EFFECT OF TEMPERATURE ON LINOLENIC ACID LOSS AND 18-3 DELTA-9-CIS, DELTA-12-CIS, DELTA-15-TRANS FORMATION IN SOYBEAN OIL

Oil was extracted from soybeans, degummed, alkali-refined and bleached. The oil w as heated at 160, 180, 200, 220 and 240-degrees-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 18:3omega3 loss and 18:3 DELTA9-cis, DELTA12-cis, DELTA15-trans (18:3c,c,t) formation were determined, and the activation energies were calculated from Arrhenius plots. Freshly prepared soy oil had 10.1% 18:3omega3 and no detectable 18:3c,c,t. Loss of 18:3omega3 followed apparent first-order kinetics. The first-order rate constants ranged from .0018 +/- .00014 min-1 at 160-degrees-C to .083 +/- .0033 min-1 at 240-degrees-C. The formation of 18:3c,c,t did not follow simple kinetics, and initial rates were estimated. The initial rates (mg per g oil per h) of 18:3c,c,t formation ranged from 0.0031 +/- 0.0006 at 160-degrees-C to 2.4 +/- .24 at 240-degrees-C. The Arrhenius activation energy for 18:3omega3 loss was 82.1 +/- 7.2 kJ mol-1. The apparent Arrhenius activation energy for 18:3c,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 18:3c,c,t formation. Selection of appropriate deodorization conditions could limit the amount of 18:3c,c,t produced.