Abstract:
This work was carried out to trace the source, and to determine the metal
contamination levels, which have harmful effects on the stability of some Sudanese vegetable oils: sesame seed, groundnut, cotton seed and sunflower.
The highest metal concentration was found for lead to reach 1.087 ppm and 0.291 ppm in crude groundnut oil and sesame seed oil, respectively,
in comparison with low standard limit 0.1 ppm of maximum. The nickel content in groundnut was 0.399 ppm, compared with the low standard limit of 0.2 ppm maximum. The highest metal concentrations were generally found in crude oils that were traditionally produced by locallymade small mills.
The metal contents of vegetable oils, extracted either by solvents or
mechanical pressure, were determined by atomic absorption spectrophotometry.
At high temperatures, the alloy, from which the mechanical pressing mills (whether traditional, small-scale, or modern large-scale), or oil
containers, or both, were made, were the main source of metal
contamination. The effect of heavy metals on the oxidative stability of the vegetable oils
were also examined by measuring physio-chemical properties.
The obtained results indicated that, the peroxide values increased gradually in all oil samples during storage. The highest increase in peroxide value was found in sunflower oil sample containing 5 ppm of Pb
of 239.122 meq / Kg at the end of the storage period. The acid value also
increased gradually in all oil samples during storage. The highest increase
in acid value was found in groundnut oil containing 5 ppm of Cu 17.935 mg KOH / g. The iodine values of all oil samples, however, were reduced gradually; the highest decrease in iodine value was found on thegroundnut oil sample containing 5ppm of Cu which reduced to 61.952 mg iodine/ 100g fat.
The refractive indeces were increased gradually in all treated oil samples
(containing 5ppm each of Fe, Ni, Cu and Pb). The highest increase in refractive index was found in sesame seed oil samples containing 5 ppm of Cu of 1.48725.
In conclusion, storage for long time-periods (nine month) at ambient temperatures (36C°) caused undesirable changes in the physical and chemical properties of edible vegetable oils contaminated with traces of heavy metals: Fe, Ni, Cu and Pb; it reduced further the nutritive value of the stored oils through the reduction of their unsaturated fatty acid contents.
