Analytical constants of fats & oilsPharmaceutical

Analytical constants of fats & oils

Session Objectives

By the end of this session, students will be able to:

Ø  Explain the determination of Analytical constants of fats & oils

Analytical constants of fats and oils

Adulteration of fats and oils is increasing day by day

Several tests are employed in the laboratory to check purity of fats and oils

Some of them are discussed here under

§  Acid value,

§  Saponification value,

§  Ester value,

§  Iodine value,

§  Acetyl value,

§  Reichert Meissl (RM) value

Acid Value

Definition: The acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the free fatty acids present in one gram of fat.

It is a relative measure of rancidity as free fatty acids are normally formed during decomposition of oil glycerides.

The value is also expressed as per cent of free fatty acids calculated as oleic acid.

Therefore, oils with increased acid number are unsafe for human consumption.

Principle: The acid value is determined by directly titrating the oil/fat in an alcoholic medium against standard potassium hydroxide/sodium hydroxide solution.

Analytical Importance: The value is a measure of the amount of fatty acids which have been liberated by hydrolysis from the glycerides due to the action of moisture, temperature and/or lypolytic enzyme lipase.

Saponification Value

Definition: The saponification value is the number of mg of potassium hydroxide required to saponify (hydrolyse) 1 gram of oil/fat.

Principle: The oil sample is saponified by refluxing with a known excess of alcoholic potassium hydroxide solution. The alkali required for saponification is determined by titration of the excess potassium hydroxide with standard hydrochloric acid.

Analytical importance:

The saponification value is an index of mean molecular weight of the fatty acids of glycerides comprising a fat.

Lower the saponification value, larger the molecular weight of fatty acids in the glycerides and vice-versa.

The value is higher for fats containing short chain fatty acids

The saponification numbers of a few fats and oils are given below

Ester value

Definition: The ester value is the number of mg of potassium hydroxide required to saponify the esters in 1.0 g of the substance

Ester value = saponification value-acid value

Iodine number

Definition: The iodine value of an oil/fat is the number of grams of iodine absorbed by 100g of the oil/fat.

Principle: The oil/fat sample taken in carbon-tetrachloride is treated with a known excess of iodine monochloride solution in glacial acetic (Wijs solution). The excess of iodine monochloride is treated with potassium iodide and the liberated iodine estimated by titration with sodium thiosulfate solution.

Analytical importance: The iodine value is a measure of the amount of unsaturation (number of double bonds) in a fat.

       lodine number is useful to know the relative unsaturation of fats, and is directly proportional to the content of unsaturated fatty acids

       Thus lower is the iodine number, less is the degree of unsaturation

       The iodine numbers of common oils/fats are given below

Analytical constants of fats and oils

Acetyl value

Defenition: The acetyl value is determined by the milligrams of potassium hydroxide require to neutralize the acetic acid produced when 1 gram of fat or oil is acetylated (hydrolyzed) with acetic anhydride.

Indicates the number of free hydroxyl groups present in the substance.


The process consists of acetylating the oil with a measured quantity of acetic anhydride in pyridine decomposing the excess anhydride by boiling with water and then, after the addition of sufficient butyl alcohol to give a homogeneous solution, titrating with alkali.

A control test with the acetic anhydride and pyridine without the oil provides a measure of the acetic anhydride available for acetylation; a similar test with the oil and the pyridine without the acetic anhydride provides a measure of the free fatty acid present.

From the values obtained, the acetyl value or the hydroxyl value of the oil is calculated.

Reichert-Meissl (RM) number

Definition: The Reichert-Meissl value is the number of millilitres of 0.1N aqueous sodium hydroxide /KOH solution required to neutralise steam volatile water soluble fatty acids distilled from 5g of an oil/fat.

Principle: The material is saponified by heating with glycerol sodium hydroxide solution and then split by treatment with dilute sulfuric acid. The volatile acids are immediately steam distilled. The soluble volatile acid in the distillate are filtered out and estimated by titration with standard sodium hydroxide solution.

Analytical Importance:

RM number is useful in testing the purity of butter since it contains a good concentration of volatile fatty acids(butyric acid, caproic acid and caprylic acid)

This is in contrast to other fats and oils which have a negligible amount of volatile fatty acids

Butter has a RM number in the range 25-30, while it is less than I for most other edible oils

Thus any adulteration of butter can be easily tested by this sensitive RM number

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