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Rancidity of Edible Oil

The document discusses the determination of acid value, which is a measure of free fatty acids present in oils and fats. It is determined by titrating a sample of the oil/fat against a standard potassium hydroxide or sodium hydroxide solution using phenolphthalein as an indicator. A higher acid value indicates more hydrolysis of triglycerides due to factors like moisture, temperature, and enzyme activity during inadequate processing or storage. The titration process and calculation of acid value from the titration results are also described.

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Usman Ghani
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847 views2 pages

Rancidity of Edible Oil

The document discusses the determination of acid value, which is a measure of free fatty acids present in oils and fats. It is determined by titrating a sample of the oil/fat against a standard potassium hydroxide or sodium hydroxide solution using phenolphthalein as an indicator. A higher acid value indicates more hydrolysis of triglycerides due to factors like moisture, temperature, and enzyme activity during inadequate processing or storage. The titration process and calculation of acid value from the titration results are also described.

Uploaded by

Usman Ghani
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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11.

0 Determination of Acid Value

11.1 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.

The acid value (AV) is a common parameter in the specification of fats and oils. It is defined as the
weight of KOH in mg needed to neutralize the organic acids present in 1g of fat and it is a measure
of the free fatty acids (FFA) in a sample of oil or fat indicates hydrolysis of triglycerides. Such
reaction occurs by the action of lipase enzyme and it is and indicator of inadequate processing and
storage conditions (i.e. high temperature, and relative humidity, tissue damage). Besides of free
fatty acids, hydrolysis of triglycerides produces glycerol.

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

11.1.2 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.

Chemical Equation for the analysis


RCOOH + KOH → ROO- K+ + H2O 

11.1.3 Apparatus: 250 ml conical flasks.

11.1.4 Reagents:

a) Ethyl alcohol: - Ninety-five per cent alcohol or rectified spirit neutral to phenolphthalein
indicator.

b) Phenolphthalein indicator solution: - Dissolve one gram of phenolphthalein in 100 ml of ethyl


alcohol.

c) Standard aqueous potassium hydroxide or sodium hydroxide solution 0.1N. Dissolve 4g of NaOH
pellets into 900ml of distilled water. Cool and make a final volume of 1000ml. Standardized the
solution before use. The solution should be colorless and stored in a brown glass bottle. For refined
oils, the strength of the alkali should be fixed to 0.1 N.

11.1.5 Procedure: Mix the oil or melted fat thoroughly before weighing. The mass of the test
sample shall be taken based on the color and expected acid value.
Weigh accurately appropriate amount of the cooled oil sample in a 250 ml conical flask and add 50
ml to 100 ml of freshly neutralised hot ethyl alcohol and about one ml of phenolphthalein indicator
solution. Boil the mixture for about five minutes and titrate while hot against standard alkali
solution shaking vigorously during the titration. The weight of the oil/fat taken for the estimation
and the strength of the alkali used for titration shall be such that the volume of alkali required for
the titration does not exceed 10 ml.
M .W V N
11.1.6 Calculation: Acid value =
W
Where, M.W = Molecular weight of NaOH (40) / KOH (56.1)

V = Volume in ml of standard potassium hydroxide or sodium hydroxide used

N = Normality of the potassium hydroxide solution or Sodium hydroxide solution;

and W = Weight in g of the sample

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