Crude oil obtained by screw pressing and solvent extraction of oilseeds will throw a deposit of so-called gums on storage. The chemical nature of these gums has been difficult to determine. They contain nitrogen and sugar and can start fermenting so they were at one stage thought to consist of glycolipids and proteins. Now we know that these gums consist mainly of phosphatides but also contain entrained oil and meal particles. They are formed when the oil absorbs water that causes some of the phosphatides to become hydrated and thereby oil-insoluble. Accordingly, hydrating the gums and removing the hydrated gums from the oil before storing the oil can prevent the formation of a gum deposit. This treatment is called water degumming. It is never applied to fruit oils like olive oil and palm oil since these oils have already been in contact with water during their production.
Water degumming is the oldest degumming treatment and also forms the basis of the production of commercial lecithin. I use the term 'commercial lecithin' here to make a distinction from the use of the word 'lecithin' as the trivial name for the compound phosphatidylcholine (PC). Similarly, phosphatidylethanolamine (PE) has the trivial name 'kephalin'. Since the water degumming process involves more water than when crude oil is allowed to absorb moisture from the atmosphere, the gums resulting from the water degumming process also remove hydrophilic substances such as sugars from the oil. Lecithin as obtained by drying the gums resulting from the water degumming process contains a mixture of different phosphatides.
The phosphatide composition of the phosphatide fraction in lecithins obtained from different oils.
Please keep in mind that Table 1 refers to lecithins, the mixture of phosphatides that has been obtained by degumming crude oil with water. Since this water degumming process does not remove all phosphatides from the oil, Table 1 does not reflect the composition of the phosphatides present in the crude oil itself. Just as a triglyceride oil is a mixture of triacylglycerols with different fatty acids, each phosphatide is also a mixture of different compounds. These compounds differ in their fatty acid composition and isomerically, in their location on the glycerol backbone. In general, the fatty acid composition of the phosphatides reflects the fatty acid composition of the oil in which these phosphatides occur but it tends to have a higher palmitic acid content and a lower oleic acid content than the oil as illustrated by Table 2.
The above table contains the data required for arriving at a conversion factor that permits the amount of phosphatides present in the oil to be calculated from its phosphorus content. For the oils represented in Table 2, this factor equals about 25 to 26. In other words, oil containing say 200 ppm of phosphorus contains about 0.5 wt% phosphatides. On the other hand, the literature often uses a factor of 31.5 or thereabouts to arrive at the acetone-insoluble component of the lecithin. This difference stems from the fact that the acetone-insoluble component of lecithin also comprises glycolipids and sugars. The factor of 31.5 is therefore very much an empirical value. It should only be used for oils that have not yet been water-degummed since on water degumming, sugars are removed. For water-degummed oils, which contain alkaline earth salts of PA and lysophosphatidic acid (LPA) and some PE and lysophosphatidylethanolamine (LPE) and do not contain any more sugars, a factor of 23 to 24 should be used to convert phosphorus to phosphatides.
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