Analytical Methods for Determination of the Oxidative Status in Oils
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In industry today standard oxidative quality parameters are based on measurements of primary and secondary oxidation products, measured by PV and AV respectively. These methods are all prone to limitations and weaknesses, and their suitability for application on marine oils is not well documented. An increase in fish oil products with added flavor, color compounds, antioxidants and vitamins has entered the market in recent years. However, no documentation on the effect of these additives on the oxidation parameters has been found. The aim of this thesis was therefore to study the effect of variations in procedures in an attempt to highlight weaknesses and further to establish the most suitable procedures for each method when performing measurements on marine oils. In addition, the effect of antioxidants and additives on the oxidation parameters in cod liver oil has been evaluated.In this thesis, PV measurements by iodometric titration and by the ferric thiocyanate method were used to measure primary oxidation products, and AV and TBARS measurements were used to measure secondary oxidation products. Uncertainty of the methods was determined by performing n -measurements at different stages of the oxidation process. Measurement by the iodometric titration method was found to have a lower detection limit of PV >2.0 mEq peroxide kg-1 oil, with an uncertainty of ± 2%. Measurement by the ferric thiocyanate method was found to have a lower detection limit of PV ≥3.6 mEq peroxide kg-1 oil, with an uncertainty of ±10%. Measurement by the AV method was found to have a lower detection limit of AV≥ 1.3, with an uncertainty of ±5%. Measurements by the TBARS method was found to have a lower detection limit of 0.7 μM TBARS /g sample, with an uncertainty of ±12%. The published method of the International Dairy Federation (IDF) for PV determination was evaluated by comparison with a modified version of the method. Factors such as type of solvent used, deaeration of reagents, premixing of reagents and addition of antioxidant were differences between the methods. It was observed significant difference in absorbance in the two methods, and it was therefore concluded that the varied factor had an influence on the method. It is necessary to perform further experiments to determine which of the varied factors that cause variations in the absorbance measurements.PV measurements by the iodometric titration method were found to be influenced by the stirring method, reagent reaction time and oxygen removal. Stirring by magnetic stirring was found to give a higher PV compared to gentle stirring. The importance of the 1 minute reagent reaction time was strengthened as the PV was found to rapidly increased at prolonged reagent reaction times. It was demonstrated that this to a higher degree is important for marine oils compared to vegetable oils, as new hydroperoxides are formed more rapidly in the unstable marine oils. A significant influence of oxygen removal in reagents was detected in cod liver oil. The findings in this thesis suggests that stirring by magnetic stirring, 1 minute reagent reaction time and deaeration of all reagents should be standard procedure when PV is determined in marine oils by the iodometric titration method.Among eight investigated antioxidants and additives, Q10, tocopherol, vitamin K1, lemon and peppermint extract was found to significantly elevate the PV measured by iodometric titration. For PV determination by the ferric thiocyanate method, lemon extract was found to significantly elevate the PV. Rosemary extract was found to significantly lower the AV measurement, while lemon extract to a very high degree elevated the AV measurement. In measurements by the TBARS method only lemon extract was found to significantly interfere with the method, leading to an elevated TBARS value. Both methods for PV detection were influenced by several of the investigated antioxidants and additives. Clearly there is need for reevaluation of the methods is use today and development of new methods. New methods for measurements of secondary decomposition products are especially needed for fish oils with added lemon extracts as today s measurements by the AV and TBARS method give highly unreliable results.