The following treatments underwent thermoxidation:(1) soybean oil without a synthetic antioxidant (SO); (2) soybean oil with 50 mg/kg of synthetic antioxidant(TBHQ); (3) soybean oil with 3,000 mg/kg of rosemary extract (RE); and (4) soybean oil with a mixture of 3,000 mg/kg of rosemary extract and 50 mg/kg of TBHQ (RE + TBHQ). The concentration of TBHQ that was used is the most common concentration utilized by the soybean oil industry. The thermoxidation tests on these four treatments were conducted on a hot plate in four different 50 mL beakers containing 30 mL of each sample with a 0.4/cm surface : volume ratio. The temperature was set at 180C, which is the most commonly used temperature for deep frying. The heating was carried out using a discontinuous approach involving 10 h of heating per day. Samples were taken at
time intervals of 0, 10 and 20 h and collected in flasks in the
presence of inert gas (N2) and stored at -18C until the time
of analysis.