Home Profile Market Place Sweeteners Lab Methods Glossary Search
Master Index Environment Application Starch Statistics Tables Write us

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 polarimeter (3.2)


6. Calculation Starch in percent of sample (average results of two samples without decimals) =

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

| Top of PageLabIndex |

Copyright© 1999. Read Disclaimer notice.
All rights reserved. International Starch Institute, Science Park Aarhus, Denmark.

Keywords: laboratory method determination starch pulp potato cassava