Investigation of the protective effect of propolis on thermal oxidation of sunflower oil at molecular level

Sunflower oil (SFO), which is considered a healthy oil due to its high unsaturated fatty acid content, is one of the most preferred vegetable oils in cooking and frying due to its cheap price. However, since the oxidation of this oil during heat treatment causes the formation of many compounds harmful to our health, there is a need for natural antioxidant substances that will increase the oxidative stability of SFO. Propolis (PRPLS), a resinous mixture of various plant products collected by honeybees to protect the hive, contains many antioxidant substances such as polyphenols and flavonoids. The aim of this study is to determine whether PRPLS has a protective effect against thermal oxidation that occurs during heat treatment at the molecular level. For this purpose, control (PRPLS and BHT free) and various concentrations of PRPLS (1000, 1500 and 2000 ppm) and BHT (100 ppm) added oils were heated in a deep fryer at 180 °C for a total of 24 hours in 8-hour period and samples taken every two hours were analyzed by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy. In addition, control and 2000 ppm PRPLS added oils were analyzed by total antioxidant status (TAS)/total oxidant status (TOS) analysis and thermogravimetric analysis-differential thermal analysis (TGA-DTA) techniques. ATR-FTIR spectroscopy results showed that the areas of the bands related to primary and secondary oxidation products (3482 and 1745 cm-1 bands) and trans-unsaturated fatty acids (987 and 965 cm-1 bands) increased while the areas of the bands related to cis fatty acids (3009 and 722 cm-1 bands) decreased in the control SFO as a result of the oxidation that occurred after heating. The addition of 2000 ppm PRPLS inhibited all these oxidation processes in oils similar to BHT. Principal component analysis (PCA), TAS/TOS analysis and DTA-TGA confirmed the conclusion that 2000 ppm PRPLS has a protective effect on the thermal oxidation of AY. The results of our study demonstrated that PRPLS can be recommended as an effective natural antioxidant to inhibit thermal oxidation in oils and it can be utilized instead of synthetic antioxidants in edible oil industry.