Determination of fatty acids with conjugated C=C bonds in pomegranate seed oil using Raman spectroscopy

For the first time, we suggest a Raman spectroscopic method for rapid evaluation of punicic acid content in pomegranate seed oil. In this contribution, we study Raman spectra of mixtures of pomegranate seed oil and sunflower oil as model systems of pomegranate seed oil with different punicic acid content using two excitation wavelengths (532 and 785 nm). The spectrum of punicic acid, calculated using the density functional theory, is also considered. We found that the ratio I₁₆₂₈/I₁₄₄₂ of the peak intensities of Raman lines at 1628 and 1442 cm^–1 is proportional to the punicic acid content. The line at 1628 cm^–1 is related to the stretching vibrations of double carbon-carbon bonds (C=C) in the sequence of three conjugated bonds in punicic acid molecule. The line at 1442 cm^–1 belongs to the scissoring C–H vibrations in CH₂ groups in molecules of all fatty acids of pomegranate seed oil. We revealed that Raman spectroscopy permits detecting punicic acid even at very low contents (up to 1 wt.%). In addition, we found that the record-ing Raman spectra using excitation wavelength of 532 allows one to detect carotenoids in pomegranate seed oil.

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