COMPARATIVE STUDY OF THE BIOLOGICAL VALUE AND OXIDATIVE STABILITY OF WALNUT AND PUMPKIN-SEED OILS
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2019
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The work is devoted to the study of the biologically active components and the oxidation stability of oils made from non-traditional raw materials such as walnuts and pumpkin seeds. The characteristics that have been determined are the content of phospholipids, carotenoids, chlorophylls, tocopherols, and sterols, the composition of fatty acids and tocopherol homologues, the acidity and oxidation stability of walnut and pumpkin-seed oils. Walnut and pumpkin-seed oils contain a significant amount of polyunsaturated fatty acids, in particular, walnut oil contains linolenic acid and has the ratio ω-3:ω-6 of polyunsaturated fatty acids, which is close to the recommended ratio. The linoleic (polyunsaturated, ω-6) and oleic (monounsaturated) fatty acids dominated in the fatty acid composition of pumpkin-seed oil, and the sum of saturated fatty acids was three times as high as that in walnut oil. An important property of walnut oil is a very high ratio of ω-3:ω-6 polyunsaturated fatty acids, 1:5, which is almost what is recommended by dietitians for the human diet. The difference in the total tocopherol content of the two oil samples was slight, but the composition of tocopherol homologues was very distinctive, i. e. β-tocopherol was the main homo;ogue in the walnut oil and α-tocopherol in the pumpkin-seed oil, respectively. The acidity of the oil samples increased quite rapidly, reaching the value close to 4 mg KOH/g of in 63 days for walnut oil, and in 70 days for pumpkin-seed oil. The oxidative stability of the two oil samples was estimated by changes of the peroxide value during 98 days of oil storage. It has been shown that the induction period of walnut oil oxidation, defined as the start of an increase of the peroxide index, was 56 days, in spite of a high content of polyunsaturated fatty acids, particularly, linolenic acid. The duration of the induction period of pumpkin-seed oil oxidation and the shelf life of this oil were 70 and 98 days, respectively, while the shelf life of walnut oil was about 90 days. The higher resistance of pumpkin-seed oil to oxidative damage is primarily due to the fatty acid composition of this oil, namely to a high content of saturated and monounsaturated fatty acids and almost twice as low a content of polyunsaturated fatty acids compared to that of walnut oil. Both oils can be recommended as a valuable source of polyunsaturated fatty acids, antioxidants, and vitamins for human nutrition.