Warmer Climate Could Reshape Olive Oil Quality, Research Shows

Can researchers foresee how a warmer climate will affect extra virgin olive oil quality? Olive oil production and its phenolic composition are both closely tied to climate.

Current research can identify broad trends, but not a single outcome. The effects depend on the cultivar, water availability and, crucially, the stage of fruit development at which heat or water stress occurs.

Monitor week by week olive development, phenolic composition, olive oil accumulation and the maturity index. Based on those data, make the decision.– Ítala Marx, Researcher in Food Chemistry and Nutrition, University of Córdoba

“What we are seeing is that higher temperatures can alter olive oil’s phenolic profile,” Ítala Marx, an MSCA alumna and researcher in food chemistry and nutrition at the University of Córdoba in Spain, told Olive Oil Times. Her work focuses on bioactive compounds in olive oil, human health and the valorization of olive byproducts.

“In some cases, they may reduce phenolic content; in others, depending on the conditions, the effect can even be favorable to olive oil quality,” she said.

One reason the impact is difficult to predict is that an olive fruit is not physiologically the same throughout the growing season.

After fruit set, the olive moves through seed development, pit hardening, mesocarp development (where most olive oil is eventually stored), olive oil accumulation and ripening. Phenolic compounds also change substantially throughout this process.

Research following phenolic metabolism during olive fruit development has found that major compounds, including oleuropein, can change markedly as the fruit develops and matures.

Heat or water stress at fruit set therefore affects the olive in a very different biological state than the same stress during olive oil accumulation or near ripening.

“The effect depends a lot on the stage of fruit maturation,” Marx said. “Depending on when higher temperatures occur, they can advance some of the stages: flowering, fruit set, fruit development, olive oil accumulation and, of course, ripening.”

“Heat can affect the olive tree in different ways,” she added. “Depending on the stage, it can affect more or less, or it can even affect positively.”

A 2025 multi-site study in Argentina examined Arbequina and Coratina grown under contrasting thermal conditions. Researchers found lower total phenolic content in warmer environments.

Verbascoside and oleuropein aglycone showed some of the clearest differences, with concentrations in the warmest environment roughly one-half to one-third of those measured in the coldest. The relationship between phenolic content and thermal time, or accumulated heat over the plant’s development, was also clearer in Arbequina than in Coratina, pointing to a genotype-dependent response.

Secoiridoids are a major family of olive oil phenolic compounds and include derivatives of oleuropein and ligstroside. They are among the compounds associated with bitterness, pungency and oxidative stability in virgin olive oil.

“But in general, if we observe the studies, we can expect that higher temperatures can negatively affect phenolic compounds, with secoiridoids being among the phenolic groups of particular interest,” Marx said.

“Secoiridoids make up a large part of the phenolic fraction of olive oil, so if this class is strongly affected, the phenolic content of the olive oil will also be affected,” she added. “And the response is not the same for every cultivar.”

Water availability adds another layer. A 2020 study on three Portuguese cultivars, Cobrançosa, Cordovil de Castelo Branco and Cordovil de Serpa, found different responses to combined heat and drought stress.

Phenolic profiles did not decline uniformly. In some cases, stress stimulated the accumulation of particular bioactive compounds.

A study on Coratina grown in Croatia compared rainfed olive trees with three irrigation regimes, including a regulated deficit strategy timed to growth stages such as flowering, fruit set, pit hardening and olive oil accumulation.

Irrigation increased olive oil yield by 58 to 74 percent compared with rainfed conditions. However, irrigation did not significantly affect total phenol and secoiridoid concentrations.

The regulated deficit approach maintained similar phenolic content while using less water than full irrigation. The result also complicates the common assumption that less water necessarily means more phenols.

“People think that, in general, lower water availability could increase phenolic content,” Marx said. “But again, it is not a given. It depends on the stage.”

“Too much water can be a problem, but too little can also be a problem,” she added. “We need to balance temperature and water stress according to the stage of fruit development.”

Water also affects what happens later, when the fruit reaches the mill.

“Maybe the phenolic concentration is not the problem, but the transformation of the phenolic compounds,” Marx said. “Some of them will remain in the water phase.”

“The amount of water in the fruit, or the amount of water used during extraction, can affect the partitioning of phenolic compounds between the aqueous and oil phases, because many of these compounds are hydrophilic.”

Experiments in Spain show how warming can shift olive development while affecting cultivars differently.

Researchers at the University of Córdoba exposed mature Picual and Arbequina trees to temperatures about 4 °C above ambient conditions. Warming advanced flowering and fruit maturation in both cultivars, while flower quality and yield were significantly reduced in Picual but not in Arbequina.

The implications go beyond changes in individual compounds. Warming can shift the biological calendar itself.

If flowering and fruit development move forward, olive oil accumulation may occur during hotter parts of the year. In a separate experimental warming study in northwest Argentina, raising air temperature by about 3 °C from final fruit set through the end of olive oil accumulation reduced fruit growth and olive oil accumulation rates in both Arbequina and Coratina.

Maximum olive oil concentration fell by about 20 percent in both cultivars.

“The specific case of heat shows a different response depending on the genotype, depending on the cultivar,” Marx said.

“If temperatures continue to increase during the years, maybe our calendar could change,” she added. “Here in Spain, we start more or less in October, with some early harvests in the middle of October. Depending on heat stress in the future, maybe the optimal harvest window could progressively shift earlier, potentially even into late September in some cases.”

Cultivar selection could become an important tool for new olive groves. Breeding programs are crossing cultivars to combine traits related to production, olive oil quality and tolerance to different growing conditions.

The University of Córdoba and IFAPA breeding program, for example, produced Sikitita from a Picual × Arbequina cross and has developed other selections combining agronomic and olive oil quality traits.

For established producers, however, replacing an olive grove is hardly an immediate solution.

“To have a large, intensive olive grove is expensive,” Marx said. “It is not possible to simply remove the existing olive trees and plant new ones.”

Instead, she said, olive growers may need to follow fruit development rather than relying primarily on traditional harvest dates.

“Monitor week by week olive development, phenolic composition, olive oil accumulation and the maturity index,” Marx said. “Based on those data, make the decision. If the conditions are right, start the harvest. Do not wait until October 20. Make the decision in real time.”

In a warmer and more variable climate, the same calendar date can correspond to different physiological stages in different years.

Marx pointed to her native Brazil.

“Arbequina, Koroneiki and Arbosana are adapting very well in Brazil,” she said. “But the phenolic composition is not necessarily the same as that observed under Mediterranean conditions.”

“In some comparisons, particularly for specific phenolic groups such as secoiridoids, concentrations in Brazilian oils can be considerably lower and may reach around half of those observed in Mediterranean oils,” Marx said. “But this is highly dependent on the cultivar, agroclimatic conditions and maturity stage.”

Marx said Brazilian olive growers and olive oil producers are adjusting their practices to local conditions.

“If I want a very good olive oil, a premium one, harvesting earlier can be one of the strategies,” Marx said. “That is the kind of adjustment olive oil producers there need to make.”

“The main thing is to study, study, study and use as much science and technology as possible to tailor olive growing to that environment.”

For Marx, producing high-quality olive oil in changing or unfamiliar conditions ultimately depends on combining adaptation with better information.

“With the correct technology and science, and I will always defend the scientific part, it is possible to grow olive trees successfully in different countries,” she said.

“If we have good olives, with the correct technology and science behind this, it is possible to produce very good olive oils.”