Direct FTIR Analysis of Free Fatty Acids in Edible Oils Using Disposable Polyethylene Films

To simplify a previously reported Fourier transform infrared (FTIR) method for the direct determination of free fatty acid (FFA), a new technique was designed to facilitate mid-FTIR transmission analysis of viscous edible oil samples using disposable polyethylene (PE) films as a spectral acquisition accessory. In the PE-film-based procedures, the viscosities of oil samples were reduced by mixing hexane for allowing the samples to readily deposit onto PE films to form oil films when the solvent evaporated (PE film as spectral background), and the path length of each oil film was normalized to a fixed path length of 0.15 mm. The relationship between FFA content (expressed as percentage of oleic acid) and the absorbance of FFA at 1711 cm⁻¹relative to a baseline at 1600 cm⁻¹(baseline was 1600 cm⁻¹) in thenormalized spectra was expressed by a linear calibration equation. To remit the interference from the absorbance due to triglyceride esters (approximately 1,746 cm⁻¹) in the wave numbers used in FFA determination (1,711 cm⁻¹), the intensity of the first overtone of the ester vibration (3,471 cm⁻¹/3,527 cm⁻¹) in FFA-free oil samples was used to determine the intensity of the interfering signal at 1,711 cm⁻¹. Moreover, a linear relationship between the two absorbances was established. Consequently, the absorbance measured at 1,711 cm⁻¹/1,600 cm⁻¹by FFA determination in the oil samples was corrected for the linear equation. FFA determination in oil samples with known FFA content (0 to 3 %) was carried out in parallel by using the American Oil Chemists’ Society titrimetric method and the PE-film-based FTIR procedures. A comparison indicated that the PE-film-based FTIR method can be used to determine FFA content in different kinds of edible oils.