Thermal regime and cultivar effects on squalene and sterol contents in olive fruits: Results from a field network in different Argentinian environments

Highlights

• Squalene (Sq) and sterols (S) are important components of virgin olive oils (VOO).

• Sq and S contents were found to vary strongly between olive growing environments.

• Variations were associated to thermal characteristics of the olive growing sites.

• Concentrations of Sq and individual sterols were higher at warmer environments.

• Results point out the need to review commercial regulations for S contents in VOO.

Abstract

Some olive cultivars and production regions around the world, although they may provide good quality olive oils, may not comply with the current commercial regulations establishing acceptable levels of sterols in extra virgin olive oils. The present study was driven by the hypothesis that olive growing environments, differing in thermal regime conditions, affect squalene and sterol contents of olive fruits. On the basis of differences in agronomic characteristics and oil composition, two olive cultivars (Arbequina and Coratina) were selected and sampled at different fruit development stages. Various models were examined to look for relationships between environmental thermal records and chemical parameters; those including the accumulated thermal time (ATT) and minimum temperatures showed the best fit. In both cultivars, the total and individual sterol contents were positively associated with ATT over all the fruit development and ripening period considered. In all cases, the data from Arbequina fit the models better than for Coratina and differences between cultivars in individual sterol contents were greater in the warmest growing environments. Overall, findings indicated a strong impact of the growing environment on squalene and sterol contents. Such an effect was associated with specific thermal characteristics of the olive growing sites; concentrations were found to be higher in the warmer northernmost sites than in the cooler southernmost ones. There was also an effect of the cultivar, particularly on the contents of β-sitosterol, campesterol and total sterols. These latter results suggest greater sensitivity of sterol metabolism to temperature in cv. Arbequina.

Introduction

The nutritional and health benefits of olive oil consumption have been well documented. They have been mainly ascribed to the high content of oleic acid and the presence of several minor compounds with multiple biological activities (Sánchez-Rodríguez et al., 2018; Lozano-Castellón et al., 2019).

Minor components in VOO account for 1.5–2% and include primarily phytosterols and non-steroidal triterpenoids (Stiti et al., 2010), hydrocarbons (Giuffrè, 2021), waxes (Giuffrè, 2013; Boualem et al., 2021), fatty alcohols (Boualem et al., 2021), tocopherols and other phenolic compounds (Uceda et al., 2008). Regarding phytosterols, the most abundant are those from the 4-demethylsterol fraction which are simply referred to as sterols. Although regulations set a minimum of 1000 mg total sterols per kg of VOO (CONSLEG; IOC, 2022), lower values have been found in oils from some olive cultivars grown in Australia and Spain (Mailer et al., 2010; Kyçyk et al., 2016). The predominant sterols in VOO are found to be β-sitosterol (70–95% of the total sterols), stigmasterol, campesterol and Δ⁵-avenasterol (Stiti et al., 2010).

The concentration of sterols in VOO is a matter of concern for many olive oil producing countries because limits for total sterol concentration and some individual sterols have been established for VOO by the European Union and International Olive Council (IOC) regulations. The limits have been stated mainly as genuineness parameter, since they could be used for fraud detection of olive oil adulterated with other edible fats and oils. Such regulations, however, have been established without fully considering the possibility of variability in sterol concentrations due to the introduction of new cultivars and, more important, to the particular environmental conditions of some growing regions, which may affect sterol composition. Despite varietal olive oils have a relatively stable sterol composition, it is well-known that it may vary according to agronomic, climatic and other environmental conditions (Ben Temime et al., 2008; Lazzez et al., 2008; Kyçyk et al., 2016; Ghisoni et al., 2019). Likewise, important variations due to crop season (Giuffrè and Louadj, 2013) and harvest date (Giuffrè et al., 2012) have been reported in individual and total sterol contents from several olive cultivars grown in Calabria (Italy). As a result, it has been found, for instance, that many VOO from varieties cultivated in Australia and the European Union do not meet the international standard value for campesterol concentration (Rivera del Alamo et al., 2004; Sanchez Casas et al., 2004; Mailer et al., 2010). Because of the negative impact this may have on the marketing of olive oil, the IOC and CODEX alimentarius's regulations have increased the acceptable percentage of campesterol (up to 4.5% of total sterols) in VOO.

Squalene, a lipophilic triterpenoid hydrocarbon present in many fats and oils, is the most abundant component of the unsaponifiable fraction of VOO (Aresta et al., 2020). Being the biosynthetic precursor of both sterols and non-steroidal triterpenoids present in the olive fruit, squalene content has been found to vary widely depending mainly on the stage of fruit development. For instance, it has been found that the developing olive drupes of cv. Arauco accumulate increasing amounts of squalene, but it drops strongly after the beginning of fruit ripening (Bodoira et al., 2015). There is a lack of studies evaluating possible effects of geographical and environmental conditions on squalene content of olive fruits. Although squalene concentration is not regulated by international quality trading standards for olive oils, its assessment is important not only for its health properties but also for the contribution to the oxidative stability of the oil (Martínez-Beamonte et al., 2020). Besides the beneficial properties above-mentioned, squalene concentration may be also used to identify fraud regarding the type of marketed olive oil, and also to determine geographical origin (Alberici et al., 2016; Camin et al., 2016).

Yield and chemical components in some olive cultivars are sensitive to environmental growth temperature. Studies conducted in the field and also under controlled temperature conditions have shown that oil and oleic acid contents of olive fruits can decrease in warm growing environments (Gracía-Inza et al., 2014; Rondanini et al., 2014; Torres et al., 2017; Mousavi et al., 2019; Miserere et al., 2022). While these studies provide strong evidence of the influence of temperature on those fruit components, the evaluation of thermal effects on minor olive oil components is largely unaddressed.

The environmental temperature emerges as a variable that could explain variations in sterol contents, as suggested by studies using olive cultivars growing in environments with different thermal regimes (Ceci and Carelli, 2010; Mailer et al., 2010; Li et al., 2019). Also, data from a controlled, field warming experiment suggest an effect of temperature on total tocopherol and sterol concentrations, with both oil components generally increasing due to moderate temperature increases (Hamze et al., 2022). Nevertheless, there is still a lack of information on the influence or contribution of each dimension or component of the thermal regime. Which component/s could have the greatest effect/s on variations in sterol content in olive fruits? Studies on this topic are important not only to understand the differences in the composition of olive oils from different producing regions, but also to assess the behavior of cultivars in a global warming scenario.

The present study was driven on the general hypothesis that squalene and sterol contents of olive fruits are affected by the thermal regime of the growing environment. To test this hypothesis, two olive cultivars (Arbequina and Coratina) varying in phenotypic plasticity were examined in several cultivation regions covering a wide latitudinal gradient in Argentina.