SOP: Tannins in Cloves and Allspice
GIFSQ — Global Institute for Food Safety & Quality
Principle
The sample is defatted and the tannins extracted with boiling water; the extract is titrated with standard potassium permanganate using indigo as indicator, and the permanganate consumed (blank minus sample) is converted to quercitannic acid by the factor 0.006235, or to oxygen absorbed by the factor 0.0008.
Apparatus required
- Soxhlet apparatus
- 2 litre porcelain dish
Reagents
a. Ether (for the Soxhlet extraction).
b. 0.1 N oxalic acid: one ml of this solution is equal to 0.006235 g of quercitannic acid or 0.0008 g of oxygen absorbed.
c. Potassium permanganate standard solution: dissolve 1.333 g of KMnO₄ in 1000 ml, and standardize against oxalic acid.
d. Indigo solution: dissolve 6 g of sodium indigotin disulphonate in 500 ml of water by heating, cool, add 50 ml of H₂SO₄, dilute to 1000 ml, and filter.
Procedure
Extract 2 g of the sample for 20 hr in a Soxhlet apparatus with ether, or use the residue from the determination of nonvolatile ether extract
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Boil the residue for 2 hr with 300 ml of water, cool, dilute to 500 ml, and filter
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Pipette 25 ml of the filtrate into a 2 litre porcelain dish; add 20 ml of indigo solution and 750 ml of water
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Titrate against standard KMnO₄ solution — add KMnO₄ solution 1 ml at a time until the blue solution changes to green, and then a few drops at a time until the colour of the solution becomes golden yellow
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Carry out a blank titration using 20 ml of indigo solution and 750 ml of water
Calculation
% Quercitannic acid = ((Sample titre − Blank titre) × 500 × 0.006235 × 100) / (25 × Weight of sample)
Where,
- Sample titre = ml of standard KMnO₄ solution used in the sample titration
- Blank titre = ml of standard KMnO₄ solution used in the blank titration
- Weight of sample = weight of the sample taken (2 g)
To find oxygen absorbed, use 0.0008 instead of 0.006235 in the above expression.
Note
According to US Standards, cloves should contain not less than 12% quercitannic acid.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.