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International Starch Institute
Science Park Aarhus, Denmark
ISI 22-1e Determination of Starch in Fruitwater by Ewers
| 1. Scope | The method is applicable to potato and cassava fruitwater and fruitjuice
(Effluent from the extraction of starch from tubers)
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LT 18. Dec. 64 Rev. LT 11. Dec. 98 |
| 2. Principle | Granular starch is hydrolysed and determined by Ewers polarimetric
method.
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| 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 Centrifuge 3.6 Conical glass beaker, 1500 ml with rack
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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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| Leave to stand for sedimentation of excess acetate. | ||
| 5. Procedure | Collect a 1000 ml sample (v1). | |
Preparation |
Centrifuge sample or let leave it to precipitate in a conical beaker (3.6). Discard the supernantant. Stir sediment with distilled water and (repeat) centrifugation. | It is essential to remove residual juice. |
Hydrolysis |
Collect the starch containing sediment and transfer 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 as is (average results of two samples
with two decimals) = Starch % = 1000000 *a 1 / (a D*2*v1)
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Value of a D at
589.3 nm for starch from: potato = +185.7 cassava = +184.0 |
| 7. Notes | Proper wash of isolated sediment and proper clarification is essential. Alternative: Clarify with Carrez solutions. | Residual juice dims polarimeter readings |
| Quick method: Centrifuge15 ml sample in conical beaker with 0.1 ml marks.
Read (or estimate) starch sediment in ml (v2). Starch dry matter = v2*0.6
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Impurities often reduce readings to a rough estimate. | |
| 8. Reference | International Standard: ISO 10520 |
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Keywords: laboratory method determination starch potato cassava fruitwater fruitjuice juice Ewers