General Properties of Lipids

Introduction

Lipids are a large and dissimilar group of naturally occurring organic compounds with the general property of being at least partially insoluble in water. In general, lipids are composed mainly of carbon and hydrogen, with relatively few other atoms such as oxygen, nitrogen, and sulfur. Understanding the properties of lipids is essential for their proper preservation, use and study. However, there are significant differences in the chemical and physical properties of different types of lipids. Here, Alfa Chemistry summarized the general properties of lipids and elaborated the properties in terms of both physical properties and chemical properties^[1-3].

Physical Properties

• Lipids are generally insoluble in water and are highly hydrophobic even if they contain certain hydrophilic parts. Their solubility in organic solvents ranges from complete solubility in hydrocarbon solvents such as sterol esters and waxes to complete non-solubility in these solvents such as glycolipids, but they are generally soluble in non-polar solvent such as ether, chloroform, benzene, etc.
• Lipids are colorless, odorless and tasteless and they are lighter than water.
• Lipids can be liquid or solid forms at room temperature.
• Lipid molecules have no ionic charge.
• The consistency of lipids depends upon the presence of saturated and/or unsaturated fatty acids.
• Solid triglycerols (Fats) have high proportions of saturated fatty acids while liquid triglycerols (Oils) have high proportions of unsaturated fatty acids.
• The melting points of saturated fatty acids rise as their number of carbon atoms increase. The presence of double bonds reduces the melting point of monounsaturated fatty acids, and their melting point of trans isomers is higher than that of cis isomers.
• When fat or oil is rubbed with water, large molecules of lipids break down into smaller molecules to form an emulsion.

Fig. 1. Relation between the melting point and the number of carbon atoms in saturated fatty acids.

Chemical Properties

• Hydrolysis: The lipids will hydrolyze in presence of acids or alkalis. Acid hydrolysis results in the formation of glycerol and long chain of fatty acid whereas alkaline hydrolysis of fats results in the formation of sodium or potassium salts of fatty acids (called as soaps) and the process is called as saponification.
• Hydrogenation: The presence of double bonds in fatty acids can occur many reactions, of which hydrogenation is the most prominent. Under the conditions of catalyst or enzyme, the double bond of unsaturated fatty acid can react with hydrogen to produce saturated fatty acids.
• Halogenation: Besides reactions with hydrogen, unsaturated fatty acids can react with halogens by addition at the double bond(s), which will discolor the halogen solution.
• Rancidity: When lipids are exposed to atmosphere (heat, light, air, moisture) for a period of time, a disagreeable odor will be developed. Such types of lipids are called rancid lipids and the phenomenon is termed as rancidity. Hydrolysis and oxidation reactions are responsible for causing rancidity, in which the occurrence of hydrolysis rancidity due to liberation of volatile acids during the hydrolysis of fats, and oxidative rancidity is caused by the oxidation of unsaturated fatty acids to aldehydes and ketones.

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References

1. Marangoni, A. G., & Narine, S. S. (Eds.). Physical properties of lipids[M]. CRC Press, 2002.
2. Aryal Sagar (2021). Lipids- definition, properties, structure, types, examples, functions. Retrieved from https://microbenotes.com/lipids-properties-structure-classification-and-functions/#properties-of-lipids
3. http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S000002BI/P000983/M016880/ET/1491301103P05M03Lipids-Propertiesfignc.pdf.