Seasonal climatic influence on carbon stable isotope compositions of ¬¬fatty acids in monovarietal extra-virgin olive oils

Stable isotope analyses held enormous potential as traceability markers of a vast variety of food products (Camin et al., 2017). This also occurred for extra virgin olive oil, since stable isotope analyses determined on oil bulk have allowed sample differentiation on the basis of geographical origin (Portarena et al., 2014; Camin et al., 2016; Portarena et al., 2017). However, the bulk isotope composition of olive oil represents an average of a relatively large number of isotopically heterogeneous compounds that are differently affected by genotype × environment interaction (Portarena et al., 2015). In this work, ?13C values on the main olive oil fatty acids (FAs) (palmitic, stearic, oleic and linoleic) have been measured from 80 monovarietal EVOOs, similarly grown and harvested at five different olive maturation stages. By collecting oils at a common site, we attempted to minimize first the effects of climate, allowing potential genotypic patterns in the ?13C of FAs to be expressed more strongly. At the same time, the five different olive ripening stages, collected per each cultivar, allowed to link FA ?13C with the seasonal climatic gradient (increasing precipitation and decreasing temperature). The results showed that climatic parameters resulted the main drivers affecting FA ?13C variability during oil accumulation. However, there was a non-homogeneity in carbon isotopic fractionation processes in the different compounds in relation to both genotype and climatic changes during oil accumulation. Oleic acid is the main oil compound produced by olive plants during all harvest season and its isotopic composition was affected, with more extent than other FAs, by seasonal climatic gradient. In particular, its ?13C values resulted mainly dependent on the amount of rain, which is the main water source available for the plants. At the start of oil accumulation, oleic acid showed the highest 13C enrichment in all cultivars. This could be attributable to the use of stored 13C enriched carbohydrates that plant used for oleic biosynthesis. In fact, during the late summer-autumn period, under low rainfall and increased air temperatures, the plant photosynthetic activity was limited and the concentration of leaf soluble sugars at canopy level was low. Indeed, plants could potentially biosynthesize oleic acid mobilizing storage carbohydrates to satisfy the continued metabolic demand (Ryan 2011, Silpi et al. 2007). Proceeding the oil accumulation, the observed oleic 13C depletion reflected the physiological responses to environmental variations. On the contrary, the isotope fractionation relative to palmitic, stearic and linoleic biosynthesis during the ripening season, implied lower isotope discrimination. In addition, the ?13C values of linoleic acid varied among the studied cultivars appearing as a useful tool for studying inter-varietal differences in olive oils. Selecting the specific oil compounds that could provide a more direct link to environmental or genotypic factor will enhance the sensitivity of fingerprinting approach in olive oil traceability studies.