Determination of Free Fatty Acids in Edible Oils by Continuous-Flow Analysis with FT-IR Spectroscopic Detection

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Abstract
A flow injection system with Fourier transform infrared spectroscopic detection was developed for the rapid determination of the free fatty acid content in edible oils. The method is based on complete deprotonation of the free fatty acids and evaluation of the intensity of the vas-(COO−) band formed at 1570 cm−1 with respect to a baseline point at 1534 cm−1. Upon deprotonation the absorption of the carboxylic acid moieties is shifted toward lower frequency, thus avoiding spectral interferences of the v-C=O from esters, ketones, or aldehydes also present in the sample. Optimum operational conditions were obtained by merging the sample stream first with a carrier stream of 25% (v/v) toluene in 2-propanol followed by merging with a 0.3 M solution of KOH in the same solvent mixture and measurement of the obtained solution after a short (2 s) reaction time. In this way saponification of the oil could be reduced to a minimum, whereas deprotonation was already completed. The obtained calibration curve was linear from 0 to 2.5% oleic acid, and the coefficient of variation of the method was 3.3%. The developed method was applied to the determination of the total amount of free fatty acids in olive, sunflower, and corn oils, delivering results that were in good agreement with those obtained by the official reference method proposed by the European Community. Furthermore, due to its high degree of automation and throughput (40 samples h−1), the developed method is suitable for quality control in routine applications.