tance is the presence of very high levels of chlorophylls, which are extremely detrimental to oil quality and must be removed before deodorization. Unlike carotenes, which are heat bleachable in the deodorizer, chlorophylls are not and must be removed by bleaching (63). Most modern oil-processing plants use a continuous bleaching system.


Canola oil, like other edible oils, is hydrogenated to improve oxidative stability and to modify the physical properties of the fat or oil. The basic principle of this method is to add hydrogen at the double bonds of the polyunsaturated fatty acids (those containing two or more double bonds), thus eliminating these sites from reaction with oxygen. Only a small portion of these fatty acids may be affected in partially hydrogenated oil stocks, depending on the type of hydrogenation system used. This process requires a catalyst (usually nickel), which may be poisoned by the presence of low levels of sulfur compounds (3-5 ppm) originating from breakdown products of glucosino-lates (64). Once hydrogenated, the oil becomes physically harder and more resistant to oxidation. In Canada, canola oil is selectively hydrogenated for the production of margarines and shortenings.

Selective hydrogenation of canola oil is carried under conditions of high temperature, low pressure (200°C, 6 psig), and agitation and affects the more unsaturated fatty acids first. This is illustrated by the fatty acid profiles during the course of hydrogenation of soybean oil shown in Figure 6. All the unsaturated fatty acids are hydrogenated at the same time but at different rates, as indicated by the reaction rate constants. Linolenic acid is hydrogenated 2.3 times faster than linoleic acid, where the latter is hydrogenated 12 times faster than oleic acid (65). This method results in the formation of higher levels of trans fatty acids, steeper solid fat index curves, and higher melting points at lower iodine values. The formation of trans fatty acids changes the physical properties of the oil as a trans double bond is equivalent in physical properties to a saturated

Figure 6. Time-dependent changes in fatty acid composition during hydrogenation of soybean oil. Source: Ref. 65.

Time (min)

Figure 6. Time-dependent changes in fatty acid composition during hydrogenation of soybean oil. Source: Ref. 65.

single bond fatty acid; for example, cis, iraws-linoleyic acid is equivalent in physical properties to oleic acid. Hardness of margarine oils is due to the higher levels of trans fatty acids present rather than to a marked decrease in unsaturation of the fatty acids (66). Because different degrees of hydrogénation are often required by the industry, hydrogénation is a batch process.


Free fatty acids or odiferous or flavor degradation products remaining in the oil are removed by deodorization. This involves steam distillation carried out at very high temperatures (240-270°C) under vacuum (3-8 mmHg). Most plants use a semicontinuous or continuous deodorizing system that is comprised of a large cylindrical tank or shell through which oil is pumped in and passed through a series of trays where it is deaerated and successively deodorized with sparging steam. In the case of canola oil, it must be essentially free of chlorophylls, phosphatidic material, and heavy metals prior to deodorization. Following deodorization, the oil is cooled, passed through a polishing filter, and sparged with nitrogen. The final product is a bland oil that is treated with citric acid (0.005-0.01%) to sequester any trace metals still remaining and other antioxidants to prevent oxidation.

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