Estimation of free fatty acids
A small quantity of free fatty acids is usually present in oils along with the triglycerides. The free fatty acid content is known as acid number/acid value. It increases during storage. The keeping quality of oil therefore relies upon the free fatty acid content.
The free fatty acid in oil is estimated by titrating it against KOH in the presence of phenolphthalein indicator. The acid number is defined as the mg KOH required to neutralize the free fatty acids present in 1g of sample. However, the free fatty acid content is expressed as oleic acid equivalents.
» 1% phenolphthalein in 95% ethanol.
» 0.1N potassium hydroxide
» Neutral Solvent
: Mix 25mL ether, 25mL 95% alcohol and 1mL of 1% phenolphthalein solution and neutralize with N/10 alkali.
||Dissolve 1-10g of oil or melted fat in 50mL of the standard solvent in a 250mL conical flask.
||Add a few drops of phenolphthalein.
||Titrate the contents against 0.1N potassium hydroxide.
||Shake constantly until a pink color which persists for fifteen seconds is obtained.
|Acid value (mg KOH/g) =
||Titrate value x Normality of KOH x 56.1
|Weight of sample (g)
The free fatty acid is calculated as oleic acid using the equation
1mL N/10 KOH = 0.028g oleic acid
To find out the exact strength of KOH, prepare 0.1N oxalic acid solution (630mg in 100mL water) and titrate against KOH with phenolphthalein as indicator. Calculate the strength of KOH by the formula V1
1. Cox H E and Pearson, D (1962) The Chemical Analysis of Foods
Chemical Publishing Co Inc New York p 420.