Ph.D. 2015- New Aspects on Stability of some Edible Oils

Abstract

Rania Ibrahim Mohammad Almoselhy: New Aspects on Stability of some Edible Oils. Unpublished Ph.D Thesis, Department of Food Science, Faculty of Agriculture, Ain Shams University, 2015. The present study was planned to evaluate the stability of four edible oils namely extra virgin olive oil (EVOO), moringa oil (MO), apricot kernel oil (AKO) and sunflower oil (SO). The oils were analyzed in order to identify the relation between their composition and stability, with great stress to investigate what components are responsible for their stability. Fourier transform infra red (FTIR) and nuclear magnetic resonance (NMR) spectroscopic determinations were also applied. MO showed the highest induction period (IP) measured by Rancimat being 190.00 hr indicating superior resistance to oxidation, followed by EVOO (59.11 hr), then AKO (26.00 hr). SO recorded only 7.45 hr. The highest resistance of MO to oxidation followed by EVOO was due to their less linoleate (LO), higher percentage of oleate (OL) and saponifiable matter, as well as antioxidant content of tocopherols, flavonoids, sterols, phenolics, chlorophyll, and carotenoids. In addition, MO contained the highest percentages of stigmasterol, campesterol, Δ5-avenasterol and δ-tocopherol which its antioxidant activity exceeds that of α- and γ-tocopherols and making clear the long IP of this oil. The highest percentages of β-sitosterol was found in EVOO, besides Δ5-avenasterol that may be mainly responsible for olive oil stability, also polyphenols were the main antioxidants in EVOO that approached half of its IP. AKO with middle IP had relatively high stability due to its higher percentage of OL, but it had higher LO and lower antioxidant content compared to MO and EVOO. The lowest IP of SO was due to its higher PUFA and lower antioxidant content. It seems that β-sitosterol and Δ5-avenasterol were also a main factor in AKO and SO oxidative stability. From FTIR spectral data of the tested oils, the shift of absorption peak around 3005 cm-1 ordered the oils, where the lowest shift was related to the lowest PUFA and the highest stability which arranged as follows: MO (3003.59 cm-1) ˃ EVOO (3004.55 cm-1) ˃ AKO (3005.52 cm-1) ˃ SO (3006.48 cm-1). This arrangement reflects the same order of oils stability evaluated by Rancimat method. Also, the higher in intensity of this peak around 3005 cm-1, the higher in USFA From this relation, it was easy to predict the order and the approximate values of the USFA in the oils. When the oil shows a peak around 3470cm-1, it is an indication that the oil is non-oxidized. The chemical shifts of solvent peaks around 2.5 and 3.3 ppm in the 1H-NMR spectra ordered the oils stability, where the lesser shift was corresponded to the oil of higher stability, this order was as obtained by Rancimat; it also agreed with the order of tocopherols content. MO showed the presence of both stigmasterol and β-sitosterol from the signals at 0.662 and 0.784 ppm, respectively. However, EVOO showed a peak at 0.780 ppm assigned to β-sitosterol. Some shifts were denoted in the spectra of the stored oils for 6 months attributed to free fatty acids indicating hydrolytic degradation of acyl groups and the formation of new compounds, as well as conjugated systems. Also, new signals were aroused after 60 months due to different kinds of alcohols, free fatty acids and diglycerides. All the 13C-NMR spectral data from the signals in the four regions of the tested oils, agreed with their GC and the other composition analysis carried out, and reflect the same composition and order of oils stability obtained by Rancimat method. In addition, its technique is particularly useful in distinguishing between mono-, di-, and triglycerides. In conclusion, FTIR and NMR spectroscopy proved their potency as new analytical methods to investigate the stability of some edible oils and the results obtained agreed with the old common methods of analyses.