Abstract
Sunflower oil is extensively used in frying in Algeria as an alternative to olive oil due to its low cost. However, the high level of unsaturated fatty acids (FA) contained in sunflower oil enhances its susceptibility to oxidation. In our study, the sunflower oil was heated at 99±2°C with incorporation of 9 L of oxygen/second for 52 h continuously in the absence of foodstuff. Heating polyunsaturated fatty acids (PUFAs) in the presence of air causes a greater degree of lipid peroxidation. The oil oxidation degree was monitored through several physicochemical analyses. The products of thermal oxidation were monitored using UV- spectrophotometric method and Fourier transform infrared spectroscopy (FT-IR). Compared to fresh oil, the free-fatty acid contents, peroxide value, density and moisture of the thermally oxidized sunflower oil increased. In addition, the iodine and saponification values decreased during thermal treatment. The treatment applied had a negative effect on FA composition; the most significant effects were on C18:2, C18:1 and C16:0 contents. Analysis of chromatographic profile of thermoxidized sunflower oil showed a reduction in linoleic acid (LA) and an increase in oleic and palmitic acids; decrease of linoleic acid content is used as an indicator of lipid oxidation. Moreover, during the early stages, conjugated dienes (CDs), absorbing at 233 nm, were formed upon decomposition of hydroperoxides. In our study, the early stages of lipid oxidation were measured by UV-spectrophotometric method. Hydroperoxides broke down into secondary products and were revealed by FT-IR; these scission products are generally odoriferous by nature. The C=O stretching band at 1739-1724 cm-1 of the aldehydes was much more intense. Formation of conjugated double bond systems and the isomerisation of cis to trans double bonds was observed in the C=C stretching region at 980 to 965 cm-1. The results obtained reveal that even fresh oil contains products of peroxidation and isomerization of C18:2, n-6; indeed, CDs can be produced during the refining process of oil. The treatment applied increased the rate of these products and conferred a marked rancid taste and a thick texture to the thermoxidized sunflower oil. Key words: Sunflower oil, Linoleic acid, thermally oxidative treatment, alteration level.
Highlights
Sunflower oil is a high-quality edible oil
The study shows that compared to fresh oil, the free-fatty acid (FFA) content, peroxide value (PV), refractive index (RI), density and moisture of the oxidized sunflower oil increased from 0.093 to 1.25%, 5.83 to 152.5 meq/kg, 1.461 to 1.476, 0.910 to 0.985 and 0.100 to 2.006 respectively
Ketones and other secondary oxidation products, revealed by Fourier transform infrared spectroscopy (FT-IR) spectroscopy, in thermoxidized oil was accelerated by constantly bubbling air into the oil during heating (Figure 2)
Summary
Sunflower oil is a high-quality edible oil. It is used in cooking, frying, and in the manufacture of margarine and shortening and considered by some as desirable as olive oil. Sunflower oil was selected in this study due to its high use in food as it is a rich source of linoleic acid. It is light in taste and appearance and has a high vitamin E content compared to other vegetable oils (Shahidi et al 1992). At high temperature and in the presence of air, many chemical reactions can be observed in oil: hydrolysis, polymerization, oxidation and isomerization (Rossell, 2001).
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