After the extraction of tissue lipids, the next stage involves the fractionation of the complex mixture into the various classes of lipids. Later, each lipid component must be isolated and analyzed. Lipids from natural sources may contain such a big amount of components that precise analysis of each one needs some preliminary purification.
The fractionation procedure to be used will depend largely on le lipid classes present in the gross extract. Most animal lipids consist of about 50 to 80% polar lipids (phospholipids and glycolipids) but plant tissues have most frequently high amounts of non-polar or neutral lipids and glycolipids, seeds being the richest in triglycerides (source of common oils). Some animal tissues such as white adipose tissue contain more than 98% triglycerides and thus are close to plant seeds.
Thus, the fractionation procedure must be adapted to the situation, some one which can work with one kind of tissue may not be effective with another source. We shall describe the most commonly used procedures for small amounts of lipids but different ways may be adopted taking into account the final aim of the study. A dry fractionation process is used by industrial operators to modify the technological properties of edible oils and fats (palm oil, butterfat, lard, fish oil).
Most frequently, groups of lipid classes are separated from each other by chromatography on columns or thin layers. Neutral lipids are thus separated from polar lipids but it is also of common practice to separate complex lipids into phospholipid and glycolipid fractions. The adopted technique depends also on the separation scale (analytical or semi-preparative). For preparative purposes, a specific methodology can be adopted, including two types of column chromatography with a progressive enrichment of the fraction. When one or two types of lipids are present (whose components have close chemical properties) short cuts may be adopted.
In the following pages, the most commonly used and efficient methods will be described.
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