Vegetable Oil Structure and Antioxidants

Ester Carvalho de Souza, Lauralice de C.F. Canale, George E. Totten:
New challenges in heat treatment and surface engineering – conference in honour of prof. Božidar Liščić (09 – 12 June 2009, Dubrovnik – Cavtat, Croatia)

Five vegetable oils: canola, soybean, corn, cottonseed and sunflower oils were characterized with respect to their composition by gas chromatography and viscosity.  The compositions of the vegetable oils suggest that they exhibited substantially different propensity for oxidation following the order of: Canola < corn < cottonseed < sunflower ≈ soybean.
The effect of molecular structure of five different vegetables oils including: corn, soybean, canola, cottonseed and sunflower oils with no antioxidants added on the oxidative stability was calculated and the following order of oxidative stability would be expected: canola < corn < cottonseed < sunflower  ≈ soybean. Soybean oil, which is one of the most commercially interesting of the vegetable oils in Brazil, was determined to be the most susceptible to oxidative degradation of those oils evaluated. Therefore, soybean oil was selected to be used for a study to evaluate the ability of an antioxidant to stabilize a vegetable oil relative to two commercially formulated petroleum quench oils: Micro Temp 157, a conventional oil, and Micro Temp 153B, a fast oil.  The antioxidants selected for this study were Irganox L57 and Irganox L109.  In addition, the potential for combinations of L57 and L109 to provide a synergistic improvement over either antioxidant individually was also examined. The results of this work showed following order of oxidative stabilization: L109 > L57 over 60 hours of accelerated testing. No synergism using a blend of L109 + L57 was observed with performance increasing with increasing amount of L109. As expected all of the antioxidant combinations evaluated were better than soybean oil with no antioxidant.
However, none of the vegetable oils evaluated exhibited the oxidative stability obtained for either petroleum oil. Cooling curve analysis results were surprising since increasing viscosity observed for vegetable oils with deceasing stability did not exhibit any large effect on cooling rates or times. The reasons for this have not been definitively shown thus far.

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