STARCH i6i 



3. Starch grains are insoluble in cold water, but in hot 

 water they swell up and form an opalescent solution which, if 

 strong enough, will on cooling eventually form a paste. 



4. Starch is precipitated from its aqueous solution by 

 alcohol or by basic lead acetate (cf. Inulin and Dextrin). 



5. Boil a little starch paste solution with a few drops of 

 dilute sulphuric acid in a test tube, and from time to time 

 remove a little of the solution, cool it and test with iodine 

 solution ; when the starch has been converted into dextrin the 

 blue colour at first formed will give way to a plum colour. 

 If boiled too long only dextrose will remain which gives no 

 colour with iodine. The solution will, however, after making 

 alkaline, reduce Fehlings' solution. 



6. Cautiously heat a little starch on a porcelain basin 

 until it has acquired a light fawn colour. Cool and extract 

 with cold water, and filter ; the dextrin produced being soluble 

 in cold water is thus separated from the starch. On adding 

 iodine to the solution a plum colour is produced. 



Estimation of Starch. 



The chief difficulty in estimating starch by determining 

 the amount of reducing sugar formed after appropriate 

 hydrolysis lies in the error caused by the presence of pentosans. 

 Lintner overcame this difficulty by estimating the pentosans 

 by the phloroglucinol method (see p. 137) and deducting a 

 proportionate amount from the reducing power after hydrolysis, 

 on the assumption that xylose and arabinose have approxi- 

 mately the same reducing power as glucose. 



A method for the determination of starch in barley or 

 wheat due to Ling, Nanji, and Harper * makes use of the fact that 

 when a paste of any of the starches, or materials containing 

 starch, is treated with barley diastase at 50°, the amylose is 

 converted into maltose and the amylopectin into aj8-hexa- 

 amylose leaving the amylohemicellulose of the cereal starches 

 as an insoluble residue. 



Cereal starches, owing to the presence of amylohemi- 

 cellulose, do not give the same percentage of maltose as other 



* Ling, Nanji, and Harper : " J. Inst. Brewing," 1924, 30, 838. 



II 



