Thermal oxidation of olive oil, sunflower oil and a mix of both oils during forty discontinuous domestic fryings of different foods

Changes in sunflower oil (SO), olive oil (OO) of 0.4 degrees acidity value and a mixture (1:1) of both oils (MO) were studied during forty domestic discontinuous deep-fat frying of various foodstuffs, mostly frozen foods. The replenishment of the oil in the fryer with fresh oil was performed after every ten uses to maintain the oil volume in the fryer. Alteration of oils was evaluated by measuring polar content (PC) and compounds related to thermoxidative and hydrolytic changes. PC increased with different rates in the three oils, thus after twenty fryings, PC was 25.3 g/100 g oil for SO, 22.4 g/100 g oil for MO and 19.7 g/100 g oil for OO. At the fortieth frying, the three oils surpassed the critical level for PC of 25 g/100 g oil, but SO and MO were abused oils with a PC value 30 g/100 g oil. Oxidized triacylglycerols (OTG), triacylglycerol dimers (TD) and triacylglycerol polymers (TP) increased after forty fryings 6.1, 34.5 and 260 times, respectively, in SO; 7.8, 50.9 and 115.8 times, respectively, in MO and 19.5, 70 and 149 times, respectively, in OO. The TP+TD/OTG ratio was used to assess the more predominant alteration through frying in the three oils. This ratio increased after forty fryings more in SO followed by MO, and then by OO, showing that oils rich in linoleic acid became more polymerized in frying than those rich in oleic acid . According to PC and the thermoxidative changes, MO behaved as a mix of OO and SO. Data indicated that during the first twenty fryings, olive oil of 0.4 degrees acidity value and the oil mix performed more satisfactorily than sunflower oil in repeated fryings of frozen foods, however, differences in frying oil behavior decreased during the last twenty fryings.