The Properties of Fats & Oils

The fatty acid composition of each oil determines its properties. The fatty acids may be distributed at random in the triglycerides or as in palm oil, specifically arranged in the triglycerides. The fatty acid composition determines the melting point, stability and saponification value of each oil.

The properties of the oil itself are important when choosing oils to make lotions, salves, bath & massage oils and other similar products. After saponification the fatty acids are randomized within the soap.

For soapmaking, it’s the fatty acid contents that matter the most. The chain length (C number) is usually cited and helps describes the molecule’s properties in relation to others in its same series.


Saturated Fatty Acids

Saturated fatty acids contain no double bonds. They are stiff molecules which tend to increase the melting point of oils. Saturated fatty acids themselves are room temperature solids. As they increase in size from lauric to stearic, the melting point of the oil increases. Saturated fatty acids in soaps have good cleaning properties and support foam. The longer chains also tend to harden soap.

Lauric acid, C 12 and Myristic acid, C 14 fatty acids tend to give very good lather. These are the softest of the saturated fatty acids.

Excessive amounts of short chain saturated fatty acids can result in a harsh soap with a strong defatting action.

Palmitic C 16 and Stearic C 18 fatty acids tend to increase the hardness of soaps. They do not lather as well as the shorter fatty acids. Excessive amounts of long chain saturated fatty acids can make soap brittle.

Palm oil contains triglycerides composed entirely of stearic acid (aka, stearin). Palm oil should never be partially melted in use because as you pour the liquid off the saponification value will shift. Either remove portions of the solid or completely melt the entire container every time you use it.


Unsaturated Fatty Acids

Unsaturated fatty acids can contain 1 or more double bonds. They are designated by the chain length followed by the number of double bonds (Oleic acid 18/1). Unsaturated fatty acids are liquids. They tend to have good cleaning power, but lather poorly. These fatty acids also tend to make milder soaps. Palmitoleic (16/1) and Oleic (18/1) tend to increase mildness in soaps. They have adequate cleaning power and perform better on dirt than on oily problems. They do not have significant defatting action. Both of these oils resist attack of the double bonds by oxygen and are resistant to rancidity. Oils high in these fatty acids are also good choices as liquid oils for most products since they lead to long shelf lives.

Linoleic acid (18/2) also increases mildness in soaps. It is very easily oxidized but oils containing it have short shelf lives due to rapid onset of rancidity. Linolenic acid (18/3) also increases mildness in soaps. It is very easily oxidized and oils containing it have short shelf lives due to rapid onset of rancidity. Avoid using liquid oils which contain more than one double bond in most products because of shelf life considerations. Antioxidants are definitely required for these oils.

Ricinolenic acid (18/1) also contains an alcohol group in the chain. This increases the viscosity of the oil and accounts for the thickness of castor oil. It also makes this fatty acid very mild on the skin and is good for super fatting soaps. It should be used in moderation because of rancidity issues.

Castor oil can be reacted with sulfuric acid to make Turkey Red Oil, the first synthetic surfactant, a predecessor to Sodium Lauryl Sulfate. Turkey Red Oil should never be used in lotions because it is an irritant if left on the skin.

Source: http://web.archive.org/web/20110811072427/http://www.snowdriftfarm.com/fatproperties2.html