Effects of ethanol extract of olive leaves on the quality of oil during frying process

Edible vegetable oils are prone to deteriorate through complex chemical reactions during frying. To promote the widespread use of low-cost by-products from the olive oil industry in the oil processing industry, the effects of ethanol extract of olive leaves (EEOL) on the quality of soybean oil during the deep frying of French fries were investigated. The in vitro antioxidant capacity of EEOL was evaluated using the scavenging rate of DPPH and ABTS⁺ free radicals as a key indicator. Then, EEOL was added to soybean oil for 30 h of intermittent frying at a constant temperature 180 ℃, a process that mimicked real-world commercial frying patterns, while soybean oil enriched by TBHQ and tea polyphenols was used for comparison. The basic physicochemical indices, color, and fatty acid composition of the soybean oils have been analyzed to comprehensively evaluate the stability of frying oil. The results showed that the total phenolic and total flavonoid contents of EEOL were (84.47±0.02) mg/g and (35.03±0.03) mg/g, respectively. The IC₅₀ of DPPH and ABTS⁺ radical scavenging activity were 15.01 μg/mL and 43.13 μg/mL, respectively. Compared with the blank group, the EEOL-added soybean oil decreased the increase of acid value by 48.24%, p-anisidine value by 7.46%, conjugated diene by 29.41%, conjugated triene by 15.03%, total polar compounds by 38.63%, while the fluctuation range of the peroxide value (0.51-1.03 mmol/kg) was lower than the blank group (0.24-1.03 mmol/kg). Meanwhile, the increase in red and yellow values of soybean oil and the oxidative loss of polyunsaturated fatty acids were also significantly delayed. In summary, the addition of EEOL as an antioxidant in frying oil showed good performance in delaying the thermal oxidation of oil and improving the quality of oil. Therefore, EEOL had the potential to act as a source of natural antioxidants to replace synthetic antioxidants to prevent oxidation of frying oil.