From Waste to Resource: Compositional Analysis of Olive Cake's Fatty Acids, Nutrients and Antinutrients  
[PERSON]  
1 Animal Science Department, University of Tias-os-Montes e Alto Douro (UTAD), 5000-801 Vila Real, Portugal; [EMAIL_ADDRESS] (T.J.); [EMAIL_ADDRESS] (C.G. ); [EMAIL_ADDRESS] (M.J.G. ); [EMAIL_ADDRESS] (D.O.-M.)  
[PERSON]  
3 Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Tras-os-Montes e Alto Douro (UTAD), 5000-801 Vila Real, Portugal; [EMAIL_ADDRESS] (M.J.); [EMAIL_ADDRESS] (C.G. ); [EMAIL_ADDRESS] (M.J.G. ); [EMAIL_ADDRESS] (D.O.-M.)  
[PERSON]  
1 Animal Science Department, University of Tias-os-Montes e Alto Douro (UTAD), 5000-801 Vila Real, Portugal; [EMAIL_ADDRESS] (T.J.); [EMAIL_ADDRESS] (C.G. ); [EMAIL_ADDRESS] (M.J.G. ); [EMAIL_ADDRESS] (D.O.-M.)  
[PERSON]  
5 Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Tras-os-Montes e Alto Douro (UTAD), 5000-801 Vila Real, Portugal; [EMAIL_ADDRESS] (M.  
The olive tree is renowned for its fruit, widely used in producing olive oil, making it a valuable cash crop in these countries. This, in turn, makes the olive industry a critical factor in developing the agro-industrial sector and a social and economic driver for the EU's southern regions [4; 5]. In 2023, there were 104 olive oil mills in Tras-os-Montes and Alto Douro (23% of the mills in Portugal) (National Institute of Statistics). Olive oil production in the 2019-2020 campaigns at Portuguese mills is estimated to have reached 140,500 tons, an all-time high (SIAZ, 2020). During the extraction process, 1000 kg of olives produces about 800 kg of waste [6]. Olive oil extraction involves several stages, such as olive washing, grinding, beating, and extraction, which is the fundamental step [7]. However, improper disposal of this by-product can represent a potential environmental problem [8]. Among the stages in the production of extra virgin olive oil (EVOO), the extraction phase has plenty of room for improvement to increase yield, product quality and the environmental burden of the entire production cycle [9]. In the traditional separation process, the resulting paste is subjected to pressure between two mats, resulting in the solid part and a liquid fraction of oil and water, known as pomace or crude olive cake. This by-product consists mainly of seed oil and is, therefore, the richest in energy (around 10.4% EE) [10; 11; 12]. Over time, this liquid fraction can be separated into two distinct parts. However, in recent decades, the olive oil industry has taken over centrifugation, offering three-phase and two-phase systems (Figure 1) [7; 13]. The fundamental difference between the two systems lies in the presence or absence of added water during extraction. In the three-phase system, a significant amount of water (usually around 50%) is added to the paste of crushed olives before centrifugation, generating three fractions at the end of the process: solid (olive skin or olive cake) and two liquids (oil and wastewater) [7; 14]. This has been the main drawback of this extraction system since these waters are highly polluting [15]. In the two-phase system, no water is added during the extraction process, which generates two distinct phases: oil and olive cake. Despite being labelled as an ecological system due to the significant reduction in the generation of wastewater and its contaminant load, it still generates very wet solid waste as a by-product [14; 15; 16]. Some mills still carry out the procedure of dehydrating the resulting olive cake followed by extraction of the remaining oil (which represents around 2% of its weight) using a solvent. Hexane is the solvent usually used for this industrial solid-to-liquid extraction and the resulting by-product, exhausted olive cake (EOC), contains smaller fragments of pulp, skins, seeds and stones, retaining around 10% moisture content [17; 18; 19].