CC BY
3*
ALT'23
The 30th International Conference on Advanced Laser Technologies
LD-O-8
Raman spectroscopy as an effective tool for evaluating the iodine values and carotenoid content in vegetable oils
S. Kuznetsov1, V. Novikov1, P. Laptinskaya1, O. Persidskaya1, E. Sagitova1
1-Prokhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilov St., 119991 Moscow, Russia
Vegetable oils are widely used in food industry, cosmetology, and medicine. Currently, the development of fast and efficient methods for monitoring their quality is relevant. For example, the thermal stability and shelf life of oils depend on the content of unsaturated fatty acids (UFA). The property of oils to increase the stability and bioavailability of carotenoids, that are vital for human health, is used in the creation of biologically active additives.
In this work, we apply Raman spectroscopy for assessing the iodine values (IV, the main measure of the number of C=C bonds in UFA) and carotenoid content in vegetable oils. Experimental Raman spectra of 35 various vegetable oils were recorded at the laser excitation with the wavelength of 532 nm. In addition, we used the Margoshes titrimetric method to obtain the exact IV of oils.
Figure 1 shows the Raman spectra of marula and tomato seed oils, which IV differ by about a factor of 1.5. The spectra of saturated triglyceride C8/C10 (a fraction of coconut oil which does not contain UFA), and of unsaturated sorbic acid are shown for comparison.
1157 1266 1440 1518 165S
K
Sorbic acid
Tomato seed oil (IV = 115 g,/100 goi]) Marula oil (IV = 85 g,/100 gnil) Triglyceride C8/C10
Wavenumber, cm 1
Figure 1. Raman spectra of marula and tomato seed oils, sorbic acid and triglyceride C8/C10 (a fraction of coconut oil).
As can be seen from Figure 1, an increase in iodine values leads to a growth of the peak intensities of Raman lines at about 1266 and 1658 cm-1, assigned to the bending vibrations of C-H bonds and stretching vibrations of C=C bonds in UFA molecules, respectively. We found that the ratios of the intensity of each of these lines to the intensity of the line at about 1440 cm-1 (bending vibrations of C-H bonds) depend on the IV of oils. In this study, we showed that for oils with IV in the range of 60 - 165 g^/100 goil, these dependences are linear. We demonstrated that recording the Raman spectra at the excitation wavelength of 532 nm makes it possible not only to evaluate IV but also allows to carry out additional diagnostics of the carotenoid content at very low concentrations. This is due to the resonant enhancement of the lines assigned to the stretching vibrations of C-C (at 1155 cm-1) and C=C (at 1525 cm-1) bonds in carotenoid molecules in this case. We established that applying the Raman spectroscopy makes it possible to detect carotenoids in studied vegetable oils with the contents less than 0.4 mg/kg.
In this work, we also showed that the use of an excitation wavelength of 532 nm provides possibility to record Raman spectra with an insignificant fluorescence background (as it is in the case of excitation at 785 nm wavelength, traditionally used when analyzing oils).
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