Determination of olive oil free fatty acid by Fourier transform infrared spectroscopy

A new procedure for determining free fatty acids (FFA) in olive oil based o n spectroscopic Fourier transform infrared-attenuated total reflectance spe ctroscopy measurements is proposed. The range of FFA contents of samples wa s extended by adding oleic acid to several virgin and pure olive oils, from 0.1 to 2.1%. Calibration models were constructed using partial least-squar es regression (PLSR). Two wavenumber ranges (1775-1689 cm(-1) and 1480-1050 cm(-1)) and several pretreatments [first and second derivative; standard n ormal variate (SNV)] were tested. To obtain good results, splitting of the calibration range into two concentration intervals (0.1 to 0.5% and 0.5 to 2.7%) was needed. The use of SNV as a pretreatment allows one to analyze sa mples of different origins. The best results were those obtained in the 177 5-1689 cm(-1) range, using 3 PLSR components. In both concentration ranges, at a confidence interval of alpha = 0.05, no significant differences betwe en the reference values and the calculated values were observed. Reliabilit y of the calibration vs. stressed oil samples was tested, obtaining satisfa ctory results. The developed method was rapid, with a total analysis time o f 5 min; it is environment friendly, and it is applicable to samples of dif ferent categories (extra virgin, virgin, pure, and pomace oil).