d) Deodorization

Steam or nitrogen stream distillation under reduced pressure is used to remove volatile compounds with undesirable flavours, most of which originate from oxidation of the oil. It is believed that this treatment is also responsible for the thermal destruction of nonvolatile off-flavour substances, whereas the resulting volatiles are distilled away.The resulting deodorized oils will also have undergone partial cis-trans isomerisation of their polyunsaturated components, which is an unfortunate consequence of this operation, which may entail heating at temperatures as high as 240ºC for extended periods of time.

This treatment unfortunately also brings along the removal of some of the natural protectants of oils, the tocopherols, as well as the sterols. The addition of citric acid is often made to chelate traces of pro-oxidant metals, hence diminish their activity and impart to the oils, even at the diminished tocopherol content, additional stability.

Although it is established that refining generally improves the oxidative stability of oils because of the removal of prooxidants such as the chlorophyls, this is not always certain. As an example, crude cottonseed oil has a greater resistance to oxidation than its refined counterpart due to the larger amounts of gossypol and tocopherols present in crude oil. However, it is undeniable to state the remarkable added benefits that come from refining edible oils. The upgrading of palm oil quality is a good example. Moreover, adding to the improvements in colour, stability and flavour, powerful toxicants, such as gossypol in cottonseed oil and aflatoxins in peanut oil, are thoroughly eliminated during the refining process.