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International Starch Institute
Science Park Aarhus, Denmark
ISI 28-1e Determination of Reducing Sugar, DE by Luff-Schoorl's method.

1. Scope The method is applicable to all starch hydrolysis products.

 

LT 1/9 1970
Rev.: LT 22.01.2002
2. Principle Iodine titration of excess copper

 

3. Reagents 3.1 Luff-Solution:

3.1.a Citric acid: 50 g C6C8O7,H2O p.a. is dissolved in 50 ml distilled water.

3.1.b. Sodium carbonate: 143.7 g Na2CO3 p.a. is dissolved in 400-500 ml boiling distilled water. 

3.1.a + 3.1.b is gently mixed after cooling of 3.1.b

3.1.c Copper sulfate: 25 g CuSO4, 5 H2O p.a. is dissolved in approx. 100 ml distilled water and transferred to the mixture of (3.1.a + 3.1.b). Dilute to 1 liter with distilled water.

3.2 Acetic acid, 0.4 N
24 ml CH3COOH p.a. up to 1 liter with distilled water.

3.3 Iodine 0.1 N (Standard titration solution)

3.4 Hydrochloric Acid 0.75 N
73 g conc. HCl up to 1 liter with distilled water

3.5 Sodium thiosulphate 0.1. (Standard titration solution)

3.6 Starch solution
2 g soluble starch is dissolved in 100 ml distilled water. Preserved with a tiny amount of HgO.

 

p.a. = pro analyse

Sample preparation

3.7 Sample solution:
The DE of sample is estimated and the quantity consuming approx. 15 ml thiosulphate (3.5) is calculated. Weigh the sample and transfer to a 250 ml metering flask. Dilute with distilled water to mark.
a = g total weight of sample used to prepare 250 ml solution 
4. Apparatus 4.1 Electrical heating plate

 

5. Procedure
Transfer with pipette 25.0 ml Luff-solution (3.1) and 25.0 ml sample-solution (3.7) to a 300 ml conical flask with standard taper-ground outer joint. Add glass spheres
Heat in 3-3 min to boiling on electrical heater.

Boil with reflux condenser in exactly 10 min. Flush condenser with a small amount of distilled water. Cool immediately in cold water.

 

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Add 50 ml acetic acid (3.2) from graduated cylinder. Add with pipet 25.0 ml iodine 0.1 N (3.3). Add from graduated cylinder 55 ml 0.75 N HCl (3.4). Shake until all is dissolved. Titrate with thiosulphate (3.5) to copper-blue with 6-8 drops of starch solution (3.6) as indicator.

Blind

Carry out a blind with 25 ml distilled water as sample solution (3.7) - same procedure
6. Calculation DE (average results of two samples to one decimal) on dry matter basis
= (c*100)/(a*d)

where

B= ml 0.1 N thiosulphate used for the blind
A= ml 0.1 N thiosulphate used for the sample
T= (B-A)*f
f= (titration) factor for the thiosulphate

c= mg dextrose per 25 ml sample solution. Read from table by interpolation using the titer T
a= sample g
d= % sample dry matter g/100g (or Brix oBx=

a = factor from table below (9. Table)
b = ml test solution for titration
g = gram sample dissolved to 250 ml
d = % dry solids in sample

 

7. Notes Alternative method: Calculation of DE from a sugar distribution obtained by HPLC.

 

8. Reference Alternative method: International Standard: ISO 5377

Schoorl, N. (1929). Suiker titratie. Chemisch Weekblad 130-134 and ZLUF 57, 566 (1929)

9. Table

Titer - ml 0.1 N thiosulphate
T

mg dextrose per 25.0 ml
c

Difference
used for interpolation

All figures relate to anhydrous dextrose.
 

1

2.4

2.4

 
 

2

4.8

2.4

 
 

3

7.2

2.5

 
 

4

9.7

2.5

 
 

5

12.2

2.5

 
 

6

14.7

2.5

 
 

7

17.2

2.6

 
 

8

19.8

2.6

 
 

9

22.4

2.6

 
 

10

25.0

2.6

 
 

11

27.6

2.7

 
 

12

30.3

2.7

 
 

13

33.0

2.7

 
 

14

35.7

2.8

 
 

15

38.5

2.8

 
 

16

41.3

2.9

 
 

17

44.2

2.9

 
 

18

47.1

2.9

 
 

19

50.0

3.0

 
 

20

53.0

3.0

 
 

21

56.0

3.1

 
 

22

59.1

3.1

 
 

23

62.2

 

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