing deep fry­ing.”

“It would be inter­est­ing to see work on indus­tri­ally deep-fried pota­toes, for instance, to check the upper num­ber of con­sec­u­tive fry­ing oper­a­tions that could be done before the oil would be con­sid­ered non-proper,” they added.

In indus­trial kitchens, researchers plan to inves­ti­gate ​“after how many fry­ing cycles would it be best to add small vol­umes of fresh vir­gin olive oil, say, near five per­cent of total oil, to avoid com­plete deple­tion of nat­ural antiox­i­dants, and there­fore pro­long the lifes­pan of the whole fry­ing medium.”

The com­po­si­tion of olive oil depends on sev­eral fac­tors, such as the pro­duc­tion region’s cli­mate, alti­tude, irri­ga­tion, soil com­po­si­tion and olives’ ripen­ing.

The research has shown that the higher amount of vir­gin olive oil metabo­lites trans­ferred to fries hap­pened using Arbequina.

“The cul­ti­var is another par­tic­u­larly impor­tant fac­tor which can influ­ence olive oil com­po­si­tion,” Guedes Torres said. ​“When we com­pared Arbequina ver­sus Koroneiki vir­gin olive oil, the for­mer stood out pre­sent­ing higher con­tents of bioac­tive com­pounds.”

“Therefore, it is likely that this could be a plau­si­ble fac­tor deter­min­ing the higher con­tents of such com­pounds trans­ferred to the fried food, by what we call a mass effect,” he added. ​“The higher the amount in medium A, the olive oil, the higher the trans­fer to medium B, the French fries.”

According to the researchers, the ben­e­fi­cial effects of deep-fry­ing with vir­gin olive oil extend to other food.

“The trans­fer of the com­po­si­tion of oil used for fry­ing to the food will always occur, as the oil gets absorbed by the food dur­ing its prepa­ra­tion,” they wrote. ​“While fry­ing, a crust is formed on the sur­face of the food by dehy­dra­tion on the food sur­face, and pores are formed by water loss due to heat trans­fer and increase of inner pres­sure of the food.”

“These pores allow oil uptake after the food is removed from the oil, regard­less of the kind of food being fried,” the researchers added. ​“But we should con­sider that this oil uptake will also depend on the food com­po­si­tion.”

Previous research has hinted at the adverse health effects that come from exces­sive French fry con­sump­tion.

Research from the University of Naples in Italy found that pota­toes fried for pro­longed peri­ods at high tem­per­a­tures have higher lev­els of acry­lamide, a com­pound con­sid­ered toxic and respon­si­ble for increas­ing a person’s can­cer risk.

The study had shown that acry­lamide lev­els were low­est in pota­toes fried in olive oil and higher in pota­toes fried in trans-fat-rich cook­ing oils.

The researchers said their next steps would be to inves­ti­gate the evo­lu­tion of the pro­file vir­gin olive oils’ bioac­tive com­pounds and the poten­tially toxic com­pounds that can form dur­ing deep fry­ing if the reuse of the oil is ​“abu­sive” and unrea­son­able.

“Most inter­est­ingly, deter­min­ing what would be this limit of abu­sive use,” Guedes Torres said. ​“This limit has been pur­sued ear­lier, but the avail­abil­ity of mod­ern high-res­o­lu­tion ana­lyt­i­cal meth­ods to assess the com­pounds being formed, and bio­log­i­cal assays to assess the tox­i­c­ity of fried foods, and not only of iso­lated com­pounds, should be inter­est­ing.”

“Furthermore, the lim­its of abu­sive use most likely will vary between oils of dif­fer­ent com­po­si­tions,” he con­cluded. ​“Therefore, assess­ment of var­ied oils, and not only the most used ones, would be inter­est­ing as a means of prospec­tion of promis­ing oils, such as vir­gin olive oils, in terms of sta­bil­ity dur­ing fry­ing and impact on the health of con­sumers.”

• Food Chemistry