Fats are predominantly triesters of fatty acids and glycerol – commonly known as triglycerides. Triglycerides that appear as solid at room temperature are normally called fats while those that are liquid are referred to as oils. Fat is a vital nutrient that provides the body with energy as well as acts as a source of essential fatty acids. Current dietary recommendations are that fats should supply no more than 30% of a person’s energy requirement. In terms of energy value a gram of fat yields 9 cal of energy compared to 6 cal/g for starch and proteins.
Saturated fatty acids are those which have only single carbon-to-carbon bonds ( C-C ). These fatty acids are very stable. Examples of saturated fatty acids are palmitic acid, C16:0 and stearic acid, C18:0. Unsaturated fatty acids contain carbon-to-carbon double bonds ( C=C ). When only one double bond is present in the carbon chain, the fatty acid is called monounsaturated ( e.g. oleic acid, C18:1 ). When there are more than one double-bond, then the fatty acid is called polyunsaturated ( e.g. linoleic acid, C18:2 and linolenic acid, C18:3 )
Generally the more unsaturated the fatty acid, the more unstable it will be. The chemical reactivity of polyunsaturated fatty acid also depends on the positions of the double bonds. Reactivity increases substantially if the double bonds are conjugated ( separated by only one single bond ) or methylene-interupted ( separated by a – CH2 unit )
The melting points of fatty acids generally increase with:-
increasing chain length
increasing saturation
changing of a cis-to a trans-isomer
Thus a short chain saturated fatty acid such as butyric acid will have a lower melting point than long chain saturated fatty acids ( e.g. palmitic acid ) and even some higher chain length unsaturated fatty acids ( e.g. oleic acid ). This property is reflected when the fatty acids form part of a triglyceride. The more saturated oil or fat may not necessarily have a higher melting point. For example, coconut oil which is about 87% saturated, but contains a high proportion of short chain fatty acids, has a melting range of 24°C – 26°C while palm oil which is about 49% saturated has a melting point in the range of 31°C – 38°C.
