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A new direct Fourier transform infrared analysis of free fatty acids in edible oils using spectral reconstitution

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Abstract

A new transmission-based Fourier transform infrared (FTIR) spectroscopic method for the direct determination of free fatty acids (FFA) in edible oils has been developed using the developed spectral reconstitution (SR) technique. Conventional neat-oil and SR calibrations were devised by spiking hexanoic acid into FFA-free canola oil and measuring the response to added FFA at 1,712 cm−1 referenced to a baseline at 1,600 cm−1(1,712 cm−1/1,600 cm−1). To compensate for the known oil dependency of such calibration equations resulting from variation of the triacylglycerol ester (C═O) absorption with differences in oil saponification number (SN), a correction equation was devised by recording the spectra of blends of two FFA-free oils (canola and coconut) differing substantially in SN and correlating the intensity of the ester (C═O) absorption at the FFA measurement location with the intensity of the first overtone of this vibration, measured at 3,471 cm−1/3,427 cm−1. Further examination of the spectra of the oil blends by generalized 2D correlation spectroscopy revealed an additional strong correlation with an absorption in the near-infrared (NIR) combination band region, which led to the development of a second correction equation based on the absorbance at 4,258 cm−1/4,235 cm−1. The NIR-based correction equation yielded superior results and was shown to completely eliminate biases due to variations in oil SN, thereby making a single FFA calibration generally applicable to oils, regardless of SN. FTIR methodology incorporating this correction equation and employing the SR technique has been automated.

SR spectra of FFA-free canola oil and coconut oil, illustrating their relative absorption profiles in the FFA measurement region (1,712 cm−1) (A) as well as the relative intensities of the first overtone of the triacylglycerol ester linkage ν(C═O) absorption (B), demonstrates that the correlating band (3,471 cm−1) can overcome oil dependency in FTIR FFA analysis of edible oils

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Acknowledgments

The authors would like to thank the Fundamental Research Funds for the Central Universities (QN 2009074) and the Young Core Instructor Support of Northwest A&F University. Thanks also go to Mr. David Pinchuk, President of Thermal-Lube Inc., for providing access to the COAT oil analysis system.

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Authors and Affiliations

  1. College of Food Science and Engineering, Northwest A&F University, 28 Xinong Road Yangling, 712100, Shaanxi, China

    Xiuzhu Yu

  2. Department of Food Science and Agricultural Chemistry, McGill University, 21,111 Lakeshore Road, Sainte-Anne-de-Bellevue, QC, H9X 3V9, Canada

    F. R. van de Voort & Jacqueline Sedman

  3. College of Science, Northwest A&F University, 22 Xinong Road Yangling, 712100, Shaanxi, China

    Jin-ming Gao

Authors
  1. Xiuzhu Yu
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  2. F. R. van de Voort
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  3. Jacqueline Sedman
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  4. Jin-ming Gao
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Correspondence to Xiuzhu Yu or Jin-ming Gao.

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Yu, X., van de Voort, F.R., Sedman, J. et al. A new direct Fourier transform infrared analysis of free fatty acids in edible oils using spectral reconstitution. Anal Bioanal Chem 401, 315–324 (2011). https://doi.org/10.1007/s00216-011-5036-x

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  • DOI: https://doi.org/10.1007/s00216-011-5036-x

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