SOP: Acid Value (Free Fatty Acids) of Oils and Fats

GIFSQ — Global Institute for Food Safety & Quality

Principle

The acid value is the number of milligrams of KOH required to neutralise the free fatty acids present in one gram of the oil or fat. The sample is dissolved in neutralised alcohol and titrated with standard alkali using phenolphthalein as indicator. If the free fatty acid content has to be expressed in percentages by weight, the calculation is based on the molecular weight of the predominant fatty acid (lauric acid for coconut or palm kernel oil; oleic acid generally).

Apparatus required

• Conical flask

Reagents

a. Neutral alcohol: boil ethanol, add a few drops of phenolphthalein and titrate against 0.01 N NaOH.

b. 0.01 and 0.1 N NaOH or KOH.

c. Phenolphthalein indicator. For oils and fats giving a dark coloured solution, use the indicators stated under the determination of saponification value (thymolphthalein or alkali blue 6B, or the methylene blue–phenolphthalein mixture).

Procedure

Weigh 10 g of the oil or melted fat

↓

Dissolve the sample in 100 ml of hot neutralised ethanol and titrate with 0.01 or 0.1 N alkali using phenolphthalein as indicator; shake vigorously during titration and keep the solution warm

Calculation

             ml of alkali × N of alkali × 56.1
Acid value = ──────────────────────────────────
                     Wt of sample (g)

The acid factors for 0.01 N alkali, to express the free fatty acid content in terms of the predominant acid in the sample, are: lauric acid = 0.002; palmitic acid = 0.00256; oleic acid = 0.00282. To convert % free fatty acids (as oleic) to acid value, multiply the former by 1.99.

Note: the source’s calculation heading reads “Acid value as oleic acid”; it is carried here as printed.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.