The estimated amount of good quality and nutritive-value oils and fats used for frying around the world is around 20 million metric tons (MT). In frying, the hot oil serves as a heat exchange medium by which heat is transferred to the material being fried. As a result of frying, the oil darkens from the formation of polar materials such as minor phenolic components; elevated FFA; high total polar materials; compounds having high foaming property, low smoke point, low iodine value, and increased viscosity; and color compounds.

Sims [62] reported has conversion of tallow, a by-product of the meat industry, into esters. The fuel properties of methyl, ethyl, and butyl esters of tallow were similar to diesel fuel, particularly ME, which were remarkably similar except for the higher liquidification temperature of tallow esters. Short-term engine performance tests with methyl, ethyl, and butyl esters gave comparable results as diesel fuel, but at higher BSFC. Blends with diesel in 50-50% proportion by volume gave inter­mediate results between esters and neat diesel fuel.

Richardson et al. [63] have tested an engine with ME of tallow. Preliminary engine tests indicated that the use of 10% and 20% blends (volume basis) performed similar to diesel fuel. However, lubricant qual­ity aspects were not studied and an endurance test was not conducted. The ignition quality of the blend was significantly better than that of diesel. Overall, it was concluded that tallow ME on 10% (volume basis) can be successfully used as diesel fuel where large amounts of tallow are produced and temperatures below 10°C are not encountered. The fuel consumption of ME of used frying oil has been measured by Mittelbach and Tritthard [64]. The ester fuel showed slightly lower hydrocarbon and carbon monoxide emissions but increased oxides of nitrogen, compared with that of diesel fuel. The particulate emissions, however, were sig­nificantly lower for used frying oil. But, they suggest long-term engine testing to prove the quality of this fuel.

The results discussed contribute to a better understanding of the structure—physical property relationships in different fatty acid esters from different vegetable oils which give the desired biodiesel quality and optimal performance of engines.