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)

 

 LT 18. Dec. 64
Rev.  LT 11. Dec. 98
2. Principle Granular starch is hydrolysed and determined by Ewers polarimetric method.

 
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

 
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.

 
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)

 
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)

 

 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

 

 Impurities often reduce readings to a rough estimate.
8. Reference International Standard: ISO 10520

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