A new instrumental method for measurement of thermal oxidative stability of oils with different saturation levels and additives

A computerized instrument (FriLife), which requires only two hours to measure the oxidative stability of oils/fats (200 g sample/test) by monitoring the progression of conductivity in a deionized water trap for oil degradation volatiles, was developed and tested. The linear slope (LS) of conductivity (range 4 to 11 µS/cm) versus time for each oil tested provided one FriLife measurement for oil stability. Conductivity measurements over time were converted to equivalent concentrations (mM) of acetic acid, and time required just to exceed 0.2 mM acetic acid or the induction time (IT) was a second FriLife measurement for oil stability. In one experiment, LS and IT were determined for soybean oils with iodine values (IV) of 70, 94, 110 and 132, and in another, LS and IT were determined for soybean oil (IV=94) alone or with additives (5 ppm dimethyl siloxane, DMS; 100 ppm tertiary-butylated hydroquinone, TBHQ; or their combination). The LS values for the soybean oils with IVs of 70, 94, 110 and 132 were all different (P < 0.05) and were, respectively, 0.12, 0.24, 0.31, and 0.46 µS/cm min. The ITs of the soybean oils ranged from 95.5 min for the most saturated oil (IV=70) to 32.4 min for the most unsaturated one (IV=132). The LSs of soybean oil (IV=94) were greatly reduced, and the ITs increased by the addition of DMS, TBHQ, or both. FriLife also was able to measure the protective effect of DMS against thermal oxidation of oils where the Oxygen Stability Index (OSI) showed no difference. Across all oils, the coefficient of variations of LS and IT generally were less than 10 and 15%, respectively. While both IT and LS are highly correlated (P < 0.05) with OSI (r ≥ |0.95|), the FriLife measurements require much less time than OSI measurement. The hydrogenated soybean oils had more tocopherol destruction (91.3% and 87.8% for IVs of 70 and 94, respectively) during a 200-min FriLife test than the soybean oil with IV of 132 (57.6%). Tested oil measurements (levels of total polar components, free fatty acids, Food Oil Sensor readings, and dienoic acids) generally showed that the more stable oils had lower levels than did the less stable oils. All levels of these measurements increased during the FriUfe test in all oils while the levels of polyunsaturated fatty adds decreased slightly. The Frillfe test Is proposed as a rapid, reproducible, accurate, and simple analytical method to analyze the oxidative stability of fats/oils.