Spectroscopic Variability and Optical Properties of Sudanese Edible Oils Using Spectroscopy Methods

Abstract

In this study, nondestructive test was applied on six types of edible oils (corn, extra virgin olive, sunflower, pressed sesame, factory sesame and peanuts) using spectroscopic methods. Raman spectroscopy and Fourier infrared spectroscopy were carried out to identify functional groups presence on oils while ultraviolet visible was used to study optical properties of edible oils. From Raman spectra of edible oils samples, the major bands can be found at (1084, 1266, 1303, 1441, 1657, 1749,) cm−1 . It is of particularly significance for edible oils that stretching of C=C bonds gave rise to strong Raman signals and C=O stretching bands were very weak; the contrary can be found in infrared spectra. FTIR spectrum of all samples in range from (400-4000) cm-1 . From comparison of IR and Raman spectra of edible oil samples, the frequencies of some characteristic peaks were identical, while the relative intensities differed markedly. On the contrary, Raman scattering signal at (560, 969) cm−1 was quite weak, but corresponding infrared absorption peak was obvious. The general appearance of the IR spectra of these vegetable oils was found to be very similar to a previously reported study. In the Raman spectrum, the spectrum of peanut and presses sesame differed from the previous studies. Thus, the combination of these two different methods has been very useful in the study of the oil which provides comprehensive information about the chemical compositions of edible oils. FTIR spectral data collected in MIR range 4000-400 cm-1 showed major peaks representing triglyceride functional groups which could be observed around 2925 cm-1 [C–H stretching (asymmetry)], 2854 cm-1 iv [C– H stretching (symmetry)], 1747 cm-1 [C=O stretching], 1463 cm-1 [C–H bending (scissoring)], 1238, 1163, 1118 and 1097 cm-1 [C–O stretching] and 722 cm-1 [C–H bending (rocking)]. A peak around 1653 cm-1 is attributed to C=C stretching (cis). A band shift at 3007 cm-1 ; assigned to C–H stretching vibration of cis- double bond (=C–H) characteristic to samples oils. The absorption spectrum of six important Sudanese oils can be divided into three main absorption regions the first region is from the beginning of the spectrum to 340 nm, the second region is (400 - 500) nm and the last region is from (600-800) nm. The shift in absorption peaks of these oils refer to different chemical group that oils contain. Absorption coefficient and refractive index of all oils under study were different because structure components of each oil were different and this confirm with Raman and FTIR results. High values of the absorption coefficient indicate the possibility of direct electronic transitions between the valence and conduction bands at these energies.