Study Links Rising Temperatures to Changes in Olive Oil Composition

An analysis of 1,643 authentic olive oil samples collected in Slovenia over 27 years has identified significant changes in their chemical composition associated with warmer weather.

The research, published in Food Chemistry, found that olive oils produced in warmer years tended to contain less oleic acid and more linoleic and palmitoleic acids.

The researchers compared annual fatty acid profiles with key meteorological indicators. Temperature emerged as one of the variables most closely associated with the observed changes.

The findings raise questions about how climate-driven changes in olive oil composition could affect protected designation specifications, quality assessments and some of the parameters regulators use to verify authenticity and detect adulteration.

The study, carried out by a team of Slovenian, Italian and Spanish researchers, also examined naturally occurring variations in carbon, oxygen and hydrogen isotopes in the oils. These variations can preserve information about the environmental conditions in which the olives were grown.

The analysis draws on a database compiled over decades by the Institute for Oliveculture at the Science and Research Center Koper (ZRS Koper).

“Our database contains 1,643 authentic Slovenian olive oils collected between 1993 and 2019, with guaranteed traceability from the olive grove to the final product,” Milena Bučar-Miklavčič of ZRS Koper, a co-author of the paper, told Olive Oil Times.

“That makes it possible to monitor the effects of climate change, improve olive oil authenticity verification, protect producers and consumers from fraud and provide an important reference for future research and international comparisons,” she added.

Across the full 27-year dataset, the researchers found a clear association between warmer conditions and changes in the oils’ fatty acid profiles.

The contrast between two years near opposite ends of the temperature range illustrates the scale of the variation.

In 1996, one of the coolest years in the study period, the olive oils contained an average of 78.9 percent oleic acid. In 2018, one of the warmest years, the average was 71.9 percent.

Comparing the annual averages recorded in those two years, linoleic acid increased from 5.6 percent to nearly 7.9 percent, while palmitoleic acid rose from 0.7 percent to 1.7 percent.

The two years illustrate a broader relationship identified across the complete dataset rather than a continuous, linear progression between 1996 and 2018.

“The changes were quite pronounced,” Bučar-Miklavčič said. “The research showed a clear connection between higher temperatures and a reduction in oleic acid, together with increases in linoleic and palmitoleic acids.”

Those figures are particularly relevant in Slovenian Istria, where the protected designation of origin specification for extra virgin olive oil sets a minimum oleic acid content of 72 percent and a maximum linoleic acid content of eight percent.

“In recent years, some producers have no longer been able to meet these limits due to climate change, even though they produce authentic, high-quality olive oil,” Bučar-Miklavčič said. “The results may therefore be important for a possible future revision of the protected designation of origin specification.”

Lower oleic acid content does not necessarily mean lower-quality olive oil.

“Quality depends on many factors, with phenolic compounds, tocopherols and other natural antioxidants playing an important role in effectively slowing oxidation,” Bučar-Miklavčič said.

She cautioned, however, that higher linoleic acid content may result in slightly lower oxidative stability, making appropriate storage conditions even more important for retailers and consumers.

Temperature is far from the only factor involved. Rainfall, relative humidity, solar radiation, cultivar, fruit maturity, altitude, geographical location, irrigation and other farming practices may also shape fatty acid composition.

“The quality of olive oil is formed through the interaction of all these factors, which is why production must be monitored as a whole,” said Nives Ogrinc of the Jožef Stefan Institute in Ljubljana, another co-author of the paper.

“Our research nevertheless showed that, among meteorological factors, temperature is one of the most important influences on fatty acid biosynthesis,” she added.

One limitation of the data was the absence of cultivar information for every sample. Many of the olive oils collected during the 27-year period were blends of different varieties, reflecting the relatively recent expansion of monovarietal olive oil production and marketing in Slovenia.

Knowing the cultivar for every sample would have allowed the researchers to more precisely distinguish between changes associated with meteorological conditions and those linked to the genetic characteristics of individual varieties.

Still, the researchers said the large number of samples provided strong evidence of a long-term relationship between weather and olive oil composition.

The implications may extend beyond a single designation of origin.

Fatty acid composition is among the parameters used by the European Union and the International Olive Council to assess olive oil purity and detect possible adulteration with other vegetable oils.

If climate change naturally alters the chemical composition of authentic olive oils, existing standards could classify genuine products as non-compliant.

“Such a possibility exists,” Bučar-Miklavčič said. “We are already observing deviations in some parameters, not only in Slovenia but also in other Mediterranean countries.”

“The purpose of the limit values is to distinguish authentic olive oil from adulterated oil,” she added. “However, if climate change causes natural changes in oil composition, it is necessary to ensure that authentic oils are not unjustifiably excluded.”

The study unfolded in two related parts.

First, the researchers used the full Slovenian database to investigate long-term changes in fatty acid composition. They then analyzed stable isotope ratios in Slovenian oils from selected harvest years between 2006 and 2008 and in 2019, extending the study to samples from Croatia, Italy, Montenegro and Spain.

Ogrinc explained that stable isotopes are naturally occurring variants of chemical elements with different atomic masses. Their relative abundance varies with environmental processes and can serve as a natural fingerprint of a product’s geographical and climatic origin.

“In the case of hydrogen and oxygen, the relationship between heavier and lighter isotopes is primarily determined by the water cycle, evaporation and precipitation,” Ogrinc told Olive Oil Times. “For carbon, it is influenced by photosynthesis and the number of sunny days. Plants absorb these isotopic signatures from the environment, and they are subsequently preserved in food.”

Among the Slovenian samples, the oxygen isotope ratio was particularly sensitive to the number of hot days and relative humidity, while the hydrogen isotope ratio was strongly associated with temperature.

The Spanish samples showed significantly higher carbon, oxygen and hydrogen isotope values than oils from Slovenia, Italy, Croatia and Montenegro.

The researchers linked the pattern to Spain’s higher average annual temperatures, lower rainfall and more intense evaporation compared with the other regions included in the study.

“The isotopic signature can be used as a natural record of climatic conditions during olive growth and as a highly effective tool for verifying the geographical origin and authenticity of olive oil,” Ogrinc said.

According to the researchers, the findings could help producers adapt cultivar selection, harvest timing and processing technologies to changing climatic conditions.

Laboratories could interpret chemical results more accurately by accounting for climate effects, while certification bodies could draw on long-term data to update protected designation specifications and improve traceability and authenticity controls.

Further research will require comparable long-term databases in other olive oil-producing countries, particularly in warmer parts of the Mediterranean, as well as more systematic monitoring of monovarietal oils to determine how individual cultivars respond to rising temperatures.

“Combining classical chemical analyses, stable isotopes, artificial intelligence and other modern approaches will be very important for improving traceability and authenticity verification,” Ogrinc and Bučar-Miklavčič concluded.