## 2 Overview of Plant-Based Oils and Waste Cooking Oils
### Plant-Based Oils
WCO exists in two distinct forms, yellow grease and brown grease, classified based on their free fatty acid (FFA) content. Yellow grease refers to waste cooking oil with FFA levels below 15%, while brown grease contains FFA levels exceeding 15% [67]. During food frying, particularly at temperatures between 170 and 200 \({}^{\circ}\)C, several chemical reactions occur in the oil, altering its composition and quality. The first reaction is hydrolysis, where moisture from the food breaks down triglycerides into glycerol, free fatty acids, monoglycerides, and diglycerides. This process affects the oil's stability and can lead to undesirable changes in taste and texture. As frying continues, primary oxidation takes place, forming unstable hydroperoxides that eventually degrade into secondary oxidation products such as aldehydes, ketones, and short-chain fatty acids, contributing to rancidity and unpleasant odors. With prolonged reuse, these secondary products can further react to form polymeric compounds, increasing the oil's viscosity and deteriorating its frying efficiency. At elevated temperatures, the oil also undergoes thermal polymerization, resulting in the formation of high molecular weight cyclic fatty acid monomers, dimers, and oligomers, which further compromise the quality and safety of the oil [68]. Continuous reuse of cooking oil, often driven by economic factors, poses significant health risks due to the accumulation of toxic compounds and oxidized materials such as polyglycerols, acylglycerols, dimeric acids, and polymeric acids [69]. The presence of these harmful substances makes reused oil unsuitable for consumption, as it has been linked to an increased risk of cardiovascular diseases, carcinogenicity, mutagenicity, genotoxicity, and cancer [68]. Given these serious health concerns, the reuse of cooking oil in households and the food industry is strongly discouraged. Figure 5 illustrates the relationship between waste cooking oil consumption and cardiovascular disease [68]. Concerns regarding the reuse of cooking oils have led to their widespread disposal, often inappropriately. Studies have shown that a significant portion of WCO is discarded through drainage systems, leading to water treatment issues such as sewer diameter reduction and blockages in infrastructure [70]. Additionally, more than 16.5 million tons of WCO are generated annually, with much of it being conventionally disposed of into sewage systems or discarded alongside municipal solid waste in landfills [71; 72; 73]. The spillage of WCO in landfills can contribute to water pollution due to its low solubility, which affects aquatic ecosystems by reducing dissolved oxygen levels and limiting sunlight penetration. Its immiscibility with water can also create a barrier that isolates soil from essential air and moisture, leading to the death of beneficial soil organisms such as earthworms and bacteria. Furthermore, the presence of WCO in the environment can attract rats and vermin, promoting their proliferation and posing further risks to human health [73]. Due to the challenges associated with waste cooking oil disposal, several innovative valorization strategies have emerged to convert WCO into green energy, thereby mitigating its environmental impact and reducing the carbon footprint. This approach aligns with multiple United Nations Sustainable Development Goals (SDGs), including SDG 3 (Good Health and Well-being), SDG 7 (Affordable and Clean Energy), SDG 11 (Sustainable Cities and Communities), SDG 13 (Climate Action), SDG 14 (Life Below Water), and SDG 15 (Life on Land) [69]. Historically,  
Figure 4: Used cooking oil production based on countries. WCO was used as livestock feed in Europe, but this practice was banned approximately 25 years ago due to its association with bovine spongiform encephalopathy (BSE) [74]. While WCO has been utilized in soap production, its application raises environmental concerns as wastewater disposal can introduce polycyclic aromatic hydrocarbons (PAHs) into the surroundings [75].