Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is a fatal neurodegenerative disease of cattle. BSE has attracted wide attention because it can be transmitted to humans. Pathogenic prions are responsible for transmissible spongiform encephalopathies (TSE), and especially for the occurrence of a new variant of Creutzfeldt-Jakob disease (nvCJD), a human brain-wasting disease. Due to this problem, the specified risk material is burned under high temperatures to avoid any hazards for humans and animals. However, another possibility could be to consider this material as a source for producing biodiesel by transesterification. In fact, production of biodiesel from the risk material could represent a more economic usage than its combustion. Siedel et al. have found that almost every single step of the process leads to a significant reduction in the concen­tration of the pathogenic prion protein (PrPSc) in the main product and by-products. They concluded that biodiesel from materials with a high concentration of pathogenic prions can be considered safe [163]. Animal fats, such as tallow or lard, have been widely investigated as a source of biodiesel [164-169]. Muniyappa et al. have found that transesterifi­cation of beef tallow produced a mixture of esters with a high concen­tration in saturated fatty acids, but with physical properties similar to esters of soybean oil [37]. Ma et al. found that 0.3% NaOH completed methanolysis of beef tallow in 15 min [170]. Some authors have found that absolute ethanol produced higher conversion and less viscosity than absolute methanol at 50°C, after 2 h [171]. Nebel and Mittelbach have found n-hexane was the most suitable solvent for extraction of fat from meat and bone meal. The extracted material was converted into fatty acid methyl esters through a two-step process [172]. Lee et al. have performed a three-step transesterification to produce biodiesel from lard and restaurant grease. They found that a porous substance, such as silica gel, improved the conversion when more than 1 M methanol was used as reaction substrate [173]. Mbaraka et al. also synthesized propylsulfonic acid-functionalized mesoporous silica materials for methanol esterification of the FFA in beef tallow, as a pretreatment step for alkyl ester production [174].

Engine tests also showed a reduction in emission, except oxides of nitrogen that increased up to 11% for the yellow grease methyl ester [157]. Cold-flow properties of the fat-based fuels were found to be less desirable than those of soy-based biodiesel, with comparable lubricity and oxidative stability [175]. To solve this problem, Kazancev et al. blended up to 25% of pork lard methyl esters with other oil methyl esters and fossil diesel fuels. In this case, the CFPP showed a value of — 5°C. In winter, only up to 5% of esters can be added to the fuel. Depressant Viscoplex 10-35 with an optimal dose of 5000 mg/kg was found to be the most effective additive to improve the cold properties [101].