206 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Soluble Starch. 



Starch grains consist of at least two substances. The French workers 

 Maquenne and Roux term them amylocellulose or amylose, the chief con- 

 stituent, and amylopectin. Amylose (granulose of previous workers) is par- 

 tially soluble in boiling water, but completely soluble in boiling water under 

 pressure. On cooling, the insoluble portion is again obtained by " reversion ". 

 The one seems to be a polymer of the other. Amylopectin is a gum-like 

 substance which swells up without dissolving in water. The gelatinisation of 

 starch paste is said to be due to the amylopectin. 



Fernbach states that soluble starch can be obtained from potato starch 

 by pouring a 1-2 per cent, suspension into a large excess of acetone and 

 shaking vigorously. A flocculent precipitate is formed, which, if filtered off, 

 ground up with acetone in a mortar and dried in vacua, dissolves in cold 

 water. 



Starch grains treated with dilute hydrochloric acid of about i o per cent, 

 for 24 hours do not lose their external appearance, but they become soluble 

 in hot water without forming a paste. Alcohol precipitates from the solution 

 a white powder, soluble in water, termed soluble starch (Brown and Morris). 



Soluble starch is most readily prepared by allowing 500 gm. of starch to* 

 stand in contact with 1000 c.c. of dilute hydrochloric acid of sp. gr. 1*037 for 

 7 days, stirring the mixture daily, pouring off the acid, washing the residue 

 free from acid with water by decantation, the last portions containing a trace 

 of ammonia, and drying it by exposure to the air. The dry product, ground 

 up in a mortar and rubbed through a fine hair sieve, is soluble in warm water 

 giving a clear solution (Lintner). 



Another method of preparing soluble starch is to treat 400 gm. of potato 

 starch with 2300 c.c. of water and 80 c.c. of N HC1 in a flask in boiling 

 water for 1-5 hours. The solution is cooled to 50, made ammoniacal and 

 800 c.c. of alcohol added. The solution is strained through muslin, and 

 whilst warm, poured into 4000 c.c. of alcohol. After 48 hours the precipi- 

 tate is filtered off, washed with alcohol and spread out to dry. 



Reactions. 



The following reactions are given by starch paste or a solution of 

 soluble starch. 



(1) Action of Alcohol. 



Starch is precipitated completely by adding an equal volume of 

 alcohol. 



(2) Action of Iodine. 



If a few drops of iodine solution be added to a starch solution, a 

 dark blue colour appears. On heating the solution, the colour disap- 

 pears, but appears again on cooling. 



The blue colour is discharged on adding 1-2 drops of caustic 

 soda. The colour reappears on neutralising with dilute hydrochloric 

 acid. 



(3) Basic Lead Acetate. 



On adding basic lead acetate to starch solutions, the starch is pre- 

 cipitated. 



