Food Chem., 2018., Nov 15;266:524-533. doi: 10.1016/j.foodchem.2018.05.117.

Oxidative stability of flaxseed oil: Effect of hydrophilic, hydrophobic and intermediate polarity antioxidants.

Mohanan A, Nickerson MT, Ghosh S.

Oxidative deterioration is a major issue limiting the utilization of flaxseed oil (FSO). Present study explored the effects of hydrophilic (tannic acid), hydrophobic (alpha-tocopherol), and intermediate polarity (ascorbyl palmitate) natural antioxidants, which displayed highest DPPH radical scavenging and iron chelating abilities among several others, on the oxidative stability of FSO. A synthetic antioxidant (TBHQ) was also used as a control. FSO oxidation was examined by peroxide and p-anisidine values during 30-day storage at 25, 40 and 60 °C, and by accelerated oxidation using a rancimat at 110 °C. On mass concentration basis, all natural antioxidants were less effective than TBHQ. Irrespective of the polarity, all natural antioxidants, except alpha tocopherol, delayed primary and secondary oxidation, and increased the oxidative stability index. The alpha-tocopherol displayed pro-oxidant effect at all concentrations. Rather than polarity, antioxidant capacities, and their ability to replace minor components from the oil-water interface were crucial for the protection of FSO.

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The selection of natural antioxidants for the prevention of lipid oxidation is challenging because it depends on multiple factors such as, fatty acid profile of the oil, minor components present in the oil and structure and functionality of the antioxidants themselves etc. Mechanism of oxidation of oils vary with their fatty acid profile. Surface active minor components (e.g., phospholipids, mono and di-glycerides and free fatty acids) can form structures with moisture to make them compete with antioxidants to be at the oil-water interface, where the oxidation is significant. With increasing chain length, the antioxidant capacity of esters of several phenols such as caffeic acid, vanillin acid, ferrulic acid etc. on bulk oils increases. In the present study, three hydrophilic (tannic acid, caffeic acid, and ascorbic), three hydrophobic (alpha tocopherol, eugenol and beta carotene) and two intermediate polarity (quercetin and ascorbyl palmitate) natural antioxidants were initially selected. Based on their free radical scavenging and iron chelating abilities, one antioxidant each from the hydrophilic, hydrophobic and intermediate polarity category was examined for their ability to prevent FSO oxidation during long term storage at different temperatures and concentrations, and using an accelerated oxidative test.

The study showed that the rate of peroxide formation and its conversion into secondary oxidation products in the FSO was determined by the nature and composition of the minor components and the antioxidant activity. It was found that the secondary antioxidant activity such as metal chelating ability, and free radical scavenging capacity were crucial for the antioxidants to protect the FSO. Natural antioxidant ascorbyl palmitate with intermediate polarity was equally good as the synthetic antioxidant, TBHQ, on a molar concentration basis, in preventing peroxide and secondary oxidation product formation and in increasing the oxidative stability index of the FSO due to their high free radical scavenging ability and poor interaction with metal ions. On the other hand, hydrophilic tannic acid, which is not well soluble in the oil, was not able to prevent lipid oxidation in FSO. Alpha-tocopherol, although oil soluble, acted as a pro-oxidant and could not protect the FSO from oxidation. Finally, it can be concluded that knowledge on antioxidant capacity, their ability to prevent peroxide formation, the composition of the minor components in the oil as well as the mechanism of action of different antioxidants in the presence of minor components are vital for the effective selection of antioxidants to prevent FSO oxidation.