Fish oil was purchased from Industrias Afines, S.L. (Pontevedra, Spain) and presented the following properties: acid value of 16.4 mg KOH/g oil, peroxide value of 2.5 meq O2/kg oil and a composition of 32.7 wt.% in oleic acid, 11.7 wt.% in linoleic acid, 3.6 wt.% in EPA and 5.7 wt.% in DHA. Palm oil was donated by the company Lípidos Santiga S.A. (Barcelona, Spain) and presented an acid value of 16.3 mg KOH/g oil, peroxide value of 2.1 meq O2/kg oil and a composition of 44.5 wt.% in palmitic acid and 38.6 wt.% in oleic acid. Waste frying oil was collected from local domestic consumers (Granada, Spain) and had an acid value of 2.4 mg KOH/g oil, peroxide value of 4.6 meq O2/kg oil and a composition of 55 wt.% in oleic acid and 26.7 wt.% in linoleic acid. Sodium hydroxide (≥ 99%), potassium hydroxide in ethanol (0.1 M), n-heptane (≥ 99%) and methyl heptadecanoate (≥ 99%) were purchased from Sigma-Aldrich, while methanol (≥ 99.8%) and diethyl ether (≥ 99%) were acquired from Scharlau. Ethanol (≥ 99%) and anhydrous sodium sulfate (≥ 99%) were provided by Panreac, whereas sulfuric acid (95%) was purchased from VWR International. Phenolphthalein (1% in ethanol) was provided by J. T. Barker.