## 1 Introduction  
Animal byproduct production, as part of the meat and poultry processing chain, is huge. For instance, it represents nearly 17 million tons per year only in the European Union [1]. Most of the waste results from over 328 million pigs, sheep, beef, goats and dairy cattle and 6 billion chickens, turkeys and other poultry that are slaughtered every year in Europe [2]. After rendering, materials classified as edible which amount up to 12 million tons, are processed in a variety of food and feed related sectors [3]. The remaining byproducts that are considered inedible have other applications for disposal such as biofuels and biodiesel for energy generation [4,5]. It is energy generation, especially biodiesel production, that is one of the most attractive and expanding applications [6]. In this sense, the production of biodiesel guarantees a better profitability of inedible animal byproducts. Animal waste also consists of the organic matter resulting from the meat processing industry as well as from human consumption. Biodiesel consists of mono-alkyl esters of long chain fatty acids produced from oil or fat, but the use of vegetable oil adds a high price to biodiesel, and this has prompted the use of animal fats as an interesting alternative. In addition to its renewability, biodiesel also constitutes an attractive alternative because it offers better lubricating properties than fossil diesel fuel and it is biodegradable and non-toxic. It also has an improved cetane number and high flash point [7]. Biodiesel also contributes to sustainability by reducing the carbon footprint due to lower CO\({}_{2}\) emission compared to fossil diesel fuel [8]. CO\({}_{2}\) is one of the most relevant gases because it contributes up to 72% of greenhouse gases [9]. Biodiesel from animal fat achieves nearly 80% fossil CO\({}_{2}\) reduction in comparison to 30% for soya [5]. In addition, the emission of polycyclic aromatic hydrocarbons is 75-90% lower than in conventional diesel, whereas total unburned hydrocarbon is 90% lower [10; 11]. The emissions of sulfur and CO are also reduced [12]. The International Energy Agency reported that bioenergy must increase up to 25% until 2025 and continue to grow, and is estimated to reach 30% of the world's road transport fuel mix by 2050 [13; 14]. Biodiesel is mostly produced from vegetable oils and animal fats and has been the object of research and development in recent decades. Research is directed towards the use of low grade feedstock, with the possibility of reusing catalysts and improving efficiency of reactors for transesterification [15]. In fact, oil and fat materials used as raw materials are of high relevance because they are estimated to represent 60-80% of the total cost of biodiesel production [16]. Therefore, it is important to select the best materials in each case because they are affected by the geographic location, climate and agriculture [11]. Several countries such as Malaysia, Indonesia, Argentina, USA, Brazil, and the Philippines, as well as countries in the EU, are using biodiesel as a good source of renewable and biodegradable energy [17]. A consumption of 17.4 million liters of biodiesel was reported in 2019 with 63% of this consumption by France, Germany, Spain, Sweden and Italy [18]. The total biodiesel world production in 2019 was approximately 35 to 45 million tons and is steadily increasing year by year [19]. The European Union is the world's largest biodiesel producer and, on an energy basis, represents nearly 75% of the total transport biofuels market. In fact, the European biodiesel industry has more than 202 plants and production in 2019 exceeded 14 million tons of biodiesel [18; 20]. US production of biodiesel was more than 5.6 million tons in 2019 and came from 91 plants with a capacity of 8.3 million tons per year [21; 22]. Biodiesel is usually blended up to 20% with fossil diesel fuels in most countries due to its complete miscibility and the unnecessary need for engine modification at such percentage.