A comparison of used cooking oils: a very heterogeneous feedstock for biodiesel

Authors: Knothe, Gerhard; Steidley, Kevin

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2008
Publication Date: 7/9/2009
Citation: Knothe, G.H., Steidley, K.R. 2009. A comparison of used cooking oils: a very heterogeneous feedstock for biodiesel. Bioresource Technology. 100(1):5796-5801

Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. Used oils such as those obtained from restaurants are of interest because they are an inexpensive source of feedstock. However, the heat and exposure to air and moisture oils and fats experience during frying and cooking changes their composition. This work investigates how the composition of fats and oils change during use in a restaurant and how this may affect fuel properties of biodiesel derived from them. It was found that changes are of random nature and properties could not be quantitatively correlated among themselves. Thus, the properties of biodiesel from these feedstocks will vary considerably.

Technical Abstract: The increased interest in and use of biodiesel renders the availability of a sufficient supply of feedstock ever more urgent. While commodity vegetable oils such as soybean, rapeseed (canola), palm and sunflower may be seen as "classical" biodiesel feedstocks, additional feedstocks are needed to meet the demand for biodiesel. Used frying oils have the advantage of being inexpensive feedstocks, however, they are of lower quality than refined vegetable oils. Another major issue facing biodiesel is the properties of the fuel derived from a certain feedstock. Feedstocks with greater amounts of polyunsaturated fatty acid chains may lead to biodiesel with oxidative stability problems, while feedstocks with greater amounts of saturated fatty acid chains may lead to biodiesel with poor low-temperature properties. In this work, used frying oils obtained from sixteen local restaurants were investigated regarding their fatty acid profile vs. the fatty acid profile of the oil or fat prior to use. The fatty acid profiles were analyzed by gas chromatography and proton nuclear magnetic resonance spectroscopy. Besides the fatty acid profile, the acid value and dynamic viscosity of the samples were determined. Dynamic viscosity was determined because of non-Newtonian behavior of some samples. The results indicate that oils and fats experience various degrees of increase in saturation during cooking/frying use, with the magnitude of these changes varying from sample to sample, i.e., a high degree of randomness of composition is found in used frying oil samples. Properties of the samples that were investigated were acid value and viscosity which consistently increased with use, also in a random fashion. Multiple independent samples obtained from the same restaurants indicate that there is little consistency of used cooking oil obtained from the same source. These results are discussed with regards to the potential fuel properties of biodiesel derived from these used frying oils.