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World annual production: over 2 million tonnes
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The starting materials are vegetable oils like palm oil, soya bean oil, sunflower oil, and rape-seed oil.


Most fats and oils are the triesters formed between propane-1,2,3,-triol (or glycerol) and long chain carboxylic acids (fatty acids)







The structures of three common fatty acids















oleic acid
stearic acid



















In vegetable oils the fatty acids are unsaturated. These oils are too runny to spread on bread. By adding hydrogen we can both change the shape of the chains, so that they can pack more tightly. Melting points rise and the oils are changed into solids. Controlling the amount of hydrogen added gives control of the consistency, producing a soft solid that can be spread straight from the fridge.

There is another advantage. The carbon-carbon double bond is reactive making polyunsaturated fats sensitive to heat, light and oxygen. Hydrogenation increases their stability.

For the hydrogenation of edible oils the catalyst used is Ni on a kieselgar (a kind of silica) support. Hydrogen is bubbled into the hot oil and the catalyst is separated at the end of the reaction. Temperatures of about 200oC are used.

Stopping the reaction after only a small amount of hydrogenation means that the oils remain liquid. These partially hydrogenated oils are used to produce cooking oils. Further hydrogenation can produce fats which are soft but solid which still contain appreciable amounts of unsaturated fatty acids and are used in margarines. If an oil is hydrogenated completely, all the carbon - carbon double bonds are eliminated and the resulting product is a brittle solid at room temperature.

The hydrogenation conditions are varied by the manufacturer to produce the physical and chemical characteristics desired in the finished product.

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