How Microorganisms Affect the Sensorial Qualities of Olive Oil

Microorganisms play a larger role than pre­vi­ously thought in affect­ing the chem­i­cal and sen­sory qual­i­ties of olive oil, research from the University of Molise sug­gests.

Yeasts are among the microor­gan­isms present in olive oil and, depend­ing on their enzy­matic activ­i­ties, they can either improve or dam­age the oil qual­ity. Some act as preser­v­a­tives, increas­ing the shelf life of the olive oil. Others act as cat­a­lysts for chem­i­cal reac­tions that lead to ran­cid­ity in the oil.

“The activ­ity of healthy olive micro­biota greatly con­tributes to the preser­va­tion of the over­all qual­ity of olive oil pro­duc­tion dur­ing the post-har­vest stor­age, pro­cess­ing phase and stor­age of the pro­duced olive oil,” researcher Biagi Zullo wrote in the report.

Gino Ciafardini, a lead­ing expert on yeast in olive oil, and Zullo have been study­ing microor­gan­isms in olive oil since 2002. In the past 16 years, they have dis­cov­ered 17 species of yeast and are cur­rently fig­ur­ing out how these microor­gan­isms affect sen­sory char­ac­ter­is­tics of the oil.

“Certain oil-born yeast species may exert a pos­i­tive effect on the sen­so­r­ial char­ac­ter­is­tics of newly pro­duced vir­gin olive oil by pro­mot­ing the deb­it­ter­ing process,” Zullo wrote. ​“Other yeasts are con­sid­ered harm­ful as they can dam­age the qual­ity of the oil through hydrol­y­sis of tri­a­cyl­glyc­erols and the pro­duc­tion of unpleas­ant fla­vors.”

The num­ber and type of yeast species are heav­ily depen­dent on the chem­i­cal com­po­si­tion of the olive oil. Oils with higher con­cen­tra­tions of phe­no­lic com­pounds tend to have fewer types of yeast in lower con­cen­tra­tions.

“The con­tent of the total polar phe­nols of the olive oil and their pro­file as antimi­cro­bial com­pounds, show a strong selec­tive pres­sure on the type and num­ber of years that sur­vive in the oil dur­ing its stor­age,” Ciafardini said. ​“In gen­eral, the olive oil’s micro­bi­o­log­i­cal qual­ity is cor­re­lated with the chem­i­cal and sen­so­r­ial qual­ity of the prod­uct.”

Yeast and other microor­gan­isms that are found in the olive oil come mainly from the fruit, itself. When the olives are pressed into a paste, these microor­gan­isms become sus­pended in the oil droplets.

“Previously, it was widely believed that vir­gin olive oil pro­vided an unsuit­able habi­tat for many microor­gan­isms,” Zullo wrote. ​“Microbiological research stud­ies have since estab­lished that freshly pro­duced vir­gin olive oil is con­t­a­m­i­nated by microor­gan­isms con­sist­ing mainly of yeast that are capa­ble of con­di­tion­ing the physic­o­chem­i­cal and sen­so­r­ial char­ac­ter­is­tics of the oil.”

Ciafardini and Zullo are cur­rently cat­e­go­riz­ing these yeast species based on their pos­i­tive or neg­a­tive effects on the oil qual­ity. Ciafardini said there is no way to con­trol the types of yeast species that end up in cer­tain olive oils.

“Through the olive oil fil­tra­tion it is not pos­si­ble to sep­a­rate the yeasts that poten­tially can harm olive oil qual­ity from those that help oil qual­ity,” he said. ​“The best way is to pro­duce vir­gin olive oil of good qual­ity, obtained from healthy fruits, hav­ing suf­fi­cient polar phe­nols con­tent able to inhibit the activ­ity of some unde­sir­able yeast enzymes.”

There may be no way to con­trol the types of yeast species in the oil, but there are vari­ables that olive oil pro­duc­ers can con­trol in order to ensure they make a higher qual­ity prod­uct.

The con­di­tion of the olives when they are processed have a big effect on the types of yeast present in the oil. For exam­ple, if the olives are begin­ning to fer­ment when they are pressed into oil, cer­tain yeast species that pro­duce ethanol and ethyl acetate will be present. These two chem­i­cals give the oil a vine­gary taste if it sits unused for too long.

Ciafardini and Zullo also found that mono­va­ri­etal olive oils tend to have fewer, some­times only one, species of yeast. Depending on the con­di­tion of the olives when they are pressed, this method of pro­duc­tion dras­ti­cally decreases the chances of harm­ful yeast exist­ing in the oil.

Certain essen­tial oils also can be added to olive oil in order to lower the con­cen­tra­tion of yeast and, in cer­tain cases, coun­ter­act its neg­a­tive effects. The essen­tial oils used in lemon and gar­lic fla­vored olive oils acted as espe­cially strong inhibitors for the growth of microor­gan­isms.

Ciafardini said that research on olive oil yeast has only reached the tip of the ice­berg. He and Zullo dis­cov­ered six of the 17 species just last year. The two believe that there is much more work to be done in order to fully under­stand how dif­fer­ent yeast species affect the qual­ity of the olive oil.

“In vir­gin olive oil… the pres­ence and activ­ity of yeasts were demon­strated dur­ing the past two decades,” Zullo wrote. ​“In fact, inves­ti­ga­tions explor­ing the olive oil micro­bi­ol­ogy are com­par­a­tively scarce; there­fore micro­biota in extra vir­gin olive oil remains poorly under­stood.”

• Food Microbiology


• National Center for Biotechnology Information