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International Starch Institute
Science Park Aarhus, Denmark
ISI 35-1e Determination of Reductive Power in Starch

1. Scope The method is applicable to thin-boiling starch

 

LT 18.12.1964
Rev. 17.03.1998
2. Principle Ferric cyanide is reduced by starch to ferrous cyanide and precipitated as a zinc compound. Excess ferric ions reduce J- and iodine is titrated with thiosulphate.

 

3. Apparatus 3.1 Analytical balance weighing to the nearest 1 mg.
3.2 Boiling water bath
3.3 Erlenmeyer flask 300 ml
3.4 Kjeldahl bulb
4. Reagents 4.1 Alkaline ferric cyanide solution 0.05M: 16.5 g K3Fe(CN)6 + 22 g water free Na2CO3 ad one litre distilled water. Keep dark.

4.2 Zinc solution: 200 ml water free acetic acid, 70 g KCl and 20 g ZnSO4, 7H2O ad one litre with distilled water

4.3 Potassium iodine KJ
4.4 Thiosulphate 0.05N: 50.0 ml

 

Short shelf life
5. Procedure Weigh approximate 250 mg (g) sample to the nearest mg and transfer to 300 ml flask (3.3). Slurry in 25 ml distilled water.
Gelatinise on boiling water bath for 5 min.
Add with one mark pipette 25.0 ml alkaline ferri cyanide (4.1) under agitation. Cover flask with Kjeldahl bulb and place in vigorously boiling water bath for 15 min. Then rapidly cooling under tap.
Ad from graduated beaker 60 ml zink solution (4.2). Shake and add approx. 4 g KJ (4.3)

 Titration

Free iodine is titrated with thiosuplphate (4.4)

 Blank

Run a blank with 25 ml distilled water without heating.
6. Calculation Calculate reducing power of sample dry matter by averaging results of two samples with two decimals.

Reducing Power, RCu = mg copper per g starch dry matter

= ((b-a) x 1000 x N x 6.354) / (g x d ), where

a = ml thiosulphate for titration of sample

b = ml thiosulphate for titration of blank

N = Normality of thiosulphate

g = g sample

d = % dry matter of sample

 

6. Note Reducing power is also expressed as a ferric cyanide number, Fecy = ml 0.1 N thiosulphate per g starch dry matter.

The RCu / Fecy is a measure for reducing groups in sample and groups formed during analysis.

 

7. Reference Schoch; Methods in Carbohydrate Chemistry, Vol. IV, 64 (1964)

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Keywords: laboratory method determination reductive power reducing thin-boiling starch