FTIR determination of free fatty acids in fish oils intended for biodiesel production

Biodiesel is commonly derived from vegetable oils and animal (fish and livestock) fats by alkali- or lipase-catalyzed transesterification reactions. Since free fatty acid (FFA) content is a critical parameter in the conversion of fish oils to methyl esters, the performance of a Fourier transform infrared (FTIR) spectroscopic method was assessed as an alternative to the conventional AOCS titrimetric method. The FTIR method involves the simultaneous extraction of FFAs and their stoichiometric conversion to their salts using a weak base, sodium hydrogen cyanamide (NaHNCN) dissolved in methanol, followed by measurement of the carboxylate band, ν(COO^?), at 1573 cm^?1 relative to a baseline at 1820 cm^?1 in the differential spectrum of the methanol extract. With minor modifications, this method was found to be capable of responding linearly to oleic acid (0–6.5%) addition, producing a FFA calibration equation having a S.D. of ±0.014% FFA. FTIR and titrimetric analytical results were compared for samples prepared by standard addition as well as for fish oils extracted from salmon skin which had been stored up to 120 days at ?20 °C. Both methods responded in a comparable manner; however, the FTIR method was more reproducible and accurate as well as simpler to carry out and was deemed to be a better primary method than the titrimetric method. The FFA content of Atlantic salmon (Salmo salar) skin lipids increased linearly from ～0.6% to 4.5% within 120 days, likely as a result of autoxidation. It was concluded that the NaHNCN-based FTIR method is a flexible, viable instrumental alternative to the AOCS titrimetric procedure for the determination of FFA content of fish tissue lipids destined for biodiesel production.