Pharmacognosy - medicinal plants (herbs)

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Theme XIV: Lipids

Fixed oils and fats

Definition of fixed oils and fats. Obtaining methods.

They are esters of glycerin with fatty acids of high molecular weight by aliphatic long chain, particularly palmitic, stearic and oleic acids. Simple esters of glycerin are often called glycerides.


The difference in consistency between the fixed oils and fats is caused by the relative proportion of liquid or solid ester of glycerin, which depends on the extension of the aliphatic chain and its degree of saturation.


The fixed oils contain a greater proportion of liquid glycerides (polyunsaturated) such as glycerin oleate, while fats are rich in solid glycerides such as glycerin stearate. The glycerides of unsaturated fatty acids have a lower melting point than saturated ones with the same carbon number.


Among fixed oils must be made a special distinction to the volatile oils, because although their chemical structure belong to this classification, physically as the name suggests, are volatile.


The difference lies in that the volatile oils don’t contain glycerin esters, they are also known as essential oils.

Obtaining methods

Both fixed oils and fats are obtained by expression extraction of the plant or animal tissue. In this process the material is subjected to hydraulic pressure and heat is used if necessary.


The oils obtained in the first expression extraction (cold pressed) have a high commercial value such as olive oil where the first expression is known by virgin olive oil, but for subsequent expressions is necessary to subject them to a process of purification.


The fixed oils are frequently treated for its clarification and finally are filtered. Some technologies obtained oil by volatile solvents that are recovered later. Fats and oils animals are usually obtained by the method of tissue heating to its melting and subsequent mechanical separation.

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Definition of fixed oils and fats. Obtaining methods. .