Effect of temperature on linolenic acid loss and 18:3 Δ9‐cis, Δ12‐cis, Δ15‐trans formation in soybean oil

Oil was extracted from soybeans, degummed, alkalirefined and bleached. The oil was heated at 160, 180, 200, 220 and 240°C for up to 156 h. Fatty acid methyl esters were prepared by boron trifluoride‐catalyzed transesterification. Gas‐liquid chromatography with a cyanopropyl CPSil88 column was used to separate and quantitate fatty acid methyl esters. Fatty acids were identified by comparison of retention times with standards and were calculated as area % and mg/g oil based on 17:0 internal standard. The rates of 18:3ω3 loss and 18:3 Δ9‐ cis , Δ12‐ cis , Δ15‐ trans (18:3 c,c,t ) formation were determined, and the activation energies were calculated from Arrhenius plots. Freshly prepared soy oil had 10.1% 18:3ω3 and no detectable 18:3 c,c,t. Loss of 18:3ω3 followed apparent first‐order kinetics. The first‐order rate constants ranged from .0018±.00014 min −1 at 160°C to .083±.0033 min −1 at 240°C. The formation of 18:3 c,c,t did not follow simple kinetics, and initial rates were estimated. The initial rates (mg per g oil per h) of 18:3 c,c,t formation ranged from 0.0031±0.0006 at 160°C to 2.4±.24 at 240°C. The Arrhenius activation energy for 18:3ω3 loss was 82.1±7.2 kJ mol −1 . The apparent Arrhenius activation energy for 18:3 c,c,t formation was 146.0±13.0 kJ mol −1 . The results indicate that small differences in heating temperature can have a profound affect on 18:3 c,c,t formation. Selection of appropriate deodorization conditions could limit the amount of 18:3 c,c,t produced.