Evaluation of oxidative stability of frying olive oil during thermal and frying processes

Frying is one of the oldest and popular preparation techniques broadly used at home and food industry. High temperatures plus the presence of air as well as the water from the food being fried cause many destructive reactions. Therefore, thermal stability and performance of oil during frying is considered as one of the important criteria in the selection of frying oil. In this regard, balanced chemical composition, presents it as a valuable option for frying purposes. Studies show lipid autoxidation considered as the most important deteriorative reaction in the olive oil. This reaction leads to the formation of a series of primary and secondary oxidation products. Hydroperoxides are the primary oxidation products of lipid oxidation. Carbonyl value (CV) does measure secondary decomposition products are more stable than peroxides and the CV seems to be a good index of oxidative changes in lipids. Therefore, the determination of carbonyl compounds in frying oils is very important for evaluating the quality of frying fats and oils. Several studies have been carried out on the oxidative stability of edible oil during frying. Rancimat test has also been considered among the accelerated methods of lipid oxidation measurement due to ease of use and reproducibility. However, although estimate of oxidative stability of edible oil according to accelerated methods, is used widely but there is always worries about accordance the results of such tests with the results found under real frying conditions and Presence of food. Therefore, necessity of simultaneous study oxidative stability is essential in order to justify and extension of result together during heating and frying. Six refined olive oil samples of different brands in 1 lit glass bottles were purchased from local shops and were stored at 4 ºC for further analysis. Fatty acid methyl ester (FAME) standards, and all chemicals and solvents used in this study were of analytical reagent grade and supplied by Merck and Sigma Chemical Companies. The ratio between monounsaturated and polyunsaturated fatty acids (M/P) was determined by gas–liquid chromatography. The spectrophotometric method was used to determine the PV. The AV was determined according to the AOCS. The TT content was determined according to the colorimetric method. The TP content was determined spectrophotometrically using Folin–Ciocalteau’s reagent. The CV of the oils was measured using 2-propanol and 2,4-decadienal as solvent and standard, respectively. A Metrohm Rancimat model 743 (Herisau, Switzerland) was used to measure the OSI and IPCV of olive oil samples. Frying process was performed in bench- top deep- fryer at 180ºC. ANOVA and regression analyses were performed according to the MATLAB and Excel software. Significant differences between means were determined by Duncan’s multiple range tests. There was good correlation between the OSI and induction period (IPcv) at the temperature range studied with a high determination coefficient (R2>0.99) in the Rancimat test. Generally, the results of the present study showed that the Rancimat method at 110 ºC correlated well with stability under frying condition and this correlation decreased as temperature increased in the Rancimat test. These observations can be explained by the fact that steps or pathway of chemical reactions that take place at low and high temperatures are different. Thus, choosing the right levels of operational parameters in the Rancimat method can produce the least possible difference between frying and the OSI test.