The melting point of fats is the temperature at which they become liquid.
Fats and oils are both mostly composed of triglycerides. A triglyceride is composed of a glycerol backbone that holds three fatty acids. Any combination of fatty acids is possible: saturated and unsaturated (including trans fats). (read more..)
The melting point of fats mainly depends on the types of fatty acids that are present.
- The proportion of saturated to unsaturated fatty acids.
Saturated fatty acids have a straight shape. This feature allows them to pack closely together, increasing the intra-molecular attraction.
Since they are more difficult to separate, it makes saturated fatty acids more solid at room temperature, when compared to unsaturated fatty acids which are bent and, therefore, cannot pack closely.
Therefore, saturated fatty acids have a higher melting point than unsaturated fatty acids. Fats with a higher concentration of saturated fatty acids require higher temperatures to melt, compared to those with a lower concentration.
Refer to point 1 in the figure below.
- The length of the fatty acid chains.
The longer the fatty acid chains (and, therefore, the higher the molecular weight) the higher the melting point.
When comparing two fatty acids of the same saturation (for example, both fully saturated), but with different lengths, the longer fatty acid will have a higher melting point.
An example of two saturated fatty acids of different lengths:
– Palmitic acid – a long hydrocarbon chain, built of 16 carbons. It melts at 145oF (63oC)
– Lauric acid – a medium hydrocarbon chain, built of 12 carbons. It melts at 110oF (43oC)
Refer to point 2 in the figure below.
- The geometry of carbon double bonds.
“Trans” fatty acids melt at higher temperatures than fatty acids with a “cis” configuration. “Cis” and “trans” configurations differ only in the position of the hydrogen atoms on the carbon double bonds: they are on the same side of the chain in “cis” configuration and opposite side in “trans” configuration. (read more..)
This explains why products high in trans fatty acids, such as margarines and shortenings, made from partially hydrogenated vegetable oils, are solid at room temperature, while vegetable oils, with “cis” carbon double bonds, are liquid.
For example, oleic acid and elaidic acid are very similar. They both have 18 carbons in the chain and the carbon double bond is on the 9th position. They differ, however, in the configuration of the hydrogen atoms at those double bonds. Oleic acid has a double bond with “cis” configuration, while elaidic acid with “trans” configuration.
For this reason, oleic acid melts at 56.1oF (13oC) and elaidic acid melts at 111oF (44oC). (1)
Refer to point 3 in the figure below.
- The position of the double bonds
The melting point (although not greatly) also depends on where the double bond is located on the chain. This can be observed while comparing similar fatty acids (the same number of double bonds and the same length of hydrocarbon chains).
For example, there are two similar unsaturated trans fatty acids, eladic and vaccenic acids. The elaidic acid has a trans double bond on the position 9 and melts at 111oF (44oC).
The vaccenic acid has double bond at the position 11 and melts at a slightly higher temperature 113oF (45oC ). Various sources publish slightly different melting points, but overall there is a small difference. (3, 4)
Refer to point 4 in the figure below.
NOTE: coconut oil is high in saturated fatty acids, unlike other oils. However, a large portion of these are medium chain fatty acids, making the melting point lower than other saturated fats with long fatty acids. This explains why coconut oil melts at a near-room temperature (e.g. if the room is slightly warmer). (5)
The melting point is also affected by the following two characteristics of triglycerides:
- The molecular configuration of triglycerides
The melting point varies in sharpness, depending on the molecular configurations of triglycerides. A fat with a sharp melting point, melts completely over a narrow range of temperatures. A fat with a broad melting point, has a wide range of temperatures to melt completely.
Simple triglycerides, such as cocoa butter, have a sharper melting point than triglyceride mixtures, such as lard and various shortening products.
Cocoa butter, for instance, mostly consists of simple triglycerides, that are made primarily of two combinations of palmitic (P), oleic (O) and stearic (S) acids: POS and SOS.
Not only does it have a sharp melting point, but it also melts at about body temperature. This explains why seconds after you take a bite on a hard chocolate bar, it turns into a thick liquid in your month. (6)
- The crystallization process
Solidified fats can have different melting points. This depends on how the fatty acid chains orient themselves in the solid state (polymorphism of fat).
When a fat solidifies, for instance by cooling during processing, it undergoes a process of crystallization – arranging the fatty acids molecules.
The melting point depends on how these molecules are arranged in the fat crystals.
The crystallization process depends on various factors, such as the temperature and time of the crystallization process, the molecular composition and configuration of the fat and other crystallization conditions. (7)