SOP: Determination of Tannins by Permanganate Titration

GIFSQ — Global Institute for Food Safety & Quality

Principle

Tannin content, a measure of the astringency of the juice and of the tendency of fresh juice to discolour, is determined by direct titration with potassium permanganate using indigo carmine as indicator. Colouring matter, ascorbic acid and sulphur dioxide interfere, so one portion of the juice is treated with hide powder, which adsorbs the tannins; the difference between the titrations of the untreated and the hide-powder-treated portions gives the tannins.

Apparatus required

• Large evaporating dish
• Burette

Reagents

a. Potassium permanganate solution: dissolve 0.785 g in 1000 ml of water.

b. Indigo-carmine solution: dissolve 2 g in 500 ml of water by boiling, cool, add 50 ml of sulphuric acid, and dilute to 1000 ml with water. Dilute this solution until exactly 5 ml is decolourised by 1 ml of the permanganate solution.

c. Hide powder.

Procedure

To 5 ml of indigo-carmine solution in a large evaporating dish, add 200 ml of water and 1 ml of the juice

↓

Titrate with the potassium permanganate solution until the colour changes through green to yellow — note this titration as A

↓

Mix another portion of the juice with 0.5 g of hide powder, let it stand for 1 hour or preferably overnight, and filter

↓

Titrate an aliquot of the filtrate in the same way — note this titration as B

Calculation

% Tannins (w/v) = (A − B)

Where,

• A = ml of permanganate used for the untreated juice portion
• B = ml of permanganate used for the hide-powder-treated portion
• the conversion of the (A − B) titration difference to per cent tannins is not fully legible in the available copy of the book; the (A − B) difference form is carried as printed — check the printed book for the complete expression

Precautions

• Some colouring matter, ascorbic acid and SO₂ interfere with the direct titration — the hide-powder treatment is the recommended correction.
• Let the hide-powder mixture stand preferably overnight for complete adsorption of tannins.

References

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