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International Starch Institute
Science Park Aarhus, Denmark
ISI 07-2e Determination of Starch in Pulp by Ewers
|1. Scope||The method is applicable to potato and cassava pulp.||LT 22 Feb. 1966
Rev. LT 11. Dec. 1997
|2. Principle||Granular starch is hydrolysed and determined by Ewers polarimetric method.||By-product from starch extraction|
|3. Apparatus||3.1 Analytical balance weighing to the nearest 1 mg.
3.2 Polarimeter with 2 dm tube
3.3 Shaking boiling water bath.
3.4 Volumetric flask, 100 ml with wide neck
3.5 Büchner funnel
|4. Reagents||4.1 Dilute hydrochloric acid 0.12 N
4.2 Lead solution, saturated
Dissolve 70 g Pb(CH3COO)2, 3 H2O in 100 ml hot distilled water.
|5. Procedure||Weigh 10.00 g (w1) wet pulp sample.|
|Slurry sample in 500 ml water and filter through Büchner funnel (3.5) and paper. Wash with 1 l water.||Do not let pulp dry out during washing|
|Transfer washed sample to volumetric flask (3.4) and flush down sample with 60 ml diluted hydrochloric acid (4.1).|
|Place volumetric flask in boiling water bath (3.3) for 15.0 min||Shake min. 3 min initially|
|Cool flask in cold water to 20 oC. Add 4 ml lead solution (4.2). Fill to mark with water. Shake and leave to stand for 45 min.||Cool rapidly|
|Filter through funnel and paper.||Repeat with more lead solution if not clear|
|Measure optical rotation (a1) of the filtrate with the
|6. Calculation||Starch in percent of sample (average results of two samples without
Starch % = 1000000 *a 1 / (a D*2*w1)
|Value of a D
at 589.3 nm for starch from:
potato = +185.7
cassava = +184.0
|Correct eventually to starch on pulp dry matter (dry 5 g sample at 130 oC
for two hours).
|Juice in pulp affects value on dry matter|
|7. Notes||Proper clarification is essential. Alternative: Clarify with Carrez solutions.||Residual juice dims polarimeter readings|
|"Bound" starch may be determined by exchanging the Büchner funnel with a screen during preparation. This procedure, however, does not properly distinguish between starch in intact cells and starch otherwise retained.||120 micron openings|
|An informative figure is the ration of starch to fibre. The fibre
fraction is found by solving the equation: "Dry matter = starch + solubles +
fibre". The solubles is process dependent and can be found by drying an aliquot of
washing water obtained in "preparation".
|8. Reference||International Standard: ISO 10520|
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Keywords: laboratory method determination starch pulp potato cassava