A variety of shea butters.
Butters, oils, and waxes all come from fats that are derived from plants or animals, and have two basic components; fatty acids and alcohols. The difference between butters and oils is primarily whether or not they are solid at room and/or body temperature. Although they are both composed of groups of fatty acids, there are differences in the molecular composition and structure of butters and oils that are responsible for these differences in melting points.
Factors that determine melting point of lipids
- Molecular weight - lower-molecular-weight fatty acids have a lower melting point, so that they are liquid at room temperature or body temperature. Higher-molecular-weight fatty acids form crystalline structures that persist to higher melting points, and so they are usually solids at room temperature and higher.
- Saturated molecular structure — longer-chain fatty acids without any double bonds are straight chain molecules (like long snakes) that are able to closely pack next to one another This close-packing induces crystallization, which requires more energy to break apart than molecules not packed together into a crystalline or semi-crystalline structure. For this reason, the melting points of these types of fatty acids are much higher. This means the "oil" will exist in a solid state at room temperature or even body temperature.
- Unsaturated molecular structure — unsaturated molecules have at least one double bond somewhere in their structure. This creates a kink or branching effect in the geometry of the molecule. This prevents unsaturated fatty acids from getting too close to one another, thereby preventing crystallization. These molecules have lots of space between themselves, which allows for more mobility of the molecules and results in a lower melting temperature. These oils may be liquid at room temperature or melt upon contact with skin.
- Stearic acid, a saturated hydrocarbon molecule with 18 carbons (relatively long-chain fatty acid) has a melting point of 69.6°C (157.28°F). Oleic acid, a monounsaturated hydrocarbon molecule, has a double bond in it that creates a kink in its geometry, which makes it more difficult for adjacent molecules to pack tightly next to one another. It has a melting point of 10.5°C (50.9°F). Polyunsaturated acids, such as linoleic and linolenic, have multiple kinks in their chains and are liquid at very low temperatures (melt point = -5°C (23°F) for linoleic acid).
- Linolenic acid, polyunsaturated fatty acid.
- Linoleic acid, polyunsaturated, omega-6 fatty acid.
- Oleic acid, monounsaturated fatty acid.
- Stearic acid, saturated fatty acid.