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.
Principle:
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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