Short-Term Pre-Milling Refrigeration Found to Retain Olive Quality

Summary

A study on the impact of dif­fer­ent refrig­er­a­tion meth­ods on olives and olive oil qual­ity found that frozen stor­age may be detri­men­tal, while refrig­er­a­tion at 4 ºC is opti­mal for short-term stor­age. The study, focus­ing on Chinese olive vari­eties, sug­gested that refrig­er­a­tion pre­serves cru­cial antiox­i­dants and fla­vors, while freez­ing can reduce nutri­tional value and color. The find­ings, applic­a­ble beyond China, could help olive pro­duc­ers world­wide, par­tic­u­larly in Mediterranean regions, mit­i­gate qual­ity loss dur­ing short-term stor­age with proper refrig­er­a­tion tech­niques.

A new study has inves­ti­gated the impact of dif­fer­ent refrig­er­a­tion meth­ods on the qual­ity of olives and the result­ing olive oil after milling.

The researchers found that frozen stor­age of olives may be detri­men­tal to the cru­cial qual­i­ties of the fruit.

They also iden­ti­fied 4 ºC as the opti­mal tem­per­a­ture for short-term stor­age of olives, par­tic­u­larly for peri­ods exceed­ing 24 hours after har­vest­ing.

While the study focused on Chinese vari­eties, includ­ing Fuao and Laixing, grown in the Gansu Valley, the researchers believe their find­ings could sup­port olive farm­ing in many other coun­tries.

Specifically, the study pub­lished by Food Chemistry com­pared the olive oil pro­files result­ing from seven days of stor­age under three dif­fer­ent con­di­tions: room tem­per­a­ture (approx­i­mately 23 ºC), refrig­er­a­tion at 4 ºC, and freez­ing at ‑20 ºC.

According to the sci­en­tists, pre­vi­ous stud­ies pri­mar­ily focused on the long-term stor­age of olives or assess­ments at fixed points in time.

“The rea­son for this study, based on a real-life pro­duc­tion prob­lem, was the dif­fi­cul­ties expe­ri­enced by a Chinese com­pany in stor­ing olives,” Xiuzhu Yu, pro­fes­sor at the College of Food Science and Engineering at Northwest A&F University in Yangling, told Olive Oil Times.

Challenging pro­duc­tion con­di­tions included occa­sional machin­ery break­downs and the need to har­vest olives early in response to adverse weather.

“This resulted in a short-term influx of olives into the mill, which far exceeded the mil­l’s capac­ity for imme­di­ate oil pro­duc­tion,” Yu noted.

This sit­u­a­tion is com­mon in many olive oil-pro­duc­ing coun­tries, par­tic­u­larly in regions where olive farm­ing is rapidly devel­op­ing.

These con­di­tions prompted the researchers to search for a stor­age solu­tion that would pre­serve olives’ unique qual­i­ties in the short term.

“The olive fruit is highly sus­cep­ti­ble to dete­ri­o­ra­tion dur­ing stor­age, such as oxi­da­tion or micro­bial growth,” Yu said. ​“That might lead to los­ing olive oil qual­ity, such as increased acid­ity or a van­ish­ing fla­vor.”

The researchers found that dif­fer­ent short-term stor­age con­di­tions pro­duce dif­fer­ent results.

“If olives are stored for more than 24 hours, then refrig­er­a­tion is rec­om­mended,” Yu said. ​“Under refrig­er­ated con­di­tions, the res­pi­ra­tion of the olives is slowed down, enzy­matic activ­ity is inhib­ited and micro­bi­o­log­i­cal growth is effec­tively con­trolled.”

Under these con­di­tions, cru­cial antiox­i­dants such as alpha-toco­pherols are retained, along with the olives’ com­plete phe­no­lic pro­file and chloro­phyll con­tent.

“At the same time, refrig­er­a­tion reduces the pro­duc­tion of unde­sir­able fla­vors such as ​‘fer­men­ta­tion’ and ​‘mold,’ and main­tains the qual­ity of aro­mas such as ​‘grassy,’” Yu remarked.

The researchers found that at room tem­per­a­ture, the high activ­ity of fat oxida­tive enzymes accel­er­ates the oxi­da­tion of fatty acids, lead­ing to an increase in acid­ity and per­ox­ide val­ues while destroy­ing volatile alde­hy­des, such as E‑2-hex­e­nal.

E‑2-hex­e­nal is a key aro­matic com­pound in extra vir­gin olive oil. It belongs to the group of volatile alde­hy­des that form dur­ing the crush­ing and malax­a­tion processes. This enzy­matic process trans­forms polyun­sat­u­rated fatty acids into var­i­ous volatile com­pounds.

Researchers also observed that stor­age at room tem­per­a­ture trig­gers the growth of yeasts and molds.

“Those pro­duce alco­holic com­pounds, pro­duc­ing a ​‘boozy’ and ​‘musty’ taste. Refrigeration inhibits micro­bial metab­o­lism and reduces the gen­er­a­tion of odors,” Yu said.

The researchers found that freez­ing olives pro­duces ice crys­tals that destroy the olive cell struc­ture, release oxida­tive enzymes, accel­er­ate the degra­da­tion of phe­no­lic sub­stances and reduce the con­tent of chloro­phyll and carotenoid pig­ments.

“That means that the olive oil becomes lighter in color, and its nutri­tional value decreases,” Yu said.

According to the researchers, after seven days of refrig­er­a­tion, cru­cial lev­els of acid­ity and per­ox­ide still meet inter­na­tional stan­dards for vir­gin olive oils and retain more than 80 per­cent of key fla­vor sub­stances.

While freez­ing was found to reduce chloro­phyll by 30 per­cent and phe­nols by 40 per­cent, these losses were lim­ited to ten per­cent when refrig­er­a­tion was applied.

“A guid­ance for con­tin­gency plans could go as fol­lows. If stor­age is required for three to seven days, pref­er­ence should be given to refrig­er­a­tion. If only one to two days, room tem­per­a­ture is still pos­si­ble, but pro­cess­ing needs to be done as soon as pos­si­ble,” Yu said.

Previous research in Spain also explored how to man­age refrig­er­ated olives once they are ready to be processed.

According to the Food Science and Engineering pro­fes­sor, the find­ings could be of inter­est far beyond olive oil pro­duc­ers in China.

“Our research on Chinese olive vari­eties offers valu­able insights that can be adapted by pro­duc­ers world­wide, includ­ing those in the Mediterranean basin,” Yu said. ​“Gansu’s val­ley cli­mate resem­bles Mediterranean regions in alti­tude and tem­per­a­ture, and the bio­chem­i­cal processes that pro­tect key nutri­ents and volatile com­pounds under refrig­er­a­tion are shared across olive cul­ti­vars.”

“Mediterranean pro­duc­ers can adopt this strat­egy to mit­i­gate qual­ity loss dur­ing short-term stor­age,” he added.

Yu warned that while the prin­ci­ples are trans­fer­able, Mediterranean pro­duc­ers should also con­sider some spe­cific dif­fer­ences.

“The vari­ety. Local cul­ti­vars, such as Picual or Arbequina, may have slightly dis­tinct fatty acid pro­files or enzyme activ­i­ties, requir­ing minor adjust­ments to stor­age dura­tion,” Yu said.

Additional con­sid­er­a­tions include the spe­cific nuances of the Mediterranean cli­mate, such as drier air and diur­nal tem­per­a­ture shifts.

“Those dif­fer­ences may influ­ence olive res­pi­ra­tion rates, but our frame­work — such as refrig­er­a­tion at 4 ºC for three to seven days — pro­vides a sci­ence-backed start­ing point,” he con­cluded.