Changes in organic acid components of volatile degradation products during oxidation of oil, and effects of organic acid on increased conductivity determined by the rancimat method

In our previous paper, we assessed the oxidative stability of olive, soybean and whale oils by the Rancimat method, which is based on the conductometric determination of volaltile degradation products of oil heated at 120 degrees C with aeration. In the present study, organic acid components of volatile degradation products of oxidized oil were analyzed to evaluate the contribution of each acid to conductivity. The volatile degradation products of olive, soybean and whale oils were collected in distilled water (referred to as collected water hereafter). Free fatty acids in the collected water were extracted with diethyl ether, and analyzed by preoperative thin-layer chromatography and gas chromatography. Organic acids in the collected water were analyzed by high-performance liquid chromatography. Model experiments to determine the increase of conductivity of various organic and fatty acids were tested by the Rancimat method. Formic, propionic, iso-butyric, n-butyric and iso-valeric acids by HPLC analysis and myristic, palmitic, stearic, oleic, linoleic and alpha-linolenic acids by GLC analysis were identified in the volatile degradation products (collected water) obtained from oxidized soybean oil. Organic acid (formic, propionic, iso-butyric and iso-valeric acids) formation was found to begin within a few minutes before the rancid point (induction time) of each oil, and organic acid production (especially in whale oil) was rapid. The order of organic acid contents in the collected water was iso-valeric acid iso-butyric acid propionic acid formic acid. On the other hand, the amounts of acetic acid were lower. Iso-valeric acid showed a greater effect on the increased conductivity than propionic, iso-butyric and formic acids. Long-chain saturated and unsaturated fatty acids had no effect on the increased conductivity