234 Alfred J. Eivart : 



The appai-ent accelerating action on malt diastase observed by 

 Rothera and Greenwood is the result of an experimental error, 

 and is probably due to the action of the tannic acid of the apple 

 pulp upon the starch solution, and upon the iodine test employed. 



On mixing 1 per cent, solutions of gallotannic acid and starch 

 a dense white precipitate is thrown down. With more dilute 

 solutions up to 0.1 per cent, a more bulky and gelatinous precipi- 

 tate forms. A slight gelatinous precipitate may form even when 

 0.04 per cent, solution is used. These results are shown even when 

 filtered starch solution is used, but the settling of the coagulum 

 or precipitate is slower. The precipitate dissolves on boiling, and 

 is precipitated on cooling, even in the presence of dilute HCl. 

 On repeatedly washing the coagulum with water all the tannic 

 acid can be removed, and it then gives no appreciable blue with 

 ferric chloride. If strong tannic acid is poured into starch solu- 

 tion, pasty masses of plastic starch form. On boiling in • w^ater 

 these break up, but without completely dissolving, especially if 

 they have been for some time in contact with the tannic acid. 

 They dissolve, forming a clear solution on warming with dilute 

 hydrochloric acid, but on cooling a white precipitate of starch is 

 formed. The coagulum is not strongly attacked by diastase. At 

 least in experiments lasting over 1-2 days at 30o C, the precipi- 

 tated starch was still undissolved by 1 per cent, malt diastase, and 

 gave a strong blue with iodine. After drying in air tlie plastic 

 masses of precipitated starch became hard and translucent. They 

 did not dissolve even on prolonged boiling with water, but dis- 

 solved rapidly on the addition of hydrochloric acid, forming a 

 clear solution, giving blue with iodine. This " insolul)le " starch 

 is probably in a different physical condition to the starch grains, 

 and appears to be very resistant to diastase. 



An excess of cohl 2 per cent, tannic acid even precipitates 

 soluble starch, but the milky liquid becomes clear again at 35° C, 

 and cloudy on cooling, Avhile the dried gummy precipitate dis- 

 solves readily in l)oiling water. Gallic acid is much less active 

 than gallotannit- acid in })rccipit atiiig ordinaiy starcli, and has 

 no prei'ipitatiiig action on soluble starch, even in toiisiderable 

 excess. In addition lleintz^ has shown that tannii' acid interferes 

 with the iodine test for staix'h. 'I'hus. if a drop of iodine solu- 

 tion is added to a mixture of a 1 ])er icnt. tannic acid and 1 

 per cent, starch solution, the blue raj. idly fades to purple, and then 



t. Jahresb. Asjiltult. Chem,, 1879, p. 499. 



