Introduction
Among agricultural crops, seeds use for the extraction of fixed oils are rated in economic and commercial importance second only to cereals. Fixed oils are obtained from fruit pericarps and in some instances such as the palm, Elaeis guineensis (palmae), two oils differing in properties and chemical composition are obtained- the palm kernel oil from the endosperm and the palmoil from the orange yellow fleshy pericarp.
A naturally occurring mixture of lipids such as olive oil or oil of theobroma may be either solid or liquid hence the term “oil” or fat ceases to have any significance.
Quantitative Tests used to evaluate Fixed oils and Fats
- Acid value: Refers to the number of milligram of potassium hydroxide (KOH) required to neutralize the free acid in 1g of the oil. In particular, low values are officially specified for those oils to be used in parenteral dosage forms.
- Saponification value:- The hydrolysis reactions of lipids can be used to determine this saponification value. It is expressed as the number of milligram of potassium hydroxide required to neutralize the free acid and hydrolyse the esters in 1g of the oil.
- Ester value:-this is the difference between the saponification and acid value.
- Iodine value:- This gives a measure of the unsaturation of the oil. Oils which partially resinify on exposure to air are known as semi drying oil or drying oil. Such oils (e.g. linseed and cotton seed oil) have high iodine values.
- Volatile acidity:- This is particularly important for animal fats such as butter. The lower fatty acids such as butyric acid are volatile in steam and this may be used for their estimation and separation.
- Unsaponifiable matter:-This estimates the quantity of compounds such as sterol which remains after the saponification of the acylglycerol and the removal of the glycerol and soaps by means of solvent. The Pharmacopoeia provides permissible limits for some oils such as evening primerose and borage oil which is a maximum of 0.3%.
- Acetyl Value:- The number of milligrammes of potassium hydroxide required to neutralize the acetic acid freed by the hydrolysis of the acetylated fats.
- The Hydroxyl value:-This depends on the number of free hydroxyl groups present. It is expressed as the number of milligramme of potassium hydroxide required to neutralize the acid combined by the acetylation of the sample.
- Peroxide value:- This is a measure of the deterioration that has taken place in an oil. Under unsuitable storage conditions such as exposure to light and air, fixed oils undergo secondary oxidation to give peroxides that then generate aldehydes and ketones. It is expressed in milliequivalents of oxygen, the quantity of peroxides contained in 1000g of the substance.
- Anisidine value:- This also measures the level of deterioration in an oil. Anisidine values are determined photometrically (350nm) and depend on the coloured complexes produced by interactions of p-anisidine with aldehydes and ketones.