Fish oil is rich in polyunsaturated omega 3 fatty acids, which makes such oils and related dietary supplements very prone to oxidation. This creates primary and secondary oxidation products, which makes oxidized oils of lower nutritional quality or even harmful. The aim of the master's thesis was to determine the content of primary, i.e. peroxide value and secondary oxidation products, i.e. p-anisidine value and malondialdehyde as well as the content of vitamin E in dietary supplements with omega 3 fatty acids. A total of 30 samples were analyzed, each sample consisting of two independent repetitions. The obtained results were compared with quality standards, data from the literature and the values marked on the label. Out of all analyzed samples, 73 % exceeded the recommended limit for the peroxide value (5 meq O2/kg), and the value range was from 1,3 to 26,1 meq O2/kg. The recommended limit for p-anisidine value (≤ 20) was exceeded by one sample, and the range of values was from 20 to 51,8. The recommended totox limit (≤ 26) was exceeded by 26 % of the samples, with values ranging from 13,2 to 70,1. Determination of p-anisidine value and totox value was not possible for krill oil samples. None of the samples contained an amount of malondialdehyde that would indicate the onset of oxidation. The concentration of present vitamin E varied greatly among the samples, between 95 and 19682 mg/kg. The measured content of vitamin E in 4 samples was not compliant with the labelled value. Based on the results, we assume that the vitamin E content had no effect on the peroxide value and totox value, while higher amounts of vitamin E showed greater protection against the formation of secondary oxidation products.
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