Saponification value
Saponification value or saponification number represents the number of milligrams of potassium hydroxide or sodium hydroxide required to saponify one gram of fat under the conditions specified. It is a measure of the average molecular weight of all the fatty acids present in the sample in form of triglycerides. The higher the saponification value, the lower the fatty acids average length, the lighter the mean molecular weight of triglycerides and vice versa. Practically, fats or oils with high saponification value are more suitable for soap making.
Determination
To determine saponification value, the sample is treated with an excess of alkali for half an hour under reflux. The KOH is consumed by reaction with triglycerides, which consume three equivalents of base. Diglycerides consume two equivalents of KOH. Monoglycerides and free fatty acids, as well as other esters such as lactones, consume one equivalent of base. At the end of the reaction the quantity of KOH is determined by titration using standard solution of hydrochloric acid. Key to the method is the use of phenolphthalein indicator, which indicates the consumption of strong base by the acid, not the weak base. The SV is calculated as following:For example, standard methods for determination of SV of vegetable and animal fats are as follows:
The SV can also be calculated from the fatty acid composition as determined by gas chromatography.
Handmade soap makers who aim for bar soap use sodium hydroxide, commonly known as lye, rather than KOH which produces soft paste, gel or liquid soaps. In order to calculate the lye amount needed to make bar soap, KOH values of SV can be converted to NaOH values by dividing KOH values by the ratio of the molecular weights of KOH and NaOH.
Calculation of average molecular weight of fats and oils
The theoretical SV of a pure triglyceride molecule can be calculated by the following equation :For instance, triolein, a triglyceride occurring in many fats and oils, has three oleic acid residues esterified to a molecule of glycerol with a total MW of 885.4. Therefore, its SV equals 190 mg KOH / g sample. In comparison, trilaurin with three shorter fatty acid residues has a MW of 639 and an SV of 263.
As it can be seen from equation , the SV of a given fat is inversely proportional to its molecular weight. Actually, as fats and oils contain a mix of different triglycerides species, the average MW can be calculated according to the following relation:
This means that coconut oil with an abundance of medium chain fatty acids contain more fatty acids per unit of weight than, for example, olive oil. Consequently, more ester saponifiable functions were present per g of coconut oil, which means more KOH is required to saponify the same amount of matter, and thus a higher SV. The calculated molecular weight is not applicable to fats and oils containing high amounts of unsaponifiable material, free fatty acids, or mono- and diacylglycerols.
Unsaponifiables
Unsaponifiables are components of a fatty substance that fail to form soaps when treated with alkali and remain insoluble in water but soluble in organic solvents. For instance, typical soybean oil contains, by weight, 1.5 – 2.5% of unsaponifiable matter. Unsaponifiables include nonvolatile components : alkanes, sterols, triterpenes, fatty alcohols, tocopherols and carotenoids as well as those that mainly result from the saponification of fatty esters. This fraction may also contain environmental contaminants and residues of plasticizers, pesticides, mineral oil hydrocarbons and aromatics.Unsaponifiable constituents are an important consideration when selecting oil mixtures for the manufacture of soaps. Unsaponifiables can be beneficial to a soap formula because they may have properties such as moisturization, conditioning, antioxidant, texturing etc. On the other hand, when proportion of unsaponifiables is too high, or the specific unsaponifiables present do not provide significant benefits, a defective or inferior soap product can result. For example, shark oil is not suitable for soap making as it may contain more than 10% of unsaponifiable matter.
For edible oils, the tolerated limit of unsaponifiable matter is 1.5%, while inferior quality crude or pomace oil could reach 3%.
Determination of unsaponifiables involves a saponification step of the sample followed by extraction of the unsaponifiable using an organic solvent. Official methods for animal and vegetable fats and oils are described by ASTM D1065 - 18, ISO 3596: 2000 or 18609: 2000, AOCS method Ca 6a-40.