Olive oil (OO) is obtained from the fruit of the olive tree, which grows widely especially in the Mediterranean basin and neighboring regions (Waterman and Lockwood 2008). Approximately 2.57 million tons of olive oil were produced worldwide in the 2022/2023 harvest period. Spain, Turkey, Greece, Italy and Tunusia were among the largest producing countries. Worldwide consumption value in the same years was approximately 2.83 million tons. Italy, USA, Spain, Turkey and Morocco were the countries that consumed the most olive oil ((IOC) 2023).
Natural olive oils is classified as extra virgin olive oil (EVOO) and virgin olive oil (VOO), especially according to acidity level, peroxide value, specific absorbance and sensory character (La Lastra, Barranco et al. 2001, Gimeno, Castellote et al. 2002). EVOO is a translucent, greenish/yellowish, aromatic and naturally consumable liquid obtained from olive tree fruit only by mechanical and physical methods without causing any change in the oil content (Ruiz-Gutiérrez, Puerta et al. 2000, Harwood and Yaqoob 2002).
Chemical composition of olive oil is depending on different factors like variety (de Mendoza, Gordillo et al. 2013), geographical region, nutritional status of the fruit, climatic conditions (Fontanazza 1988), fruit maturity, fruit storage, extraction type and oil storage methods (Di Giovacchino, Sestili et al. 2002) and it may vary. Approximately 98–99% of its chemical composition consists of so-called saponifiable fractions (major components). Phenolic compounds, called the unsaponifiable fraction (minor components), constitute 1–2% of olive oil (Bengana, Bakhouche et al. 2013). These major and minor substances in its structure ensure that the oxidative stability and therefore the lifespan of olive oil is high (Diraman, Söbüçovali et al. 2015).
One of the most important factors in why olive oil has a significant consumption amount is undoubtedly its taste. At this point, volatile components (VC) play an important role. The formation of VCs in olive oil is mostly related to the first stage of production and they contain many components such as aldehydes, alcohols, ketones, acids and terpenes (de Mendoza, Gordillo et al. 2013).
The most important of the major components of olive oil is its fatty acid composition. Among the fatty acids in EVOOs, the most important are oleic acid, palmitic acid, linoleic acid, stearic acid, palmitoleic acid (Haddam, Chimi et al. 2014). If all the hydrogens in the carbon chain structure of fatty acids are complete, it is called saturated fatty acid, and if the carbons are connected to each other with double bonds due to missing hydrogen, it is called unsaturated fatty acid.
Lipid oxidation is a chain of reactions that can occur in vegetable and animal fats consumed as food, has negative effects on human health and encourages the formation of free radicals (Karakuş 2008). It has been shown that monounsaturated fatty acid consumption is associated with a decrease in plasma LDL (Low Density Lipoprotein) level and an increase in HDL (High Density Lipoprotein) level (Lapointe, Couillard et al. 2006, Fitó, Guxens et al. 2007).
Among the important minor components of olive oils, the phenolic substance content and the three main phenolic compounds in its composition, hydroxytyrosol (HT), tyrosol (T) and oleuropein (O), come to the fore. It has been stated that the antioxidant activity of hydroxytyrosol and oleuropein is stronger than antioxidants such as vitamin E, dimethyl sulfoxide and butylated hydroxy toluene (González-Santiago, Martín-Bautista et al. 2006, Bovicelli 2007, Roche, Dufour et al. 2009, Granados-Principal, Quiles et al. 2010, Perona, Fitó et al. 2011).
Thanks to the mentioned properties, the importance of olive oil in many diseases, including chronic diseases and cancer, was emphasized (Alonso, Ruiz-Gutierrez et al. 2006). It has been reported that it protects against cardiovascular diseases, regulates hypertension and hemostatic components (platelet function, thrombogenesis and fibrinolysis) and shows anti-inflammatory effects (Lopez-Miranda, Delgado‐Lista et al. 2007, López-Miranda, Pérez-Jiménez et al. 2010). Olive oil consumption is also associated with a decrease in the risk of obesity, metabolic syndrome and type 2 diabetes by regulating glucose metabolism (Lopez‐Miranda, Delgado‐Lista et al. 2007, Ruiz-Canela and Martínez-González 2011).
In this study, the quality parameters, fatty acid compositions, antioxidant properties, phenolic contents and volatile components of EVOO samples (Edremit, Halhalı, Memecik) produced from different types of olives grown in different regions of Turkey, one of the world's leading olive oil producing countries, were examined. The seller and end consumer behavior of these oils usually results in a storage period of 3–6 months. Therefore, the changes caused by the process in olive oil properties were re-examined at 3 and 6 months. In addition, in order to accurately simulate the storage process, the oils were stored in amber colored bottles at + 4oC and + 25oC and in transparent bottles at + 25oC and then analyzed. Previous valuable studies on olive oils explain either all phytochemical properties without the effect of storage or the change in some properties as a result of storage. As in this study, there is no other study in which the changes in many phytochemical properties as a result of storage are discussed together and more important components are tried to be highlighted through statistical studies, taking into account process variables.