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International Starch Institute
Science Park Aarhus, Denmark
ISI 23-1e Determination of Alkali Lability

1. Scope The method is applicable to starch, dextrin, acid and enzyme modified starch

 

LT 17 Dec.1964
Rev. LT 30 Aug 1999
2. Principle Acid is released by reorganising aldehyde end-groups in an alkaline environment. Ester groups such as acetate and adipate are hydrolysed by alkali. The acid is titrated as a comparative measurement of aldehyde-groups and thereby also of starch molecule length.

 

3. Apparatus 3.1 Analytical balance weighing to the nearest 0.1 mg.
3.2 Stopwatch
3.3 Water bath
3.4 Beaker 400 ml
3.5 Conical flask 250 ml and rubber stopper with one hole fitted with a glass tube closed in one end by a rubber tubing. The free end of the rubber tubing is closed and a slit is cut in the tubing. The slit is functioning as a valve for the whole assembly. Vigorously boiling
3.6 pH-metre or titrator

 

4. Reagents 4.1 Sodium hydroxide 0.4N, carbonate free.
4.2 Hydrochloric acid 0.2N
4.3 Sodium hydroxide 0.1N
4.4 Hydrochloric acid 0.1N

 

5. Procedure Weigh 0.5 g sample to the nearest mg in a 250 ml conical flask (3.5)
Add 10 ml distilled water and stir well.

Gelatinising

Add 25.0 ml (4.1) alkaline and stir well Avoid lump forming
When the starch is evenly gelatinised 65 ml 95 - 100 C hot distilled water is added. The flask is closed with the rubber stopper and valve. The flask is heated exactly 60 minutes in a vigorously boiling water bath (3.4). Cool flask immediately in running cold water.

Measuring

Flush the cooked starch into a 400 ml beaker and titrate to pH 8.0 with acid (4.2).  Note volume of acid  in ml (V).
Run a blind with 100 ml distilled water and 25.0 ml (4.1) alkaline. Note volume of acid in ml (B).

 

6. Calculation Calculate Alkali Lability of sample by averaging results of two samples with no decimal.

Alkali (Lability) Number =  ml 0.1N NaOH/g of starch dry matter = (V-B)*N*1000/(g*d), where

V = ml HCl used for titration of sample
B = ml HCl used for titration of blind
N = Normality of HCl used for titration
g = Sample weight in g
d = Sample dry matter in per cent

 

7. Note The Alkali Number is corrected in case of alkaline or acid sample. 1 g of sample is gelatinised in 50 ml hot distilled water. When evenly gelatinised the gel is cooled and titrated immediately to pH 8.0 with either acid (4.4) or alkaline (4.3). The titer as ml acid (4.4) used is added to the Alkali Number or ml alkaline (4.3) used is subtracted the Alkali Number

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Keywords: laboratory method determination alkali lability starch dextrin modified starch