Improving oxidative stability of flaxseed oil by microemulsified polar antioxidants

In this study, the effect of plant extracts and standard antioxidants on the oxidative stability of water-in-flaxseed oil microemulsions was investigated. Fatty acid profile, tocopherol composition, total carotenoid content of cold-pressed flaxseed oil (FO) and antioxidant capacity of the plant extracts used in the study was determined. The emulsification was managed by magnetic stirring and the water content of the oils was adjusted to 1 ml/L. Pomegranate peel, rosemary and grape seed water extracts and standard antioxidants gallic acid and α-tocopherol were added separately to these emulsions. The prepared microemulsions were allowed to oxidize at 50 °C in an oven for 20 days. At intervals, peroxide value (PV), thiobarbutyric acid reactive substances (TBARS), free fatty acid content (FFA) and antioxidant capacity (ABTS) were determined. According to the results, it was found that FO contains 55.9 % -linolenic acid as the primary fatty acid while oleic and linoleic acids were measured as 18.3 and 16.0%, respectively. γ-Tocopherol was found to be the predominant tocopherol vitamer with the concentration of 399 mg/kg whereas the other tocopherols were measured to be lower levels. Both PV and TBARS tests have revealed that FO microemulsion that containing just water+surfactant (Polyglycerol polyricinoleate) had higher oxidative stability compared to the bulk FO, surfactant containing FO and α-tocopherol+water containing FO. On the other hand, either gallic acid or plant extract added FO samples had lower PV and TBARS values compared to all other samples. Among plant extract added samples, the highest oxidative stability was measured for grape seed extract which had even lower PV and TBARS results compared to gallic acid. In the meantime, antioxidant capacity and color disappearance which was the indication of carotenoid disappearance were also minimum in this sample. As conclusion, it has been shown that although they are technically emulsions, microemulsions resemble to bulk FO with clear appearance and thermodynamic stability. More importantly, it is postulated with this theses that oxidative stability can be significantly improved by using solvent-free plant extracts in the form of FO microemulsions without changing the appearance.