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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
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| 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.
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| 5. Procedure | Weigh 10.00 g (w1) wet pulp sample. | |
Preparation |
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 |
Hydrolysis |
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 | |
Clarification |
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 | |
Measurement |
Measure optical rotation (a1) of the filtrate with the
polarimeter (3.2)
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| 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).
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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".
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| 8. Reference | International Standard: ISO 10520 |
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Keywords: laboratory method determination starch pulp potato cassava