Study: Virgin Olive Oils Protect Ready-To-Eat Salads from Some Bacteria

Researchers found virgin olive oils high in polyphenols lowered the risk of contamination from common food-borne pathogens by up to 90 percent.
By Paolo DeAndreis
Nov. 16, 2021 08:50 UTC

Eating salad with a vir­gin olive oil dress­ing may deac­ti­vate some of its poten­tially harm­ful con­tents, includ­ing some com­mon types of bac­te­ria.

New research shows that vir­gin olive oils might serve as an effec­tive bar­rier against pathogens com­monly found in ready-to-eat and quick salad bags.

We found that in a range going from a few sec­onds to 15 min­utes, some olive oils can greatly reduce the harm­ful poten­tial of the bac­te­ria. At the same time, they do not destroy ben­e­fi­cial pro­bi­otics.- Severino Zara, agri­cul­tural researcher, University of Sassari

Scientists from the University of Sassari in Sardinia and the Italian National Research Council inves­ti­gated quick salad bags, which are respon­si­ble for a mas­sive increase in salad con­sump­tion world­wide.

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Quick salad bags are min­i­mally processed foods con­sumed with­out the need for addi­tional treat­ment,” the researchers wrote. Their min­i­mal pro­cess­ing guar­an­tees the preser­va­tion of the product’s sen­so­r­ial fea­tures but results in a shorter shelf-life.”

The high micro­bi­o­log­i­cal risk is the main prob­lem asso­ci­ated with the con­sump­tion of these prod­ucts as micro­bi­o­log­i­cal con­t­a­m­i­na­tion is very com­mon in veg­eta­bles grow­ing in soil,” they added. Moreover, their mois­ture con­tent, pH, trans­porta­tion and stor­age may favor the occur­rence of post-pro­duc­tion pathogen con­t­a­m­i­na­tions.”

The researchers found that the risk of harm­ful con­t­a­m­i­na­tion drops sig­nif­i­cantly by apply­ing nor­mal quan­ti­ties of vir­gin olive oil to the quick salad bags and eat­ing the salad after about 15 min­utes. With cer­tain vir­gin olive oils, the risk can drop by as much as 90 per­cent.

The researchers eval­u­ated the antimi­cro­bial prop­er­ties of vir­gin olive oils from 13 dif­fer­ent Italian olive cul­ti­vars, some of which are native to Sardinia and oth­ers that grow in sev­eral regions of the coun­try, such as Coratina or Sivigliana.

For the study, all of the olives came from a sin­gle research grove in Oristano, a province on the island’s west­ern coast. The fruits were har­vested simul­ta­ne­ously and trans­formed into vir­gin olive oils accord­ing to the International Olive Council guide­lines at the same oil mill, which is also a uni­ver­sity research facil­ity.

We found that vir­gin olive oil can be con­sid­ered a nat­ural antimi­cro­bial,” Severino Zara, the co-author of the study and an agri­cul­tural researcher at the University of Sassari, told Olive Oil Times.

While research shows a grow­ing cat­a­log of stud­ies cer­ti­fy­ing the excel­lent nutraceu­ti­cal qual­i­ties of extra vir­gin olive oils, our research hints at another spe­cific pro­file of vir­gin olive oils, which has not been suf­fi­ciently inves­ti­gated yet,” he added. Some of them can destroy up to 90 per­cent of the bac­te­r­ial sur­face charge of many of the most com­mon pathogens.”

Tests were con­ducted on sal­ads com­monly found on super­mar­ket shelves. The leafy green veg­eta­bles were then exam­ined, and any con­t­a­m­i­nants were removed. After, the sal­ads were inoc­u­lated with pathogens such as Candida, Staphylococcus aureus, Salmonella and Escherichia coli along with a few pro­bi­otics.

The antimi­cro­bial activ­ity of the vir­gin olive oils was tested using quan­ti­ties which are equal to a nor­mal serv­ing,” Zara said. This means that we inves­ti­gated what hap­pens to 100 grams of con­t­a­m­i­nated salad when a con­sumer pours a typ­i­cal vir­gin olive oil serv­ing on it.”

We found that in a range going from a few sec­onds to 15 min­utes, some olive oils can greatly reduce the harm­ful poten­tial of the bac­te­ria,” he added. At the same time, they do not destroy ben­e­fi­cial pro­bi­otics.”

However, not all vir­gin olive oils acted in the same way. Scientists had already noted dif­fer­ent antimi­cro­bial qual­i­ties in the olive oils from cer­tain cul­ti­vars when ana­lyz­ing their effect in a lab envi­ron­ment. They then pro­ceeded to test their impact in a real-life sit­u­a­tion such as a salad dress­ing.

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Since we har­vested and trans­formed the olives which were grown with the same cli­mate and in the same area, we had the ideal con­di­tions to com­pare the effects of the dif­fer­ent cul­ti­vars,” Zara said.

When exper­i­ment­ing in vitro, we had seen how cer­tain vir­gin olive oils would fare within an hour of pathogen inoc­u­la­tion and found that in many vir­gin olive oils, most bac­te­ria would almost instantly die,” he added. In oth­ers, some bac­te­ria would still be active after one hour from expo­sure.”


As a result of the in vitro exper­i­ments and the sub­se­quent real-life sim­u­la­tion, sci­en­tists iden­ti­fied the local olive cul­ti­var Bosana and Sivigliana as more effec­tive in elim­i­nat­ing pathogens.

Even pathogens such as Salmonella, which is a gram-neg­a­tive bac­terium, and it is way more resilient than oth­ers, have been greatly reduced and cur­tailed by the vir­gin olive oil effect,” Zara said.

The most effec­tive cul­ti­vars con­tained higher lev­els of polyphe­nols.

While in dif­fer­ent sea­sons and loca­tions polyphe­nol lev­els in olive oil might change, cul­ti­vars such as Bosana sport a pro­file which results in a high polyphe­no­lic con­tent because of its genetic char­ac­ter­is­tics,” Zara said.

Given the very mild effect of the addi­tion of vir­gin olive oil on the pro­bi­otic con­tent of the salad, which main­tains their nutraceu­ti­cal qual­i­ties intact, researchers believe that extra vir­gin olive oils high in polyphe­nols should be for­mally con­sid­ered antimi­cro­bial.

Further stud­ies are needed to con­firm the results obtained,” the researchers wrote. Indeed, although this work strongly sup­ports the uti­liza­tion of olive oils, not only for their nutraceu­ti­cal, but also for their antimi­cro­bial prop­er­ties, it is impor­tant to under­line that its uti­liza­tion could have some lim­its (also impor­tant to be exam­ined closely) mostly due to the organolep­tic mod­i­fi­ca­tions of olive oils when they are added as dress­ings if ready-to-eat food are not quickly con­sumed.”

What we can say is that those who eat using highly polyphe­no­lic olive oils are more pro­tected against con­t­a­m­i­na­tions from very com­mon pathogens, even more, when eat­ing ready-to-eat food,” Zara con­cluded.


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