## 2 Overview of Plant-Based Oils and Waste Cooking Oils  
### Plant-Based Oils  
Vegetable oils are primarily obtained from plant seeds and can be categorized as either edible or non-edible. The extraction process varies depending on the plant source and intended application, with different seeds exhibiting distinct oil yield rates. Examples of high-yield oil-producing plants are soybean seed, safflower seed, neon seed, and canola seed. Common extraction methods include solvent extraction, ultrasound-assisted extraction, and mechanical pressing techniques such as hot-press and cold-press. Among these, the hot-press method has been identified as particularly effective, especially for canola oil extraction, as it enhances yield and efficiency during production [9; 47]. Figure 1 presents a schematic illustration of the mechanical press extraction method [48]. To ensure the quality of oil is not compromised during the hot-press extraction process, parameter optimization was investigated in Ref. [48], focusing on pressing time and pressure. It was reported that the hydraulic cold-pressed extraction (HCPE) technique achieves a high oil yield of up to 86.31% within a short extraction time of 30 min. Although ultrasound-assisted extraction has been shown to produce a higher oil yield, it requires more time, is less user-friendly, and necessitates an additional solvent extraction step post-processing [48]. Since this work does not primarily focus on oil extraction methods, further details on optimal extraction techniques that preserve both the physical and chemical properties of the oil can be found in Refs. [47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57]. Vegetable oils have been widely utilized in various industries, including the production of fragrances, perfumes, and luxury soaps. Their application in the power sector as an alternative insulating liquid to mineral oil has gained prominence due to their biodegradability and reduced environmental impact in the event of transformer oil spills. The insulating properties of vegetable-based oils (natural esters) are summarized in Table 1, where they are compared with mineral oil. Notably, natural esters exhibit high fire and flash points, making them a safer choice for use in sensitive environments. Natural esters differ from mineral oils in terms of molecular structure. While natural esters are composed of triglyceride ester molecules, mineral oils consist of aromatic hydrocarbon molecules, as illustrated in Figure 2 [58]. Due to the molecular structure of natural esters, which contain unsaturated fatty acids, natural esters are more prone to oxidation compared to mineral oils. This susceptibility to oxidation remains a key challenge in their application as transformer insulating liquids [26]. It is important to state that while the oxidation mechanisms of mineral oil and natural esters are similar, they differ at the terminal reaction stage. In natural esters, free electrons cause molecules to bond with each other rather than forming non-soluble polar byproducts, as observed in mineral oils [59]. Consequently, oxidation in natural esters primarily results in increased viscosity. Notably, in free-breathing transformers, the viscosity should not exceed a 10% increase over a 10-year operational period [60]. The practical application of natural esters in transformer insulation has been in use for several decades across various regions, including both subpolar and hot climates. However, mineral oil remains the dominant insulating liquid [3,61]. The slow adoption of natural esters can be attributed to factors such as transformer design, regional climate conditions, compatibility issues, and, most notably, cost [62,63]. The high cost of natural esters, despite their environmental benefits, has driven researchers to explore the potential of waste cooking oils as a more affordable and sustainable alternative, bypassing the planting, harvesting, and extraction stages to reduce costs. Figure 1: Hydraulic process for the extraction of seed oils.